This file documents the use and the internals of Assuan.
This is Edition 2.5.5, last updated 7 December 2017, of The ‘Developing with Assuan’ Manual, for Version 2.5.5.
Published by the Free Software Foundation
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Boston, MA 02110-1301 USA
Copyright © 2001–2013 Free Software Foundation, Inc.
Copyright © 2001–2015 g10 Code GmbH
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”.
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This manual documents how to exploit the Assuan library, a simple interprocess communcation library.
Next: Description of the Assuan protocol., Previous: Introduction, Up: Introduction [Contents][Index]
Assuan is an extensible inter-process communication (IPC) protocol and library. It is designed for point-to-point communication and it doesn’t provide a naming system. To contact a server, either the client must know how to locate the server, e.g., via a well-known Unix domain socket, or, if the server is transient, how to start it. In the latter case, Assuan provides functionality to start the server process.
In Assuan, communication is typically either via a pipe or a 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 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 is useful: the client is separated from the server: they run 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.
Next: Implementation, Previous: Introduction to Assuan, Up: Introduction [Contents][Index]
The architecture of the modular GnuPG system is based on several highly specialized modules which form a network of clients and servers. A common framework for intermodule communication is therefore needed and implemented as a library.
Goals:
Design criteria:
Next: Preparation, Previous: Description of the Assuan protocol., Up: Introduction [Contents][Index]
The implementation is line based with a maximum line size of 1000 octets. The default IPC mechanism is Unix Domain Sockets.
On connect, the server responds either with okay or an error status. To perform an authentication check, the server may send an Inquiry response prior to the first Okay. 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 with the server’s credentials. This avoids attacks based on wrong socket permissions.
The server may choose to delay the first response in case of an error. The server, however, never closes the connection, however, the lower protocol may do so after some time of inactivity or when the connection enters an error state.
All textual messages are assumed to be in UTF-8 unless otherwise noted.
Next: Client requests, Up: Implementation [Contents][Index]
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, which is 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; these are encoded as %25, %0D and %0A, respectively. Only uppercase letters should be used in the hexadecimal representation. Other characters may be percent escaped for easier debugging. All 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>
The server needs further information from the client. The client should respond with data (using the “D” command and terminated by “END”). Alternatively, the client may cancel the current operation by responding with “CAN”.
Consider the following examples (lines prefixed with S indicate text that the server sends; lines prefixed with C indicate text that the client sends):
S: INQUIRE foo C: D foo bar C: D bar baz C: END [Server continues normal work]
This implements a callback to the client:
S: INQUIRE foo C: END [Server continues]
and:
S: INQUIRE foo C: CAN [Server terminates the operaion and in most cases returns an ERR to the client.]
But, CAN may also mean “I have no data for you, try to get it from elsewhere.”
Note: lines longer 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.)
Next: Error codes, Previous: Server responses, Up: Implementation [Contents][Index]
The server waits for client requests after sending 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 pair 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 unescaped (i.e., literal) 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>
Sends raw data to the server. There must be exactly one space after
the ’D’. The values for ’%’, CR and LF must be percent escaped.
These are encoded as %25, %0D and %0A, respectively. Only uppercase
letters should be used in the hexadecimal representation. Other
characters may be percent escaped for easier debugging. All 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 supported by the Assuan library:
BYE
Close the connection. The server will respond 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.
Previous: Client requests, Up: Implementation [Contents][Index]
Libassuan is used with gpg-error style error codes. It is recommended
to set the error source to a different value from the default
GPG_ERR_SOURCE_UNKNOWN
by calling function assuan_set_gpg_err_source early.
Next: Generalities, Previous: Implementation, Up: Introduction [Contents][Index]
To use ASSUAN, you have to make some changes to your sources and the build system. The necessary changes are small and explained in the following sections.
Next: Building sources, Up: Preparation [Contents][Index]
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 namespace 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_*
namespace
directly, and the gpg_err*
and gpg_str*
namespaces
indirectly.
Next: Building sources using Automake, Previous: Header, Up: Preparation [Contents][Index]
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)
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)
Next: Multi Threading, Previous: Building sources, Up: Preparation [Contents][Index]
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.
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)
Previous: Building sources using Automake, Up: Preparation [Contents][Index]
The libassuan
library is designed so that it can be used in a
threaded application, if some rules are followed.
