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INFO-DIR-SECTION GNU Libraries
START-INFO-DIR-ENTRY
* Assuan: (assuan). An IPC library for non-persistent servers.
END-INFO-DIR-ENTRY
This file documents the use and the internals of Assuan.
This is Edition 2.2.0, last updated 28 November 2014, of `The
`Developing with Assuan' Manual', for Version 2.2.0.
Published by the Free Software Foundation
51 Franklin Street, Fifth Floor
Boston, MA 02110-1301 USA
Copyright (C) 2002, 2003, 2006, 2007, 2011 Free Software Foundation,
Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3 of the License, or (at your
option) any later version. The text of the license can be found in the
section entitled "Copying".
File: assuan.info, Node: Top, Next: Introduction, Up: (dir)
Introduction
************
This manual documents how to exploit the Assuan library, a simple
interprocess communcation library.
* Menu:
* Introduction:: An introduction to and the motivation behind Assuan.
* Assuan:: Description of the Assuan protocol.
* Implementation:: Overview of the implementation.
* Preparation:: What you should do before using the library.
* Generalities:: Initialization code and data types used.
* Client code:: How to develop an Assuan client.
* Server code:: How to develop an Assuan server.
* External I/O Loop:: How to use external I/O event loops.
* Utilities:: Utility functions.
* Socket wrappers:: Socket wrapper functions.
Miscellaneous
* Library Copying:: GNU Lesser General Public License says
how you can copy and share Assuan.
* Copying:: How you can copy and share this manual.
Indices
* Index:: Index of concepts and symbol names.
File: assuan.info, Node: Introduction, Next: Assuan, Prev: Top, Up: Top
1 Introduction to Assuan
************************
In an ideal world, Assuan would not be necessary. Assuan's primary use
is to allow a client to interact with a non-persistent server. Using
Assuan, this is accomplished by forking a subprocess and communicating
with it via, for example, a pipe or Unix domain socket. This method is
neither elegant nor efficient, especially when there is a lot of data
spread across several transactions: not only is there a penalty for an
increased number of context switches, but also a significant amount of
data is MEMCPYed from the client to a file descriptor and from the file
descriptor to the server. Despite these and other disadvantages, this
type of client/server communication can be useful: the client is
completely separate from the server; they are in different address
spaces. This is especially important in situations where the server
must have a known degree of reliability and data must be protected: as
the Assuan protocol is well defined and clients cannot corrupt the
servers' address space, auditing becomes much easier.
Assuan was developed for use by the GNU Privacy Guard, GnuPG, to
prevent potentially buggy clients from unwittingly corrupting sensitive
transactions or compromising data such as a secret key. Assuan permits
the servers, which do the actual work, e.g. encryption and decryption
of data using a secret key, to be developed independently of the user
interfaces, e.g. mail clients and other encryption front ends. Like a
shared library, the interface is well defined and any number of front
ends can use it; however, unlike a shared library, the client cannot
see or touch the server's data. As with any modular system, Assuan
helps keep the components small, understandable and less error prone.
Assuan is not, however, limited to use with GnuPG servers and
clients: it was designed to be flexible enough to meet the demands of
many transaction based environments with non-persistent servers.
File: assuan.info, Node: Assuan, Next: Implementation, Prev: Introduction, Up: Top
2 Description of the Assuan protocol.
*************************************
The architecture of the modular GnuPG system is based on several highly
specialized modules which compose a network of client/server
communication. A common framework for intermodule communication is
therefore needed and should be implemented in a library.
Goals:
* Common framework for module communication
* Easy debugging
* Easy module testing
* Extensible
* Optional authentication and encryption facility
* Usable to access external hardware
Design criteria:
* Client/Server with back channel
* Use a mainly text based protocol
* Escape certain control characters
* Allow indefinite data length
* Request confidentiality for parts of the communication
* Dummy module to allow direct linking of client and server
* Inline data or descriptor passing for bulk data
* No protection against DoS needed
* Subliminal channels are not an issue
File: assuan.info, Node: Implementation, Next: Preparation, Prev: Assuan, Up: Top
3 Implementation
****************
The implementation is line based with a maximum line size of 1000
octets. The default IPC mechanism is Unix Domain Sockets.
On a connect request the server responds either with an okay or an
error status. For authentication-check the server may send an Inquiry
response prior to the first Okay, and it may also issue Status
messages. The server must check that the client is allowed to connect,
this is done by requesting the credentials for the peer and comparing
them to those of the server. This avoids attacks based on wrong socket
permissions.
It may choose to delay the first response in case of an error. The
server never closes the connection - however the lower protocol may do
so after some time of inactivity or when the connection is in an error
state.
All textual messages are assumed to be in UTF-8 unless otherwise
noted.
* Menu:
* Server responses:: Description of server responses.
* Client requests:: Description of client requests.
* Error codes:: List of error and status codes.
File: assuan.info, Node: Server responses, Next: Client requests, Up: Implementation
3.1 Server responses
====================
`OK [<arbitrary debugging information>]'
Request was successful.
`ERR ERRORCODE [<human readable error description>]'
Request could not be fulfilled. The possible error codes are
defined by `libgpg-error'.
`S KEYWORD <status information depending on keyword>'
Informational output by the server, still processing the request.
A client may not send such lines to the server while processing an
Inquiry command. KEYWORD shall start with a letter or an
underscore.
`# <string>'
Comment line issued only for debugging purposes. Totally ignored.
`D <raw data>'
Raw data returned to client. There must be exactly one space after
the 'D'. The values for '%', CR and LF must be percent escaped;
this is encoded as %25, %0D and %0A. Only uppercase letters
should be used in the hexadecimal representation. Other
characters may be percent escaped for easier debugging. All these
Data lines are considered one data stream up to the OK or ERR
response. Status and Inquiry Responses may be mixed with the Data
lines.
`INQUIRE KEYWORD <parameters>'
Server needs further information from the client. The client
should answer with a command which is allowed after an inquiry.
Note that the server does not confirm that client command but
either continues processing or ends processing with an error
status. Not all commands are allowed.
A client should only check the first letter of each line and then
skip over to the next token (except for data lines where the raw data
starts exactly after 2 bytes). Lines larger than 1000 bytes should be
treated as a communication error. (The rationale for having a line
length limit is to allow for easier multiplexing of several channels).
File: assuan.info, Node: Client requests, Next: Error codes, Prev: Server responses, Up: Implementation
3.2 Client requests
===================
The server waits for client requests after he sent an Okay or Error.
The client should not issue a request in other cases.
COMMAND <parameters>
COMMAND is a one word string without preceding white space.
Parameters are command specific, CR, LF and the percent signs should be
percent escaped as described above. To send a backslash as the last
character it should also be percent escaped. Percent escaping is
allowed anywhere in the parameters but not in the command. The line
ends with a CR, LF or just a LF.
Not yet implemented feature: If there is a need for a parameter list
longer than the line length limit (1000 characters including command and
CR, LF), the last character of the line (right before the CR/LF or LF)
must be a non-escape encoded backslash. The following line is then
expected to be a continuation of the line with the backslash replaced by
a blank and the line ending removed.
D <raw data>
Raw data to the server. There must be exactly one space after the
'D'. The values for '%', CR and LF must be percent escaped; this is
encoded as %25, %0D and %0A. Only uppercase letters should be used in
the hexadecimal representation. Other characters may be percent escaped
for easier debugging. All these Data lines are considered one data
stream up to the `OK' or `ERR' response. Status and Inquiry Responses
may be mixed with the Data lines.
END
Lines beginning with a `#' or empty lines are ignored. This is
useful to comment test scripts.
Although the commands are application specific, some of them are
used by all protocols and partly directly supported by the Assuan
library:
`BYE'
Close the connection. The server will reply with `OK'.
`RESET'
Reset the connection but not any existing authentication. The
server should release all resources associated with the connection.
`END'
Used by a client to mark the end of raw data. The server may send
`END' to indicate a partial end of data.
`HELP'
Lists all commands that the server understands as comment lines on
the status channel.
`QUIT'
Reserved for future extensions.
`OPTION'
Set options for the connection. The syntax of such a line is
OPTION NAME [ [=] VALUE ]
Leading and trailing spaces around NAME and VALUE are allowed but
should be ignored. For compatibility reasons, NAME may be
prefixed with two dashes. The use of the equal sign is optional
but suggested if VALUE is given.
`CANCEL'
This command is reserved for future extensions.
`AUTH'
This command is reserved for future extensions. Not yet specified
as we don't implement it in the first phase. See Werner's mail to
gpa-dev on 2001-10-25 about the rationale for measurements against
local attacks.
`NOP'
No operation. Returns OK without any action.
File: assuan.info, Node: Error codes, Prev: Client requests, Up: Implementation
3.3 Error codes
===============
Libassuan is used with gpg-error style error codes. It is recommended
to set the error source to a different value than the default
`GPG_ERR_SOURCE_UNKNOWN' by calling *note function
assuan_set_gpg_err_source:: early.
