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NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | EXAMPLES | SEE ALSO | AUTHORS | BUGS | COLOPHON |
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UNTITLED() LOCAL UNTITLED()
libpipeline — pipeline manipulation library
<pipeline.h>
libpipeline is a C library for setting up and running pipelines of
processes, without needing to involve shell command-line parsing
which is often error-prone and insecure. This relieves
programmers of the need to laboriously construct pipelines using
lower-level primitives such as fork and execve.
The general way to use libpipeline involves constructing a
pipeline structure and adding one or more pipecmd structures to
it. A pipecmd represents a subprocess (or “command”), while a
pipeline represents a sequence of subprocesses each of whose
outputs is connected to the next one's input, as in the example ls
| grep pattern | less. The calling program may adjust certain
properties of each command independently, such as its environment
and nice(3) priority, as well as properties of the entire pipeline
such as its input and output and the way signals are handled while
executing it. The calling program may then start the pipeline,
read output from it, wait for it to complete, and gather its exit
status.
Strings passed as const char * function arguments will be copied
by the library.
Functions to build individual commands
pipecmd *pipecmd_new(const char *name)
Construct a new command representing execution of a program
called name.
pipecmd *pipecmd_new_argv(const char *name, va_list argv)
pipecmd *pipecmd_new_args(const char *name, ...)
Convenience constructors wrapping pipecmd_new() and
pipecmd_arg(). Construct a new command representing
execution of a program called name with arguments.
Terminate arguments with nullptr (before C23: (void *) 0).
pipecmd *pipecmd_new_argstr(const char *argstr)
Split argstr on whitespace to construct a command and
arguments, honouring shell-style single-quoting, double-
quoting, and backslashes, but not other shell evilness like
wildcards, semicolons, or backquotes. This is included only
to support situations where command arguments are encoded
into configuration files and the like. While it is safer
than system(3), it still involves significant string parsing
which is inherently riskier than avoiding it altogether.
Please try to avoid using it in new code.
typedef void pipecmd_function_type (void *);
typedef void pipecmd_function_free_type (void *);
pipecmd *pipecmd_new_function(const char *name,
pipecmd_function_type *func,
pipecmd_function_free_type *free_func, void *data)
Construct a new command that calls a given function rather
than executing a process.
The data argument is passed as the function's only argument,
and will be freed before returning using free_func (if
non-NULL).
pipecmd_* functions that deal with arguments cannot be used
with the command returned by this function.
pipecmd *pipecmd_new_sequencev(const char *name, va_list cmdv)
pipecmd *pipecmd_new_sequence(const char *name, ...)
Construct a new command that itself runs a sequence of
commands, supplied as command * arguments following name and
terminated by nullptr (before C23: (void *) 0). The
commands will be executed in forked children; if any exits
non-zero then it will terminate the sequence, as with "&&"
in shell.
pipecmd_* functions that deal with arguments cannot be used
with the command returned by this function.
pipecmd *pipecmd_new_passthrough(void)
Return a new command that just passes data from its input to
its output.
pipecmd *pipecmd_dup(pipecmd *cmd)
Return a duplicate of a command.
void pipecmd_arg(pipecmd *cmd, const char *arg)
Add an argument to a command.
void pipecmd_argf(pipecmd *cmd, const char *format, ...)
Convenience function to add an argument with printf
substitutions.
void pipecmd_argv(pipecmd *cmd, va_list argv)
void pipecmd_args(pipecmd *cmd, ...)
Convenience functions wrapping pipecmd_arg() to add multiple
arguments at once. Terminate arguments with nullptr (before
C23: (void *) 0).
void pipecmd_argstr(pipecmd *cmd, const char *argstr)
Split argstr on whitespace to add a list of arguments,
honouring shell-style single-quoting, double-quoting, and
backslashes, but not other shell evilness like wildcards,
semicolons, or backquotes. This is included only to support
situations where command arguments are encoded into
configuration files and the like. While it is safer than
system(3), it still involves significant string parsing
which is inherently riskier than avoiding it altogether.
Please try to avoid using it in new code.
void pipecmd_get_nargs(pipecmd *cmd)
Return the number of arguments to this command. Note that
this includes the command name as the first argument, so the
command ‘echo foo bar’ is counted as having three arguments.