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.
libassuan
context.
assuan_set_assuan_log_stream
to setup a default log stream.
libassuan
does not serialize invocation of callback functions
across contexts.
Next: How to develop an Assuan client, Previous: Preparation, Up: Introduction [Contents][Index]
Next: Initializing the library, Up: Generalities [Contents][Index]
ASSUAN uses a so-called context to store a connection’s state. The following data type is used for that:
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.
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.
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.
Next: Default Log Handler, Previous: Data Types used by the library, Up: Generalities [Contents][Index]
Libassuan makes use of Libgpg-error and assumes that Libgpg-error has
been initialized. In general gpgrt_check_version
should be
called to guarantee this; the Libgpg-error manual for details.
Libassuan itself 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:
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.
This is a pointer to a struct assuan_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.
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.
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);
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:
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.
This function sets the default logging handler for log messages
generated by contexts allocated with assuan_new
.
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.
Next: How to work with contexts, Previous: Initializing the library, Up: Generalities [Contents][Index]
The default log handler can be configured by the following functions:
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.
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.
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.
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.
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.
Next: How to communicate with the peer, Previous: Default Log Handler, Up: Generalities [Contents][Index]
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.
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.
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.
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:
Store the arbitrary pointer value pointer into the context ctx. This is useful to provide command handlers with additional application context.
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); ... }
Set the the flag for context ctx to value. Values for flags are usually 1 or 0 but certain flags might need other values.
The flags are all named and collected in an enum
for better readability.
Available flags are:
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.
Return the value of flag in context 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);
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);
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
.
Set the default system hooks to use. There is currently no way to reset to the default system hooks.
The socket subsystem uses an internal context which uses the default system hooks. This function allows to change these system hooks. The function is not thread-safe and only useful if a certain order of assuan and assuan socket initializations are required.
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
.
Previous: How to work with contexts, Up: Generalities [Contents][Index]
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:
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.
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:
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.
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.
This function registers an I/O monitor io_monitor for the context ctx with the hook value hook_data.
Next: How to develop an Assuan server, Previous: Generalities, Up: Introduction [Contents][Index]
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:
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_INVALID_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:
Make a connection to the Unix domain socket name using the
already-initialized 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_FDPASSING
,
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:
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:
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 ASSUAN_INVALID_FD
can be used as a runtime test to
check whether descriptor passing is available on the platform:
0
is returned if descriptor passing is available, otherwise an
error with the error code GPG_ERR_NOT_IMPLEMENTED
is returned.
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")
.
Next: How to use external I/O event loops, Previous: How to develop an Assuan client, Up: Introduction [Contents][Index]
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 off by
allocating a new Assuan context with assuan_new
.
See 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:
This 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
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
generally don’t need to know this, because assuan_pipe_connect
,
which is called by the client to connect to such a server,
automagically sets this variable.
This 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, which enables 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 returning an error value.
On the Windows platform the following function needs to be called after
assuan_init_socket_server
:
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 technically only needed for
Windows, but it does no harm to use it on other systems.
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; } }
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
.
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.
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.
Register function fnc with context ctx to be called right
before the standard handler for the BYE
command is being called.
Register function fnc with context ctx to be called right
before the standard handler for the RESET
command is being called.
Register function fnc with context ctx to be called right
before the standard handler for the RESET
command is being called.
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.
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.
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.
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.
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.
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.
Next: Utility functions, Previous: How to develop an Assuan server, Up: Introduction [Contents][Index]
The above implementations of an Assuan client and server are synchronous, insofar as 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:
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.
Next: External I/O event loops in the server., Up: How to use external I/O event loops [Contents][Index]
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.
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.
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).
assuan_release
.
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.
Previous: External I/O event loops in the client., Up: How to use external I/O event loops [Contents][Index]
Currently, no Assuan server exists which uses external I/O event loops. However, the following guidelines should lead to a usable implementation:
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.
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.
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.
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.
Finish a pending command and return the error code rc to the client.
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.
Next: Socket wrapper functions, Previous: How to use external I/O event loops, Up: Introduction [Contents][Index]
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.
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.
A server may use this function to request specific data from a client.