File: assuan.info, Node: Preparation, Next: Generalities, Prev: Implementation, Up: Top
4 Preparation
*************
To use ASSUAN, you have to perform some changes to your sources and the
build system. The necessary changes are small and explained in the
following sections.
* Menu:
* Header:: What header file you need to include.
* Building sources:: How to build sources using the library.
* Automake:: How to build sources with the help of Automake.
* Multi Threading:: How `libassuan' can be used in a MT environment.
File: assuan.info, Node: Header, Next: Building sources, Up: Preparation
4.1 Header
==========
All interfaces (data types and functions) of `libassuan' are defined in
the header file `assuan.h'. You must include this in all source files
using the library, either directly or through some other header file,
like this:
#include <assuan.h>
The name space of `libassuan' is `assuan_*' for function and type
names and `ASSUAN*' for other symbols. In addition the same name
prefixes with one prepended underscore are reserved for internal use
and should never be used by an application.
Because `libassuan' makes use of the GPG Error library, using
`libassuan' will also use the `GPG_ERR_*' name space directly, and the
`gpg_err*' and `gpg_str*' name space indirectly.
File: assuan.info, Node: Building sources, Next: Automake, Prev: Header, Up: Preparation
4.2 Building sources
====================
If you want to compile a source file including the `assuan.h' header
file, you must make sure that the compiler can find it in the directory
hierarchy. This is accomplished by adding the path to the directory in
which the header file is located to the compilers include file search
path (via the `-I' option).
However, the path to the include file is determined at the time the
source is configured. To solve this problem, `libassuan' ships with a
small helper program `libassuan-config' that knows the path to the
include file and other configuration options. The options that need to
be added to the compiler invocation at compile time are output by the
`--cflags' option to `libassuan-config'. The following example shows
how it can be used at the command line:
gcc -c foo.c $(libassuan-config --cflags)
Adding the output of `libassuan-config --cflags' to the compiler's
command line will ensure that the compiler can find the `assuan.h'
header file.
A similar problem occurs when linking the program with the library.
Again, the compiler/linker has to find the library files. For this to
work, the path to the library files has to be added to the library
search path (via the `-L' option). For this, the option `--libs' to
`libassuan-config' can be used. For convenience, this option also
outputs all other options that are required to link the program with
the `libassuan' libraries (in particular, the `-lassuan' option). The
example shows how to link `foo.o' with the `libassuan' library to a
program `foo'.
gcc -o foo foo.o $(libassuan-config --libs)
Of course you can also combine both examples to a single command by
specifying both options to `libassuan-config':
gcc -o foo foo.c $(libassuan-config --cflags --libs)
File: assuan.info, Node: Automake, Next: Multi Threading, Prev: Building sources, Up: Preparation
4.3 Building sources using Automake
===================================
It is much easier if you use GNU Automake instead of writing your own
Makefiles. If you do that you do not have to worry about finding and
invoking the `libassuan-config' script at all. `libassuan' provides an
Automake macro that does all the work for you.
-- Macro: AM_PATH_LIBASSUAN ([MINIMUM-VERSION], [ACTION-IF-FOUND],
[ACTION-IF-NOT-FOUND])
Check whether `libassuan' (at least version MINIMUM-VERSION, if
given) exists on the host system. If it is found, execute
ACTION-IF-FOUND, otherwise do ACTION-IF-NOT-FOUND, if given.
Additionally, the function defines `LIBASSUAN_CFLAGS' to the flags
needed for compilation of the program to find the `assuan.h'
header file, and `LIBASSUAN_LIBS' to the linker flags needed to
link the program to the `libassuan' library.
You can use the defined Autoconf variables like this in your
`Makefile.am':
AM_CPPFLAGS = $(LIBASSUAN_CFLAGS)
LDADD = $(LIBASSUAN_LIBS)
File: assuan.info, Node: Multi Threading, Prev: Automake, Up: Preparation
4.4 Multi Threading
===================
The `libassuan' library is designed so that it can be used in a
threaded application, if some rules are followed.
* Run the initialization functions before you actually start to use
threads. Specifically, the functions `assuan_set_gpg_err_source',
`assuan_set_malloc_hooks' and `assuan_set_log_cb' should not be
called concurrently with `assuan_new'. Use `assuan_new_ext'
instead or ensure proper serialization.
* Only one thread at a time may access an `libassuan' context.
* If you use the default log handler, use
`assuan_set_assuan_log_stream' to setup a default log stream.
* If you have callback functions shared by multiple functions, the
callback function must be reentrant for that purpose. `libassuan'
does not serialize invocation of callback functions across
contexts.
File: assuan.info, Node: Generalities, Next: Client code, Prev: Preparation, Up: Top
5 Generalities
**************
* Menu:
* Data Types:: Data types used by `libassuan'.
* Initializing the library:: How to initialize the library.
* Default Log Handler:: How to configure the default log handler.
* Contexts:: How to work with contexts.
* Reading and Writing:: How to communicate with the peer.
File: assuan.info, Node: Data Types, Next: Initializing the library, Up: Generalities
5.1 Data Types used by the library
==================================
ASSUAN uses a context to keep the state for a connection. The
following data type is used for that:
-- Data type: assuan_context_t
The `assuan_context_t' type is a pointer to an object maintained
internally by the library. Contexts are allocated with
`assuan_new' or `assuan_new_ext' and released with
`assuan_release'. Other functions take this data type to access
the state created by these functions.
-- Data type: assuan_fd_t
The `assuan_fd_t' is a file descriptor (in Unix) or a system
handle (in Windows). The special value `ASSUAN_INVALID_FD' is
used to specify invalid Assuan file descriptors.
-- Function: assuan_fd_t assuan_fdopen (int FD)
Create an assuan file descriptor from a POSIX (libc) file
descriptor FD. On Unix, this is equivalent to `dup', while on
Windows this will retrieve the underlying system handle with
`_get_osfhandle' and duplicate that.
File: assuan.info, Node: Initializing the library, Next: Default Log Handler, Prev: Data Types, Up: Generalities
5.2 Initializing the library
============================
In general the library requires no initialization. There are however
some initialization hooks provided which are often useful. These
should be called as early as possible and in a multi-threaded
application before a second thread is created.
These functions initialize default values that are used at context
creation with `assuan_new'. As there can only be one default, all
values can also be set directly with `assuan_new_ext' or with
context-specific functions after context creation.
If your application uses its own memory allocation functions or
wrappers it is good idea to tell `libassuan' about it so it can make
use of the same functions or wrappers:
-- Data type: struct assuan_malloc_hooks
This structure is used to store the memory allocation callback
interface functions. It has the following members, whose semantics
are identical to the corresponding system functions:
`void *(*malloc) (size_t cnt)'
This is the function called by ASSUAN to allocate memory for
a context.
`void *(*realloc) (void *ptr, size_t cnt)'
This is the function called by ASSUAN to reallocate memory
for a context.
`void (*free) (void *ptr)'
This is the function called by ASSUAN to release memory for a
context.
-- Data type: assuan_malloc_hooks_t
This is a pointer to a `struct assuan_malloc_hooks'.
-- Function: void assuan_set_malloc_hooks
(assuan_malloc_hooks_t MALLOC_HOOKS)
This function sets the default allocation hooks for new contexts
allocated with `assuan_new'. You need to provide all three
functions. Those functions need to behave exactly as their
standard counterparts `malloc', `realloc' and `free'. If you
write your own functions, please take care to set `errno' whenever
an error has occurred.
-- Function: assuan_malloc_hooks_t assuan_get_malloc_hooks ()
This function gets the default allocation hooks for new contexts
allocated with `assuan_new'. The result structure is statically
allocated and should not be modified.
The ASSUAN library uses `libgpg-error' error values, which consist
and error code and an error source. The default source used by
contexts allocated with `assuan_new' can be set with the following
function.
-- Function: void assuan_set_gpg_err_source
(gpg_err_source_t ERR_SOURCE)
This function sets the default error source for errors generated by
contexts allocated with `assuan_new'.
One way to call this function is
assuan_set_gpg_err_source (GPG_ERR_SOURCE_DEFAULT);
-- Function: gpg_err_source_t assuan_get_gpg_err_source (void)
This function gets the default error source for errors generated by
contexts allocated with `assuan_new'.
To integrate assuan logging and diagnostics into your own logging
system, you may use the following two functions:
-- Data type: int (*assuan_log_cb_t) (assuan_context_t CTX,
void *HOOK_VALUE, unsigned int CAT, const char *MSG)
The user-provided callback function takes a context CTX, for which
the message MSG was generated, and a hook value HOOK_VALUE that
was supplied when the log handler was registered for the context
with `assuan_set_log_cb', and a category CAT. The category is one
of:
`ASSUAN_LOG_INIT'
`ASSUAN_LOG_CTX'
`ASSUAN_LOG_ENGINE'
`ASSUAN_LOG_DATA'
RFU
`ASSUAN_LOG_SYSIO'
Log lowlevel I/O data.
`ASSUAN_LOG_CONTROL'
Log the control channel.
The user may then, depending on the category, write the message to
a log file or treat it in some other way.