Added in libpipeline 1.1.0.
void pipecmd_nice(pipecmd *cmd, int value)
Set the nice(3) value for this command. Defaults to 0.
Errors while attempting to set the nice value are ignored,
aside from emitting a debug message.
void pipecmd_discard_err(pipecmd *cmd, int discard_err)
If discard_err is non-zero, redirect this command's standard
error to /dev/null. Otherwise, and by default, pass it
through. This is usually a bad idea.
void pipecmd_chdir(pipecmd *cmd, const char *directory)
Change the working directory to directory while running this
command.
Added in libpipeline 1.3.0.
void pipecmd_fchdir(pipecmd *cmd, int directory_fd)
Change the working directory to the directory given by the
open file descriptor directory_fd while running this
command.
Added in libpipeline 1.4.0.
void pipecmd_setenv(pipecmd *cmd, const char *name, const char
*value)
Set environment variable name to value while running this
command.
void pipecmd_unsetenv(pipecmd *cmd, const char *name)
Unset environment variable name while running this command.
void pipecmd_clearenv(pipecmd *cmd)
Clear the environment while running this command. (Note
that environment operations work in sequence;
pipecmd_clearenv followed by pipecmd_setenv causes the
command to have just a single environment variable set.)
Beware that this may cause unexpected failures, for example
if some of the contents of the environment are necessary to
execute programs at all (say, PATH).
Added in libpipeline 1.1.0.
void pipecmd_pre_exec(pipecmd *cmd, pipecmd_function_type *func,
pipecmd_function_free_type *free_func, void *data)
Install a pre-exec handler. This will be run immediately
before executing the command's payload (process or
function). Pass NULL to clear any existing pre-exec
handler. The data argument is passed as the function's only
argument, and will be freed before returning using free_func
(if non-NULL).
This is similar to pipeline_install_post_fork, except that
is specific to a single command rather than installing a
global handler, and it runs slightly later (immediately
before exec rather than immediately after fork).
Added in libpipeline 1.5.0.
void pipecmd_sequence_command(pipecmd *cmd, pipecmd *child)
Add a command to a sequence created using
pipecmd_new_sequence().
void pipecmd_dump(pipecmd *cmd, FILE *stream)
Dump a string representation of a command to stream.
char *pipecmd_tostring(pipecmd *cmd)
Return a string representation of a command. The caller
should free the result.
void pipecmd_exec(pipecmd *cmd)
Execute a single command, replacing the current process.
Never returns, instead exiting non-zero on failure.
Added in libpipeline 1.1.0.
void pipecmd_free(pipecmd *cmd)
Destroy a command. Safely does nothing if cmd is NULL.
Functions to build pipelines
pipeline *pipeline_new(void)
Construct a new pipeline.
pipeline *pipeline_new_commandv(pipecmd *cmd1, va_list cmdv)
pipeline *pipeline_new_commands(pipecmd *cmd1, ...)
Convenience constructors wrapping pipeline_new() and
pipeline_command(). Construct a new pipeline consisting of
the given list of commands. Terminate commands with nullptr
(before C23: (void *) 0).
pipeline *pipeline_new_command_argv(const char *name, va_list
argv)
pipeline *pipeline_new_command_args(const char *name, ...)
Construct a new pipeline and add a single command to it.
pipeline *pipeline_join(pipeline *p1, pipeline *p2)
Joins two pipelines, neither of which are allowed to be
started. Discards want_out, want_outfile, and outfd from
p1, and want_in, want_infile, and infd from p2.
void pipeline_connect(pipeline *source, pipeline *sink, ...)
Connect the input of one or more sink pipelines to the
output of a source pipeline. The source pipeline may be
started, but in that case pipeline_want_out() must have been
called with a negative fd; otherwise, calls
pipeline_want_out(source, -1). In any event, calls
pipeline_want_in(sink, -1) on all sinks, none of which are
allowed to be started. Terminate arguments with nullptr
(before C23: (void *) 0).
This is an application-level connection; data may be
intercepted between the pipelines by the program before
calling pipeline_pump(), which sets data flowing from the
source to the sinks. It is primarily useful when more than
one sink pipeline is involved, in which case the pipelines
cannot simply be concatenated into one.