This function sends an ’INQUIRE’ command back to the client and
returns the client’s response in a newly allocated buffer. You need
to pass at least the server’s context (ctx) and a description of
the required data (keyword). All other parameters may be
NULL
or 0
, but 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 specifies an upper
size limit 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.
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
.
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.
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
.
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.
Return the file descriptor sent by the client using the last INPUT
command. Returns ASSUAN_INVALID_FD
if no file descriptor is available.
Return the file descriptor sent by the client using the last
OUTPUT
command. Returns ASSUAN_INVALID_FD
if no file descriptor is
available.
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.
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.
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.
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.
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.
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.
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.
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.
Next: GNU Lesser General Public License, Previous: Utility functions, Up: Introduction [Contents][Index]
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.
Initialize the socket wrappers. Must be called once at startup if any of the socket wrapper functions are used.
Deinitialize the socket wrappers.
Wrapper for close which does a closesocket on Windows if needed.
Wrapper around socket.
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.
Directly connect to port on host given as a name. The
current implementation requires that flags has either
ASSUAN_SOCK_SOCKS
or ASSUAN_SOCK_TOR
set. On success a
new TCP STREAM socket is returned; on error ASSUAN_INVALID_FD
and ERRNO set. If credentials is not NULL
, it is a
string used for password based SOCKS authentication. Username and
password are separated by a colon. reserved should be 0. To
test whether the proxy is available host and port may be
given as NULL/0: If the proxy is available the function returns a
valid socket which is in the state after credentials sub-negotiation.
The caller now knows that the SOCKS proxy is available and has been
authenticated; normally the caller closes the socket then.
Wrapper around bind. Under Windows this creates a file and writes the port number and a random nonce to this file.
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.
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.
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.
To control certain properties of the wrapper two additional functions are provided:
Set the flags name for socket fd to value. See below for a list of valid names. Returns 0 on success; on failure sets ERRNO and returns -1.
Store the current value of the flag name for socket fd at r_value. See below for a list of valid names. Returns 0 on success; on failure sets ERRNO and returns -1.
The supported flags are:
cygwin
This flag has an effect only on Windows. If the value is 1, the socket is set into Cygwin mode so that Cygwin clients can connect to such a socket. This flag needs to be set before a bind and should not be changed during the lifetime of the socket. There is no need to set this flag for connecting to a Cygwin style socket because no state is required at the client. On non-Windows platforms setting this flag is ignored, reading the flag always returns a value of 0.
tor-mode
socks
If value is 1 globally enable SOCKS5 mode for new connections
using IPv6 or IPv4. fd must be set to ASSUAN_INVALID_FD
A
future extension may allow to disable SOCKS5 mode for a specified
socket but globally disabling SOCKS5 mode is not possible. Using the
flag “tor-mode” expects the SOCKS5 proxy to listen on port 9050, the
flag “socks” expects the proxy to listen on port 1080.
Connections to the loopback address are not routed though the SOCKS proxy. UDP requests are not supported at all. The proxy will be connected at address 127.0.0.1; an IPv6 connection to the proxy is not yet supported.
Next: GNU General Public License, Previous: Socket wrapper functions, Up: Introduction [Contents][Index]
Copyright © 1991, 1999 Free Software Foundation, Inc. 59 Temple Place – Suite 330, Boston, MA 02111-1307, 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.]
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users.
This license, the Lesser General Public License, applies to some specially designated software—typically libraries—of the Free Software Foundation and other authors who decide to use it. You can use it too, but we suggest you first think carefully about whether this license or the ordinary General Public License is the better strategy to use in any particular case, based on the explanations below.
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We call this license the Lesser General Public License because it does Less to protect the user’s freedom than the ordinary General Public License. It also provides other free software developers Less of an advantage over competing non-free programs. These disadvantages are the reason we use the ordinary General Public License for many libraries. However, the Lesser license provides advantages in certain special circumstances.
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If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).
To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the library's name and an idea of what it does. Copyright (C) year name of author This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the library, if necessary. Here is a sample; alter the names:
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!
Next: Index, Previous: GNU Lesser General Public License, Up: Introduction [Contents][Index]
Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it 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. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’. This is free software, and you are welcome to redistribute it under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read http://www.gnu.org/philosophy/why-not-lgpl.html.
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