If MSG is a null pointer, then no message should be logged, but
the function should return 1 if it is interested in log messages
with the category CAT. If it is not interested, 0 should be
returned. This allows `libassuan' to suppress the generation of
expensive debug output.
-- Function: void assuan_set_log_cb (assuan_log_cb_t LOG_CB,
void *LOG_CB_DATA)
This function sets the default logging handler for log messages
generated by contexts allocated with `assuan_new'.
-- Function: void assuan_get_log_cb (assuan_log_cb_t *LOG_CB,
void **LOG_CB_DATA)
This function gets the default logging handler for log messages
generated by contexts allocated with `assuan_new'.
You do not need to set a log handler, as ASSUAN provides a
configurable default log handler that should be suitable for most
purposes. Logging can be disabled completely by setting the log
handler to a null pointer.
File: assuan.info, Node: Default Log Handler, Next: Contexts, Prev: Initializing the library, Up: Generalities
5.3 Default Log Handler
=======================
The default log handler can be configured by the following functions:
-- Function: void assuan_set_assuan_log_prefix (const char *TEXT)
Set the prefix to be used at the start of a line emitted by assuan
on the log stream to TEXT. The default is the empty string.
-- Function: const char * assuan_get_assuan_log_prefix (void)
Return the prefix to be used at the start of a line emitted by
assuan on the log stream. The default implementation returns the
empty string.
-- Function: void assuan_set_assuan_log_stream (FILE *FP)
This sets the default log stream to which `libassuan' should log
messages not associated with a specific context to FP. The
default is to log to `stderr'. This default value is also changed
by using `assuan_set_log_stream' (to set a logging stream for a
specific context) unless this function has been used. Obviously
this is not thread-safe and thus it is highly recommended to use
this function to setup a proper default.
-- Function: FILE * assuan_get_assuan_log_stream (void)
Return the stream which is currently being using for global
logging.
The log stream used by the default log handler can also be set on a
per context basis.
-- Function: void assuan_set_log_stream (assuan_context_t CTX,
FILE *FP)
Enable debugging for the context CTX and write all debugging
output to the stdio stream FP. If the default log stream (used
for non-context specific events) has not yet been set, a call to
this functions implicitly sets this stream also to FP.
File: assuan.info, Node: Contexts, Next: Reading and Writing, Prev: Default Log Handler, Up: Generalities
5.4 How to work with contexts
=============================
Some operations work globally on the library, but most operate in a
context, which saves state across operations. To allow the use of
`libassuan' in mixed environments, such as in a library using GPGME and
an application using GPGME, the context is very extensive and covers
utilitary information like memory allocation callbacks as well as
specific information associated with client/server operations.
-- Function: gpg_error_t assuan_new (assuan_context_t *CTX_P)
The function `assuan_new' creates a new context, using the global
default memory allocation, log handler and `libgpg-error' source.
It is equivalent to
gpg_error_t err;
assuan_log_cb_t log_cb;
void *log_cb_data;
assuan_get_log_cb (&log_cb, &log_cb_data);
err = assuan_new_ext (ctx_p, assuan_get_gpg_err_source (),
assuan_get_malloc_hooks (), log_cb, log_cb_data);
As you can see, this is not thread-safe. Take care not to modify
the memory allocation hooks or log callback handler concurrently
with `assuan_new'.
The function returns an error if a memory allocation error occurs,
and 0 with the new context in CTX_P otherwise.
-- Function: gpg_error_t assuan_new_ext (assuan_context_t *CTX_P,
gpg_err_source_t ERR_SOURCE,
assuan_malloc_hooks_t MALLOC_HOOKS, assuan_log_cb_t LOG_CB,
void *LOG_CB_DATA)
The function `assuan_new_ext' creates a new context using the
supplied `libgpg-error' error source ERR_SOURCE, the memory
allocation hooks MALLOC_HOOKS and the log handler LOG_CB with the
user data LOG_CB_DATA.
After the context has been used, it can be destroyed again.
-- Function: void assuan_release (assuan_context_t ctx)
The function `assuan_release' destroys the context CTX and
releases all associated resources.
Other properties of the context beside the memory allocation handler,
the log handler, and the `libgpg-error' source can be set after context
creation. Here are some of them:
-- Function: void assuan_set_pointer (assuan_context_t CTX,
void *POINTER)
Store the arbitrary pointer value POINTER into the context CTX.
This is useful to provide command handlers with additional
application context.
-- Function: void* assuan_get_pointer (assuan_context_t CTX)
This returns the pointer for context CTX which has been set using
the above function. A common way to use it is by setting the
pointer before starting the processing loop and to retrieve it
right at the start of a command handler:
static int
cmd_foo (assuan_context_t ctx, char *line)
{
ctrl_t ctrl = assuan_get_pointer (ctx);
...
}
-- Function: void assuan_set_flag (assuan_context_t CTX,
assuan_flag_t FLAG, int VALUE)
Set the the FLAG for context CTX to VALUE. Values for flags are
usually 1 or 0 but certain flags might need other values.
-- Data type: assuan_flag_t
The flags are all named and collected in an `enum' for better
readability. Currently only one flag is defined:
`ASSUAN_NO_WAITPID'
When using a pipe server, by default Libassuan will wait
for the forked process to die in `assuan_release'. In
certain cases this is not desirable. By setting this
flag, a call to `waitpid' will be suppressed and the
caller is responsible to cleanup the child process.
`ASSUAN_CONFIDENTIAL'
Use to return the state of the confidential logging mode.
`ASSUAN_NO_FIXSIGNALS'
Do not modify signal handler for `SIGPIPE'.
`ASSUAN_CONVEY_COMMENTS'
If enabled comment lines are passed to the status
callback of the `assuan_transact'.
`ASSUAN_FORCE_CLOSE'
Setting this flag forces the next command to assume that
the connection has been closed. This breaks the command
processing loop and may be used as an implicit BYE
command. VALUE is ignored and thus it is not possible
to clear this flag.
-- Function: int assuan_get_flag (assuan_context_t CTX,
assuan_flag_t FLAG)
Return the value of FLAG in context CTX.
-- Function: void assuan_begin_confidential (assuan_context_t CTX)
Put the logging feature into confidential mode. This is to avoid
logging of sensitive data.
This is identical to:
assuan_set_flag (ctx, ASSUAN_CONFIDENTIAL, 1);
-- Function: void assuan_end_confidential (assuan_context_t CTX)
Get the logging feature out of confidential mode. All data will be
logged again (if logging is enabled).
This is identical to:
assuan_set_flag (ctx, ASSUAN_CONFIDENTIAL, 0);
-- Data type: struct assuan_system_hooks
This structure is used to store the system callback interface
functions. It has the following members, whose semantics are
similar to the corresponding system functions, but not exactly
equivalent.
`int version'
The user should set this to `ASSUAN_SYSTEM_HOOKS_VERSION'.
This indicates to the library which members of this structure
are present in case of future extensions. The user should
initialize the whole structure with zero bytes.
`void (*usleep) (assuan_context_t ctx, unsigned int usec)'
This is the function called by ASSUAN to sleep for `USEC'
microseconds.
`int (*pipe) (assuan_context_t ctx, assuan_fd_t fd[2], int inherit_idx)'
This is the function called by ASSUAN to create a pipe. The
returned file descriptor `fd[inherit_idx]' must be
inheritable by the child process (under Windows, this
requires some extra work).
`int (*close) (assuan_context_t ctx, assuan_fd_t fd)'
This is the function called by ASSUAN to close a file
descriptor created through the system functions.
`ssize_t (*read) (assuan_context_t ctx, assuan_fd_t fd, void *buffer, size_t size)'
This is the function called by ASSUAN to read data from a file
descriptor. It is functionally equivalent to the system
`read' function.
`ssize_t (*write) (assuan_context_t ctx, assuan_fd_t fd, const void *buffer, size_t size)'
This is the function called by ASSUAN to write data to a file
descriptor. It is functionally equivalent to the system
`write' function.
`int (*recvmsg) (assuan_context_t ctx, assuan_fd_t fd, assuan_msghdr_t msg, int flags)'
This is the function called by ASSUAN to receive a message
from a file descriptor. It is functionally equivalent to the
system `recvmsg' function.
`int (*sendmsg) (assuan_context_t ctx, assuan_fd_t fd, const assuan_msghdr_t msg, int flags);'
This is the function called by ASSUAN to send a message to a
file descriptor. It is functionally equivalent to the system
`sendmsg' function.
`int (*spawn) (assuan_context_t ctx, pid_t *r_pid, const char *name, const char **argv, assuan_fd_t fd_in, assuan_fd_t fd_out, assuan_fd_t *fd_child_list, void (*atfork) (void *opaque, int reserved), void *atforkvalue, unsigned int flags)'
This is the function called by ASSUAN to spawn a child
process. The `stdin' and `stdout' file descriptors are
provided in `fd_in' and `fd_out' respectively, but can be set
to `ASSUAN_INVALID_FD', in which case they are set to
`/dev/null'. On systems which use `fork' and `exec', the
`atfork' function should be called with `atforkvalue' and `0'
for flags in the child process right after `fork' returns.
`fd_child_list' is a `ASSUAN_INVALID_FD' terminated array (or
`NULL') and specifies file descriptors to be inherited by the
child process.
A special situation occurs if `name' is a null pointer, in
which case the process should just fork but not call `exec'.
In this case, `*argv' should be set to `"client"' in the
parent process and `"server"' in the child process.
Flags is the bit-wise OR of some (or none) of the following
flags:
`ASSUAN_SPAWN_DETACHED'
If set and there is a need to start the server it will
be started as a background process. This flag is useful
under W32 systems, so that no new console is created and
pops up a console window when starting the server. On
W32CE systems this flag is ignored.
`pid_t (*waitpid) (assuan_context_t ctx, pid_t pid, int action, int *status, int options)'
This is the function called by ASSUAN to wait for the spawned
child process PID to exit, or, if ACTION is 1, to just
release all resources associated with PID (required on Windows
platforms). If ACTION is 0, this is equivalent to `waitpid'.
`int (*socketpair) (assuan_context_t ctx, int namespace, int style, int protocol, assuan_fd_t filedes[2])'
This is the function called by ASSUAN to create a socketpair.
It is equivalent to `socketpair'.
-- Function: void assuan_set_system_hooks
(assuan_system_hooks_t SYSTEM_HOOKS)
Set the default system hooks to use. There is currently no way to
reset to the default system hooks.
-- Function: void assuan_ctx_set_system_hooks (assuan_context_t CTX,
assuan_system_hooks_t SYSTEM_HOOKS)
Set the system hooks for context CTX. There is currently no way
to reset to the default system hooks, create a new context for
that.
The following system hook collections are defined by the library for
your convenience:
`ASSUAN_SYSTEM_NPTH'
System hooks suitable for use with the nPth library.
`ASSUAN_SYSTEM_NPTH_IMPL'
The implementation of system hooks for use with the nPth library.
This must be invoked once somewhere in the application, and defines
the structure that is referenced by `ASSUAN_SYSTEM_NPTH'.
`ASSUAN_SYSTEM_PTH'
System hooks suitable for use with the GNU Pth library.
`ASSUAN_SYSTEM_PTH_IMPL'
The implementation of system hooks for use with the GNU Pth
library. This must be invoked once somewhere in the application,
and defines the structure that is referenced by
`ASSUAN_SYSTEM_PTH'.
File: assuan.info, Node: Reading and Writing, Prev: Contexts, Up: Generalities
5.5 How to communicate with the peer
====================================
What would be an IPC library without the ability to read and write
data? Not very useful. Libassuan has high level functions to take
care of of the more boring stuff, but eventually data needs to be
written and read.
The basic read and write functions are:
-- Function: gpg_error_t assuan_read_line (assuan_context_t CTX,
char **LINE, size_t *LINELEN)
Read the next line written by the peer to the control channel and
store a pointer to the buffer holding that line at the address
LINE. The valid length of the lines is stored at the address of
LINELEN. This buffer is valid until the next read operation on
the same context CTX. You may modify the context of this buffer.
The buffer is invalid (i.e. must not be used) if an error is
returned. This function returns `0' on success or an error value.
-- Function: gpg_error_t assuan_write_line (assuan_context_t CTX,
const char *LINE)
Write the string LINE to the other end on the control channel.
This string needs to be a proper formatted Assuan protocol line and
should not include a linefeed. Sending linefeed or `Nul'
characters is not possible and not allowed by the assuan protocol.
This function shall not be used for sending data (`D') lines. This
function returns `0' on success or an error value.
To actually send bulk data lines a specialized function is available:
-- Function: gpg_error_t assuan_send_data (assuan_context_t CTX,
const void *BUFFER, size_t LENGTH)
This function is used by a server or a client to send LENGTH bytes
of bulk data in BUFFER to the other end on the control channel.
The data will be escaped as required by the Assuan protocol and
may get buffered until a line is full. To flush any pending data,
BUFFER may be passed as `NULL' and LENGTH be `0'.
When used by a client, this flush operation does also send the
`END' command to terminate the response on an `INQUIRE' request.
Note that the function `assuan_transact' takes care of sending
this `END' itself.
This function returns `0' on success or an error value.
The input and output of data can be controlled at a higher level
using an I/O monitor.
-- Data type: unsigned int (*assuan_io_monitor_t)
(assuan_context_t CTX, void *HOOK_VALUE, int INOUT, const char *LINE,
size_t LINELEN)
The monitor function is called right after a line has been
received, if INOUT is `ASSUAN_IO_FROM_PEER', or just before it is
send, if INOUT is `ASSUAN_IO_TO_PEER'. The HOOK_VALUE is provided
by the user when registering the I/O monitor function with a
context using `assuan_set_io_monitor'. The callback function
should return the bitwise OR of some (or none) of the following
flags:
`ASSUAN_IO_MONITOR_NOLOG'
Active logging of this line is suppressed. This can reduce
debug output in the case of a frequent message.
`ASSUAN_IO_MONITOR_IGNORE'
The whole output line is discarded.
-- Function: void assuan_set_io_monitor (assuan_context_t CTX,
assuan_io_monitor_t IO_MONITOR, void *HOOK_DATA)
This function registers an I/O monitor IO_MONITOR for the context
CTX with the hook value HOOK_DATA.
File: assuan.info, Node: Client code, Next: Server code, Prev: Generalities, Up: Top
6 How to develop an Assuan client
*********************************
Depending on the type of the server you want to connect you need to use
different functions.
If the peer is not a simple pipe server but one using full-duplex
sockets, the full-fledged variant of the above function should be used:
-- Function: gpg_error_t assuan_pipe_connect
(assuan_context_t *CTX,const char *NAME, const char *ARGV[],
assuan_fd_t *FD_CHILD_LIST, void (*ATFORK) (void *, int),
void *ATFORKVALUE, unsigned int FLAGS)
A call to this functions forks the current process and executes the
program NAME, passing the arguments given in the NULL-terminated
list ARGV. A list of file descriptors not to be closed may be
given using the `ASSUAN_INVLID_FD' terminated array FD_CHILD_LIST.
If NAME is a null pointer, only a fork but no exec is done. Thus
the child continues to run. However all file descriptors are
closed and some special environment variables are set. To let the
caller detect whether the child or the parent continues, the
parent returns with `"client"' returned in ARGV and the child
returns with `"server"' in ARGV. This feature is only available
on POSIX platforms.
If ATFORK is not NULL, this function is called in the child right
after the fork and the value ATFORKVALUE is passed as the first
argument. That function should only act if the second argument it
received is `0'. Such a fork callback is useful to release
additional resources not to be used by the child.
FLAGS is a bit vector and controls how the function acts:
`ASSUAN_PIPE_CONNECT_FDPASSING'
If cleared a simple pipe based server is expected. If set a
server based on full-duplex pipes is expected. Such pipes
are usually created using the `socketpair' function. It also
enables features only available with such servers.
`ASSUAN_PIPE_CONNECT_DETACHED'
If set and there is a need to start the server it will be
started as a background process. This flag is useful under
W32 systems, so that no new console is created and pops up a
console window when starting the server. On W32CE systems
this flag is ignored.
If you are using a long running server listening either on a TCP or a
Unix domain socket, the following function is used to connect to the
server:
-- Function: gpg_error_t assuan_socket_connect (assuan_context_t *CTX,
const char *NAME, pid_t SERVER_PID, unsigned int FLAGS)
Make a connection to the Unix domain socket NAME and return a new
Assuan context at CTX. SERVER_PID is currently not used but may
become handy in the future; if you don't know the server's process
ID (PID), pass `ASSUAN_INVALID_PID'. With FLAGS set to
`ASSUAN_SOCKET_CONNECT_FDPASSIN', `sendmsg' and `recvmesg' are
used for input and output and thereby enable the use of descriptor
passing.
Connecting to a TCP server is not yet implemented. Standard URL
schemes are reserved for NAME specifying a TCP server.
Now that we have a connection to the server, all work may be
conveniently done using a couple of callbacks and the transact function:
-- Function: gpg_error_t assuan_transact (assuan_context_t CTX,
const char *COMMAND,
gpg_error_t (*DATA_CB)(void *, const void *, size_t),
void *DATA_CB_ARG,
gpg_error_t (*INQUIRE_CB)(void*, const char *),
void *INQUIRE_CB_ARG,
gpg_error_t (*STATUS_CB)(void*, const char *),
void *STATUS_CB_ARG)
Here CTX is the Assuan context opened by one of the connect calls.
COMMAND is the actual Assuan command string. It shall not end
with a line feed and its length is limited to `ASSUAN_LINELENGTH'
(~1000 bytes)
DATA_CB is called by Libassuan for data lines; DATA_CB_ARG is
passed to it along with the data and the length. [FIXME: needs
more documentation].
INQUIRE_CB is called by Libassuan when the server requests
additional information from the client while processing the
command. This callback shall check the provided inquiry name and
send the data as requested back using the `assuan_send_data'. The
server passed INQUIRY_CB_ARG along with the inquiry name to the
callback.
STATUS_CB is called by Libassuan for each status line it receives
from the server. STATUS_CB_ARG is passed along with the status
line to the callback.
The function returns `0' success or an error value. The error
value may be the one one returned by the server in error lines or
one generated by the callback functions.
Libassuan supports descriptor passing on some platforms. The next
two functions are used with this feature:
-- Function: gpg_error_t assuan_sendfd (assuan_context_t CTX,
assuan_fd_t FD)
Send the descriptor FD to the peer using the context CTX. The
descriptor must be sent before the command is issued that makes
use of the descriptor.
Note that calling this function with a CTX of `NULL' and FD of
`-1' can be used as a runtime test to check whether descriptor
passing is available on the platform. If it is, 0 is returned,
otherwise an error with the error code `GPG_ERR_NOT_IMPLEMENTED'.
-- Function: gpg_error_t assuan_receivefd (assuan_context_t CTX,
assuan_fd_t *FD)
Receive a descriptor pending for the context CTX from the peer.
The descriptor must be pending before this function is called. To
accomplish this, the peer needs to use `assuan_sendfd' before the
trigger is sent (e.g. using `assuan_write_line ("INPUT FD")'.
File: assuan.info, Node: Server code, Next: External I/O Loop, Prev: Client code, Up: Top
7 How to develop an Assuan server
*********************************
Implementing a server for Assuan is a bit more complex than a client.
However, it is a straightforward task we are going to explain using a
commented example.
The list of the implemented server commands is defined by a table like:
static struct {
const char *name;
int (*handler) (assuan_context_t, char *line);
} command_table[] = {
{ "FOO", cmd_foo },
{ "BAR", cmd_bar },
{ "INPUT", NULL },
{ "OUTPUT", NULL },
{ NULL }};
For convenience this table is usually put after the actual command
handlers (`cmd_foo', `cmd_bar') or even put inside `command_handler'
(see below). Note that the commands `INPUT' and `OUTPUT' do not
require a handler because Libassuan provides a default handler for
them. It is however possible to assign a custom handler.
A prerequisite for this example code is that a client has already
connected to the server. Often there are two modes combined in one
program: A pipe-based server, where a client has forked the server
process, or a Unix domain socket based server that is listening on the
socket.
void
command_handler (int fd)
{
gpg_error_t rc;
int i;
assuan_context_t ctx;
rc = assuan_new (&ctx);
if (rc)
{
fprintf (stderr, "server context creation failed: %s\n",
gpg_strerror(rc));
return;
}
if (fd == -1)
{
assuan_fd_t filedes[2];
filedes[0] = assuan_fd_from_posix_fd (0);
filedes[1] = assuan_fd_from_posix_fd (1);
rc = assuan_init_pipe_server (ctx, filedes);
}
else
rc = assuan_init_socket_server (ctx, fd, ASSUAN_SOCKET_SERVER_ACCEPTED);
if (rc)
{
fprintf (stderr, "server init failed: %s\n", gpg_strerror (rc));
return;
}
This is the first part of the command handler. We start of by
allocating a new Assuan context with `assuan_new'. *Note function
assuan_new::.
In case this is called as a pipe based server, FD will be based as
FD and the code assumes that the server's `stdin' and `stdout' file
handles are connected to a pipe. The initialization is thus done using
the function:
-- Function: gpg_error_t assuan_init_pipe_server
(assuan_context_t CTX, assuan_fd_t FILEDES[2])
The function takes the two file descriptors from FILEDES and
returns a new Assuan context at R_CTX. As usual, a return value
of `0' indicates success and a failure is indicated by a returning
an error value. In case of error, `NULL' will be stored at R_CTX.
In case the server has been called using a bi-directional pipe
(socketpair), FILEDES is ignored and the file descriptor is taken
from the environment variable `_assuan_connection_fd'. You won't
need to know that because `assuan_pipe_connect', used by the
client to connect to such a server, automagically sets this
variable.
-- Function: gpg_error_t assuan_init_socket_server
(assuan_context_t CTX, assuan_fd_t FD, unsigned int FLAGS)
The function takes the file descriptor FD which is expected to be
associated with a socket and an Assuan context CTX. The following
bits are currently defined for FLAGS:
`ASSUAN_SOCKET_SERVER_FDPASSING'
If set, `sendmsg' and `recvmesg' are used for input and
output and thus enabling the use of descriptor passing.
`ASSUAN_SOCKET_SERVER_ACCEPTED'
If set, FD refers to an already accepted socket. That is,
Libassuan won't call ACCEPT for it. It is suggested to set
this bit as it allows better control of the connection state.
As usual, a return value of `0' indicates success and a failure is
indicated by a returning an error value.
On the Windows platform the following function needs to be called after
`assuan_init_socket_server':
-- Function: void assuan_set_sock_nonce ( assuan_context_t CTX,
assuan_sock_nonce_t *NONCE)
Save a copy of NONCE in context CTX. This should be used to
register the server's nonce with a context established by
`assuan_init_socket_server'. It is actually only needed for
Windows but it does not harm to use it on other systems as well.
After error checking, the implemented assuan commands are registered
with the server.
for (i = 0; command_table[i].name; i++)
{
rc = assuan_register_command (ctx,
command_table[i].name,
command_table[i].handler, NULL);
if (rc)
{
fprintf (stderr, "register failed: %s\n", gpg_strerror (rc));
assuan_release (ctx);
return;
}
}
-- Data type: gpg_error_t (*assuan_handler_t) (assuan_context_t CTX,
char *LINE)
This is the function invoked by ASSUAN for various command related
callback functions. Some of these callback functions have a
different type, but most use `assuan_handler_t'.
-- Function: gpg_error_t assuan_register_command
(assuan_context_t CTX, const char *CMD_STRING,
assuan_handler_t HANDLER, const char *HELP_STRING)
This registers the command named CMD_STRING with the Assuan
context CTX. HANDLER is the function called by Libassuan if this
command is received from the client. NULL may be used for HANDLER
to use a default handler (this only works with a few pre-defined
commands). Note that several default handlers have already been
registered when the context has been created: `NOP', `CANCEL',
`OPTION', `BYE', `AUTH', `RESET' and `END'. It is possible, but
not recommended, to override these commands.
HELP_STRING is a help string that is used for automatic
documentation. It should contain a usage line followed by an empty
line and a complete description.
-- Function: gpg_error_t assuan_register_post_cmd_notify
(assuan_context_t CTX, void (*FNC)(assuan_context_t),
gpg_error_t ERR)
Register a function to be called right after a command has been
processed. ERR is the result code from the last internal assuan
operation and not the one returned by the handler. It may be used
for command-related cleanup.
-- Function: gpg_error_t assuan_register_bye_notify
(assuan_context_t CTX, assuan_handler_t HANDLER)
Register function FNC with context CTX to be called right before
the standard handler for the `BYE' command is being called.
-- Function: gpg_error_t assuan_register_reset_notify
(assuan_context_t CTX, assuan_handler_t HANDLER)
Register function FNC with context CTX to be called right before
the standard handler for the `RESET' command is being called.
-- Function: gpg_error_t assuan_register_cancel_notify
(assuan_context_t CTX, assuan_handler_t HANDLER)
Register function FNC with context CTX to be called right before
the standard handler for the `RESET' command is being called.
-- Function: gpg_error_t assuan_register_option_handler
(assuan_context_t CTX,
gpg_error_t (*FNC)(assuan_context_t, const char*, const char*))
Register function FNC with context CTX for processing options.
That function is being called with the context, the name and the
value of the option. Leading and trailing spaces are removed from
the name and the value. The optional leading two dashes of the
name are removed as well. If no value has been given, an empty
string is passed. The function needs to return `0' on success or
an error code.
-- Function: gpg_error_t assuan_register_input_notify
(assuan_context_t CTX, assuan_handler_t HANDLER)
Although the input function may be overridden with a custom
handler, it is often more convenient to use the default handler
and to know whether an `INPUT' command has been seen and
successfully parsed. The second argument passed to that function
is the entire line. Because that line has already been parsed
when the function gets called, a file descriptor set with the
`INPUT' command may already be used. That file descriptor is
available by calling `assuan_get_input_fd'. If the notification
function returns an error, the input fd does not change.
-- Function: gpg_error_t assuan_register_output_notify
(assuan_context_t CTX, assuan_handler_t HANDLER)
Although the output function may be overridden with a custom
handler, it is often more convenient to use the default handler
and to know whether an `OUTPUT' command has been seen and
successfully parsed. The second argument passed to that function
is the entire line. Because that line has already been parsed
when the function gets called, a file descriptor set with the
`OUTPUT' command may already be used. That file descriptor is
available by calling `assuan_get_output_fd'. If the notification
function returns an error, the output fd does not change.
-- Function: gpg_error_t assuan_set_hello_line (assuan_context_t CTX,
const char *LINE)
This is not actually a register function but may be called also
after registering commands. It changes the "Hello" line, sent by
the server to the client as a first response, from a default
string to the string LINE. For logging purposes, it is often
useful to use such a custom hello line which may tell version
numbers and such. Linefeeds are allowed in this string, however,
each line needs to be shorter than the Assuan line length limit.
Now that everything has been setup, we can start to process our clients
requests.
for (;;)
{
rc = assuan_accept (ctx);
if (rc == -1)
break;
else if (rc)
{
fprintf (stderr, "accept problem: %s\n", gpg_strerror (rc));
break;
}
rc = assuan_process (ctx);
if (rc)
{
fprintf (stderr, "processing failed: %s\n", gpg_strerror (rc));
continue;
}
}
assuan_release (ctx);
}
For future extensibility and to properly detect the end of the
connection the core of the server should loop over the accept and
process calls.
-- Function: gpg_error_t assuan_accept (assuan_context_t CTX)
A call to this function cancel any existing connection and waits
for a connection from a client (that might be skipped, depending
on the type of the server). The initial handshake is performed
which may include an initial authentication or encryption
negotiation. On success `0' is returned. An error value will be
returned if the connection could for some reason not be
established. An error code of `GPG_ERR_EOF' indicates the end of
the connection.
-- Function: gpg_error_t assuan_process (assuan_context_t CTX)
This function is used to handle the Assuan protocol after a
connection has been established using `assuan_accept'. It is the
main protocol handler responsible for reading the client commands
and calling the appropriate handlers. The function returns `0' on
success or an error value if something went seriously wrong.
Error values from the individual command handlers, i.e.
operational error, are not seen here.
That is all needed for the server code. You only need to come up with
the code for the individual command handlers. Take care that the line
passed to the command handlers is allocated statically within the
context and calls to Assuan functions may modify that line. You are
also allowed to modify that line which makes parsing much easier.
File: assuan.info, Node: External I/O Loop, Next: Utilities, Prev: Server code, Up: Top
8 How to use external I/O event loops
*************************************
The above implementations of an Assuan client and server are
synchronous, insofar the main routines block until a request or client
connection is completely processed. In some programs, for example GUI
applications, this is undesirable. Instead, Assuan operations should
be non-blocking, and the caller should be able to poll all involved
file descriptors to determine when the next Assuan function can be
invoked without blocking.
To make this possible, client and server have to adhere to some
rules:
* Either partner should always write full lines. If partial lines
are written, the remainder of the line should b sent without delay.
* Either partner should eagerly receive status messages. While
receiving and sending bulk data may be delayed, the status
communication channel is different: Both partners may send status
messages in blocking mode at any time the protocol allows them to
send such status messages. To ensure that these send operations
do not actually block the sender, the recipient must be ready to
receive without undue delay.
* If descriptor passing is used over a socket, the descriptor must be
sent after the corresponding command without undue delay.
Together, these restrictions allow to limit the need for asynchronous
I/O operations to bulk data and the inbound status file descriptor.
In addition to the above rules, client and server should adhere to
the following implementation guidelines.
* Menu:
* External I/O Loop Client:: External I/O event loops in the client.
* External I/O Loop Server:: External I/O event loops in the server.
File: assuan.info, Node: External I/O Loop Client, Next: External I/O Loop Server, Up: External I/O Loop
8.1 External I/O event loops in the client.
===========================================
The reference implementation for using external I/O event loops in the
client is the GPGME library, which exports its own external I/O event
loop mechanism and utilizes the Assuan library transparently for the
user. The following steps document how GPGME achieves this.
1. Before connecting, set up pipes for bulk data transfer (using the
INPUT/OUTPUT commands, for example). These are passed to the
server either by inheritance (using a pipe server) or by FD
passing (using a socket server).
2. Then you need to connect to the server. GPGME uses a pipe server,
so it just spawns a server process, which is a non-blocking
operation. FIXME: Currently, using a client with external event
loop over a socket connection is not supported. It is easy to
support (we just need a variation of `assuan_socket_connect' which
takes an already connected socket FD and turns it into an Assuan
context), so if you need this let us know.
3. After connecting, get the inbound status FD with
`assuan_get_active_fds' (the first one returned is the status FD).
This FD can be duplicated if it is convenient (GPGME does this to
be able to close this FD and associated callback handlers without
disrupting Assuan's internals).
4. Then register the Assuan inbound status FD and all bulk data FDs
with the I/O event mechanism. In general, this requires setting up
callback handlers for these FDs and registering them with the main
event loop.
5. When bulk data FDs become ready, you can simply perform the
corresponding read or write operations. When the inbound status FD
becomes ready, you can receive the next server line with
assuan_read_line().
6. You should close and unregister the bulk data FDs when you wrote
all data (for outbound FDs) or receive an EOF (for inbound FDs).
When you receive an ERR from the server, or an OK for the final
operation, you can unregister the inbound status FD and call
`assuan_release'.
7. As noted above, all send operations on the outbound status FD are
done immediate with blocking. In GPGME, this has never caused any
problems.
8. The `INQUIRE' function can be handled in two ways: If the
requested data is immediately available, the client can just send
the data blockingly. If the requested data needs to be fetched
from a blocking source, a callback handler can be registered for
the FD with the main event loop. GPGME does not support the
`INQUIRE' function, so we do not have any practical experience
with this.
Currently, the client can not cancel a pending operation gracefully.
It can, however, disconnect from the server at any time. It is the
responsibility of the server to periodically send status messages to
the client to probe if the connection remains alive.
File: assuan.info, Node: External I/O Loop Server, Prev: External I/O Loop Client, Up: External I/O Loop
8.2 External I/O event loops in the server.
===========================================
Currently, no Assuan server exists which uses external I/O event loops.
However, the following guidelines should lead to a usable
implementation:
1. For socket servers: You can not use `assuan_accept', so you should
just implement the bind/connect/listen/accept stage yourself. You
can register the listen FD with your main event loop, accept the
connection when it becomes ready, and finally call
`assuan_init_socket_server' with the final argument being
`ASSUAN_SOCKET_SERVER_ACCEPTED' to create an Assuan context for
this connection. This way you can also handle multiple
connections in parallel. The reference implementation for this
approach is DirMngr.
For pipe servers: `assuan_init_pipe_server' creates an Assuan
context valid for the pipe FDs.
2. Once you have a context for a single connection, you can get the
inbound status FD with `assuan_get_active_fds' (the first one
returned is the status FD). This FD can be duplicated if it is
convenient. Every time the inbound status FD is readable, you
should invoke the function `assuan_process_next' (see below) to
process the next incoming message. `assuan_process_next'
processes as many status lines as can be received by a single
`read' operation. When it returns, the inbound status FD may
still be readable, but Assuan does not check this.
The function `assuan_process_next' returns 0 if it can not make
progress reliably, and it returns true in `done' if the client
closed the connection. See below for more information on this
function.
3. The command will be dispatched by `assuan_process_next' just as
with `assuan_process', however, you will want to implement the
command handlers in such a way that they do not block. For
example, the command handler may just register the bulk data FDs
with the main event loop and return.
When the command is finished, irregardless if this happens
directly in the command handler or later, you must call
`assuan_process_done' with an appropriate error value (or 0 for
success) to return an appropriate status line to the client. You
can do this at the end of the command handler, for example by
ending it with `return assuan_process_done (error_code);'.
Another possibility is to invoke `assuan_process_done' from the
place in the code which closes the last active bulk FD registered
with the main event loop for this operation.
It is not possible to use `assuan_inquire' in a command handler, as
this function blocks on receiving the inquired data from the client.
Instead, the asynchronous version `assuan_inquire_ext' needs to be used
(see below), which invokes a callback when the client provided the
inquired data. A typical usage would be for the command handler to
register a continuation with `assuan_inquire_ext' and return 0.
Eventually, the continuation would be invoked by `assuan_process_next'
when the client data arrived. The continuation could complete the
command and eventually call `assuan_process_done'.
Cancellation is supported by returning an appropriate error value to
the client with `assuan_process_done'. For long running operations,
the server should send progress status messages to the client in
regular intervals to notice when the client disconnects.
-- Function: gpg_error_t assuan_process_next (assuan_context_t CTX,
int *DONE)
This is the same as `assuan_process' but the caller has to provide
the outer loop. He should loop as long as the return code is zero
and DONE is false.
-- Function: gpg_error_t assuan_process_done (assuan_context_t CTX,
gpg_error_t RC)
Finish a pending command and return the error code RC to the
client.
-- Function: gpg_error_t assuan_inquire_ext (assuan_context_t CTX,
const char *KEYWORD, size_t MAXLEN,
gpg_error_t (*CB) (void *cb_data, gpg_error_t rc, unsigned char *buffer, size_t buffer_len),
void *CB_DATA)
This is similar to `assuan_inquire' but the caller has to provide
the outer loop (using `assuan_process_next'). The caller should
specify a continuation with CB, which receives CB_DATA as its
first argument, and the error value as well as the inquired data as
its remaining arguments.
File: assuan.info, Node: Utilities, Next: Socket wrappers, Prev: External I/O Loop, Up: Top
9 Utility functions
*******************
There are a lot of helper functions to make writing Assuan code easier.
Some of these functions provide information not available with the
general functions.
-- Function: gpg_error_t assuan_write_status (assuan_context_t CTX,
const char *KEYWORD, const char *TEXT)
This is a convenience function for a server to send a status line.
You need to pass it the KEYWORD and the content of the status line
in TEXT.
-- Function: gpg_error_t assuan_inquire (assuan_context_t CTX,
const char *KEYWORD, unsigned char **R_BUFFER,
size_t *R_LENGTH, size_t MAXLEN)
A server may use this function to inquire data from a client. It
sends an `INQUIRE' command back to the server and returns the
response conveniently in a newly allocated buffer. You need to
pass at least the server's context CTX and the KEYWORD describing
the requested data. All other parameters may be `NULL' or `0',
although this is rarely useful.
On success the result is stored in a newly allocated buffer stored
at R_BUFFER. The length of the data is stored at R_LENGTH. If
MAXLEN has not been given as `0', it describes an upper size
limited of the expected data. If the client returns too much data
the function fails and an error with the error code
`GPG_ERR_ASS_TOO_MUCH_DATA' will be returned.
-- Function: FILE* assuan_get_data_fp (assuan_context_t CTX)
Return a stdio stream for the Assuan context CTX. This stream may
then be used for data output (assuan_write_data). The stream is
valid until the end of the current handler. Calling `fclose' for
that stream is not required. Assuan does all the buffering needed
to insert the status line as well as the required line wrapping
and quoting for data lines.
This function is only available on systems supporting either
`funopen' or `fopencookie'. If it is not supported `NULL' is
returned and `errno' is set to `ENOSYS'.
-- Function: gpg_error_t assuan_set_okay_line (assuan_context_t CTX,
const char *LINE)
Set the text used for the next `OK' response to LINE. This is
sometimes useful to send additional human readable information
along with the OK line. The string is automatically reset at the
end of the current handler.
-- Function: gpg_error_t assuan_command_parse_fd
(assuan_context_t CTX, char *LINE, assuan_fd_t *RFD)
This is the core of the default `INPUT' and `OUTPUT' handler. It
may be used in custom commands as well to negotiate a file
descriptor. If LINE contains `FD=N', it returns N in RFD assuming
a local file descriptor. If LINE contains just `FD' it returns a
file descriptor at RFD; this file descriptor needs to have been
sent by the client right before using `assuan_sendfd'.
On W32 systems the returned file descriptor is a system handle and
not a libc low level I/O file descriptor. Thus applications need
to use `_open_osfhandle' before they can pass this descriptor to
standard functions like `fdopen' or `dup'.
-- Function: const char * assuan_get_command_name
(assuan_context_t CTX)
Return the name of the command currently processed by a handler.
The returned string is valid until the next call to an Assuan
function on the same context. Returns `NULL' if no handler is
executed or the command is not known.
-- Function: assuan_fd_t assuan_get_input_fd (assuan_context_t CTX)
Return the file descriptor sent by the client using the last
`INPUT' command. Returns `ASSUAN_INVALID_FD' if no file
descriptor is available.
-- Function: assuan_fd_t assuan_get_output_fd (assuan_context_t CTX)
Return the file descriptor sent by the client using the last
`OUTPUT' command. Returns `ASSUAN_INVALID_FD' if no file
descriptor is available.
-- Function: gpg_error_t assuan_close_input_fd (assuan_context_t CTX)
Close the file descriptor set by the last `INPUT' command. This
function has the advantage over a simple `close' that it can do
some sanity checks and make sure that a following
`assuan_get_input_fd' won't return an already closed descriptor.
-- Function: gpg_error_t assuan_close_output_fd (assuan_context_t CTX)
Close the file descriptor set by the last `OUTPUT' command. This
function has the advantage over a simple `close' that it can do
some sanity checks and make sure that a following
`assuan_get_input_fd' won't return an already closed descriptor.
-- Function: gpg_error_t assuan_set_error (assuan_context_t CTX,
gpg_error_t ERR, const char *TEXT)
This is a helper to provide a more descriptive error text with
`ERR' lines. For this to work, the text needs to be stored in the
context CTX while still being in the command handler. This
function is commonly called this way
return assuan_set_error (ctx, err, "commands needs 5 arguments");
The value ERR is passed through and thus the return value of the
command handler in the example. The provided text further explains
that error to humans.
-- Function: pid_t assuan_get_pid (assuan_context_t CTX)
This function returns the pid of the connected connected peer. If
that pid is not known `ASSUAN_INVALID_PID' is returned. Note that
it is not always possible to learn the pid of the other process.
For a pipe based server the client knows it instantly and a
mechanism is in place to let the server learn it. For socket based
servers the pid is only available on systems providing the
`SO_PEERCRED' socket option (1).
-- Data type: assuan_peercred_t
This structure is used to store the peer credentials. The
available members depend on the operating system.
`pid_t pid'
The process ID of the peer.
`uid_t uid'
The user ID of the peer process.
`gid_t gid'
The group ID of the peer process.
-- Function: gpg_error_t assuan_get_peercred (assuan_context_t CTX,
assuan_peercred_t *PEERCRED)
Return user credentials of the peer. This will work only on certain
systems and only when connected over a socket. On success, a
pointer to the peer credentials is stored in PEERCRED. The
information is only valid as long as the state of the connection
is unchanged (at least until the next assuan call to the same
context).
As of now only the server is able to retrieve this information.
Note, that for getting the pid of the peer `assuan_get_pid' is
usually better suited.
-- Function: int assuan_get_active_fds (assuan_context_t CTX,
int WHAT, assuan_fd_t *FDARRAY, int FDARRAYSIZE)
Return all active file descriptors for the context CTX. This
function can be used to select on the file descriptors and to call
`assuan_process_next' if there is an active one. The first
descriptor in the array is the one used for the command connection.
Currently WHAT needs to be `0' to return descriptors used for
reading, `1' will eventually be used to return descriptors used for
writing. FDARRAY is an array of integers provided by the caller;
FDARRAYSIZE gives the size of that array.
On success the number of active descriptors are returned. These
active descriptors are then stored in FDARRAY. On error `-1' is
returned; the most likely reason for this is a too small FDARRAY.
Note that on W32 systems the returned file descriptor is a system
handle and not a libc low level I/O file descriptor.
-- Function: int assuan_pending_line (assuan_context_t CTX)
A call to this function return true if a full line has been
buffered and thus an entire assuan line may be read without
triggering any actual I/O.
---------- Footnotes ----------
(1) to our knowledge only the Linux kernel has this feature
File: assuan.info, Node: Socket wrappers, Next: Library Copying, Prev: Utilities, Up: Top
10 Socket wrapper functions
***************************
It is sometimes useful to support Unix domain sockets on Windows. To do
this in a portable way, Assuan provides a set of wrapper functions which
may be used on any system but will enhance Windows to support these
socket types. The actual implementation is based on local TCP sockets
and fully transparent for the client. Server code needs to utilize two
extra functions to check the permissions.
-- Function: gpg_error_t assuan_sock_init (void)
Initialize the socket wrappers. Must be called once at startup if
any of the socket wrapper functions are used.
-- Function: gpg_error_t assuan_sock_deinit (void)
Deinitialize the socket wrappers.
-- Function: int assuan_sock_close (assuan_fd_t FD)
Wrapper for close which does a closesocket on Windows if needed.
-- Function: assuan_fd_t assuan_sock_new (int DOMAIN, int TYPE,
int PROTO);
Wrapper around socket.
-- Function: int assuan_sock_connect (assuan_fd_t SOCKFD,
struct sockaddr *ADDR, int ADDRLEN)
Wrapper around connect. For Unix domain sockets under Windows this
function also does a write immediately after the the connect to
send the nonce as read from the socket's file. Under Unix this
function check whether the socket file is a redirection file and
connects to the redirected socket instead; see
`assuan_sock_set_sockaddr_un' for details on the redirection file
format.
-- Function: int assuan_sock_bind ( assuan_fd_t SOCKFD,
struct sockaddr *ADDR, int ADDRLEN)
Wrapper around bind. Under Windows this creates a file and writes
the port number and a random nonce to this file.
-- Function: int assuan_sock_set_sockaddr_un ( const char *FNAME,
struct sockaddr *ADDR, int *R_REDIRECTED)
This is a helper function to initialize the Unix socket domain
address structure ADDR and store the file name FNAME there. If
R_REDIRECTED is not NULL the function checks whether FNAME already
exists, is a regular file, and not a socket. In that case FNAME
is read to see whether this is a redirection to a socket file. If
that is the case 1 is stored at R_REDIRECTED. If the file does
not look like a redirection file 0 will be stored there and FNAME
will be used in the regular way.
The format of a redirection file is
%Assuan%
socket=NAME
With NAME being is the actual socket to use. No white spaces are
allowed, both lines must be terminated by a single linefeed, and
extra lines are not allowed. Environment variables are
interpreted in NAME if given in `${VAR}' notation. No escape
characters are defined; if the string `${' shall be used in file
name, an environment variable with that content may be used. The
length of the redirection file is limited to 511 bytes which is
more than sufficient for any known implementation of Unix domain
sockets.
-- Function: int assuan_sock_get_nonce ( struct sockaddr *ADDR,
int ADDRLEN, assuan_sock_nonce_t *NONCE)
This is used by the server after a bind to return the random
nonce. To keep the code readable this may also be used on POSIX
system.
-- Function: int assuan_sock_check_nonce ( assuan_fd_t FD,
assuan_sock_nonce_t *NONCE)
If the option `ASSUAN_SOCKET_SERVER_ACCEPTED' has been used,
Libassuan has no way to check the nonce of the server. Thus an
explicit check of the saved nonce using this function is required.
If this function fails the server should immediately drop the
connection. This function may not be used if Libassuan does the
accept call itself (i.e. `ASSUAN_SOCKET_SERVER_ACCEPTED' has not
been used) because in this case Libassuan calls this function
internally. See also `assuan_set_sock_nonce'.
Actually this mechanism is only required on Windows but for
cleanness of code it may be used on POSIX systems as well, where
this function is a nop.
File: assuan.info, Node: Library Copying, Next: Copying, Prev: Socket wrappers, Up: Top
Appendix A GNU LESSER GENERAL PUBLIC LICENSE
********************************************
Version 2.1, February 1999
Copyright (C) 1991, 1999 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02110-1301, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
[This is the first released version of the Lesser GPL. It also counts
as the successor of the GNU Library Public License, version 2, hence the
version number 2.1.]
A.0.1 Preamble
--------------
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A.0.2 How to Apply These Terms to Your New Libraries
----------------------------------------------------
If you develop a new library, and you want it to be of the greatest
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ONE LINE TO GIVE THE LIBRARY'S NAME AND AN IDEA OF WHAT IT DOES.
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Yoyodyne, Inc., hereby disclaims all copyright interest in the library
`Frob' (a library for tweaking knobs) written by James Random Hacker.
SIGNATURE OF TY COON, 1 April 1990
Ty Coon, President of Vice
That's all there is to it!
File: assuan.info, Node: Copying, Next: Index, Prev: Library Copying, Up: Top
GNU General Public License
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Version 3, 29 June 2007
Copyright (C) 2007 Free Software Foundation, Inc. `http://fsf.org/'
Everyone is permitted to copy and distribute verbatim copies of this
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Preamble
========
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Also add information on how to contact you by electronic and paper
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PROGRAM Copyright (C) YEAR NAME OF AUTHOR
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
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The hypothetical commands `show w' and `show c' should show the
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You should also get your employer (if you work as a programmer) or
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the GNU GPL, see `http://www.gnu.org/licenses/'.
The GNU General Public License does not permit incorporating your
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GNU Lesser General Public License instead of this License. But first,
please read `http://www.gnu.org/philosophy/why-not-lgpl.html'.
File: assuan.info, Node: Index, Prev: Copying, Up: Top
Index
*****
[index ]
* Menu:
* AM_PATH_LIBASSUAN: Automake. (line 13)
* assuan_accept: Server code. (line 252)
* assuan_begin_confidential: Contexts. (line 108)
* assuan_close_input_fd: Utilities. (line 85)
* assuan_close_output_fd: Utilities. (line 91)
* assuan_command_parse_fd: Utilities. (line 54)
* assuan_context_t: Data Types. (line 10)
* assuan_ctx_set_system_hooks: Contexts. (line 211)
* assuan_end_confidential: Contexts. (line 115)
* assuan_fd_t: Data Types. (line 17)
* assuan_fdopen: Data Types. (line 22)
* assuan_flag_t: Contexts. (line 76)
* assuan_get_active_fds: Utilities. (line 144)
* assuan_get_assuan_log_prefix: Default Log Handler. (line 13)
* assuan_get_assuan_log_stream: Default Log Handler. (line 27)
* assuan_get_command_name: Utilities. (line 69)
* assuan_get_data_fp: Utilities. (line 34)
* assuan_get_flag: Contexts. (line 105)
* assuan_get_gpg_err_source: Initializing the library.
(line 68)
* assuan_get_input_fd: Utilities. (line 75)
* assuan_get_log_cb: Initializing the library.
(line 113)
* assuan_get_malloc_hooks: Initializing the library.
(line 50)
* assuan_get_output_fd: Utilities. (line 80)
* assuan_get_peercred: Utilities. (line 131)
* assuan_get_pid: Utilities. (line 108)
* assuan_get_pointer: Contexts. (line 59)
* assuan_init_pipe_server: Server code. (line 76)
* assuan_init_socket_server: Server code. (line 90)
* assuan_inquire: Utilities. (line 19)
* assuan_inquire_ext: External I/O Loop Server.
(line 84)
* assuan_malloc_hooks_t: Initializing the library.
(line 38)
* assuan_new: Contexts. (line 14)
* assuan_new_ext: Contexts. (line 37)
* assuan_peercred_t: Utilities. (line 117)
* assuan_pending_line: Utilities. (line 161)
* assuan_pipe_connect: Client code. (line 16)
* assuan_process: Server code. (line 262)
* assuan_process_done: External I/O Loop Server.
(line 77)
* assuan_process_next: External I/O Loop Server.
(line 71)
* assuan_read_line: Reading and Writing. (line 15)
* assuan_receivefd: Client code. (line 118)
* assuan_register_bye_notify: Server code. (line 164)
* assuan_register_cancel_notify: Server code. (line 174)
* assuan_register_command: Server code. (line 141)
* assuan_register_input_notify: Server code. (line 191)
* assuan_register_option_handler: Server code. (line 180)
* assuan_register_output_notify: Server code. (line 203)
* assuan_register_post_cmd_notify: Server code. (line 157)
* assuan_register_reset_notify: Server code. (line 169)
* assuan_release: Contexts. (line 45)
* assuan_send_data: Reading and Writing. (line 36)
* assuan_sendfd: Client code. (line 107)
* assuan_set_assuan_log_prefix: Default Log Handler. (line 9)
* assuan_set_assuan_log_stream: Default Log Handler. (line 18)
* assuan_set_error: Utilities. (line 98)
* assuan_set_flag: Contexts. (line 72)
* assuan_set_gpg_err_source: Initializing the library.
(line 61)
* assuan_set_hello_line: Server code. (line 215)
* assuan_set_io_monitor: Reading and Writing. (line 72)
* assuan_set_log_cb: Initializing the library.
(line 108)
* assuan_set_log_stream: Default Log Handler. (line 35)
* assuan_set_malloc_hooks: Initializing the library.
(line 42)
* assuan_set_okay_line: Utilities. (line 47)
* assuan_set_pointer: Contexts. (line 54)
* assuan_set_sock_nonce: Server code. (line 111)
* assuan_set_system_hooks: Contexts. (line 206)
* assuan_sock_bind: Socket wrappers. (line 39)
* assuan_sock_check_nonce: Socket wrappers. (line 76)
* assuan_sock_close: Socket wrappers. (line 21)
* assuan_sock_connect: Socket wrappers. (line 29)
* assuan_sock_deinit: Socket wrappers. (line 18)
* assuan_sock_get_nonce: Socket wrappers. (line 70)
* assuan_sock_init: Socket wrappers. (line 14)
* assuan_sock_new: Socket wrappers. (line 25)
* assuan_sock_set_sockaddr_un: Socket wrappers. (line 44)
* assuan_socket_connect: Client code. (line 56)
* assuan_transact: Client code. (line 78)
* assuan_write_line: Reading and Writing. (line 25)
* assuan_write_status: Utilities. (line 12)
* gpg_error_t (*assuan_handler_t) (assuan_context_t CTX, char *LINE): Server code.
(line 134)
* int (*assuan_log_cb_t) (assuan_context_t CTX, void *HOOK_VALUE, unsigned int CAT, const char *MSG): Initializing the library.
(line 76)
* introduction: Top. (line 6)
* LGPL, Lesser General Public License: Library Copying. (line 6)
* struct assuan_malloc_hooks: Initializing the library.
(line 21)
* struct assuan_system_hooks: Contexts. (line 122)
* unsigned int (*assuan_io_monitor_t) (assuan_context_t CTX, void *HOOK_VALUE, int INOUT, const char *LINE, size_t LINELEN): Reading and Writing.
(line 55)
Tag Table:
Node: Top941
Node: Introduction2007
Node: Assuan4073
Node: Implementation5151
Node: Server responses6309
Node: Client requests8235
Node: Error codes11250
Node: Preparation11589
Node: Header12167
Node: Building sources12955
Node: Automake14864
Node: Multi Threading16012
Node: Generalities16974
Node: Data Types17378
Node: Initializing the library18481
Ref: function assuan_set_gpg_err_source20981
Node: Default Log Handler23295
Node: Contexts25061
Ref: function assuan_new25641
Node: Reading and Writing35838
Node: Client code39289
Ref: function assuan_sendfd44268
Ref: function assuan_receivefd44800
Node: Server code45173
Node: External I/O Loop57261
Node: External I/O Loop Client59079
Node: External I/O Loop Server62169
Node: Utilities66753
Ref: Utilities-Footnote-174823
Node: Socket wrappers74887
Node: Library Copying79060
Node: Copying107230
Node: Index144854
End Tag Table