The result is similar to tee(1), except that output can be
sent to more than two places and can easily be sent to
multiple processes.
void pipeline_command(pipeline *p, pipecmd *cmd)
Add a command to a pipeline.
void pipeline_command_argv(pipeline *p, const char *name, va_list
argv)
void pipeline_command_args(pipeline *p, const char *name, ...)
Construct a new command and add it to a pipeline in one go.
void pipeline_command_argstr(pipeline *p, const char *argstr)
Construct a new command from a shell-quoted string and add
it to a pipeline in one go. See the comment against
pipecmd_new_argstr() above if you're tempted to use this
function.
void pipeline_commandv(pipeline *p, va_list cmdv)
void pipeline_commands(pipeline *p, ...)
Convenience functions wrapping pipeline_command() to add
multiple commands at once. Terminate arguments with nullptr
(before C23: (void *) 0).
void pipeline_want_in(pipeline *p, int fd)
void pipeline_want_out(pipeline *p, int fd)
Set file descriptors to use as the input and output of the
whole pipeline. If non-negative, fd is used directly as a
file descriptor. If negative, pipeline_start() will create
pipes and store the input writing half and the output
reading half in the pipeline's infd or outfd field as
appropriate. The default is to leave input and output as
stdin and stdout unless pipeline_want_infile() or
pipeline_want_outfile() respectively has been called.
Calling these functions supersedes any previous call to
pipeline_want_infile() or pipeline_want_outfile()
respectively.
void pipeline_want_infile(pipeline *p, const char *file)
void pipeline_want_outfile(pipeline *p, const char *file)
Set file names to open and use as the input and output of
the whole pipeline. This may be more convenient than
supplying file descriptors, and guarantees that the files
are opened with the same privileges under which the pipeline
is run.
Calling these functions (even with NULL, which returns to
the default of leaving input and output as stdin and stdout)
supersedes any previous call to pipeline_want_in() or
pipeline_want_outfile() respectively.
The given files will be opened when the pipeline is started.
If an output file does not already exist, it is created
(with mode 0666 modified in the usual way by umask); if it
does exist, then it is truncated.
void pipeline_ignore_signals(pipeline *p, int ignore_signals)
If ignore_signals is non-zero, ignore SIGINT and SIGQUIT in
the calling process while the pipeline is running, like
system(3). Otherwise, and by default, leave their
dispositions unchanged.
int pipeline_get_ncommands(pipeline *p)
Return the number of commands in this pipeline.
pipecmd *pipeline_get_command(pipeline *p, int n)
Return command number n from this pipeline, counting from
zero, or NULL if n is out of range.
pipecmd *pipeline_set_command(pipeline *p, int n, pipecmd *cmd)
Set command number n in this pipeline, counting from zero,
to cmd, and return the previous command in that position.
Do nothing and return NULL if n is out of range.
pid_t pipeline_get_pid(pipeline *p, int n)
Return the process ID of command number n from this
pipeline, counting from zero. The pipeline must be started.
Return -1 if n is out of range or if the command has already
exited and been reaped.
Added in libpipeline 1.2.0.
FILE *pipeline_get_infile(pipeline *p)
FILE *pipeline_get_outfile(pipeline *p)
Get streams corresponding to infd and outfd respectively.
The pipeline must be started.
void pipeline_dump(pipeline *p, FILE *stream)
Dump a string representation of p to stream.
char *pipeline_tostring(pipeline *p)
Return a string representation of p. The caller should free
the result.
void pipeline_free(pipeline *p)
Destroy a pipeline and all its commands. Safely does
nothing if p is NULL. May wait for the pipeline to complete
if it has not already done so.
Functions to run pipelines and handle signals
typedef void pipeline_post_fork_fn (void);
void pipeline_install_post_fork(pipeline_post_fork_fn *fn)
Install a post-fork handler. This will be run in any child
process immediately after it is forked. For instance, this
may be used for cleaning up application-specific signal
handlers. Pass NULL to clear any existing post-fork
handler.
See pipecmd_pre_exec for a similar facility limited to a
single command rather than global to the calling process.
void pipeline_start(pipeline *p)
Start the processes in a pipeline. Installs this library's
SIGCHLD handler if not already installed. Calls error
(FATAL) on error.
The standard file descriptors (0, 1, and 2) must be open
before calling this function.
int pipeline_wait_all(pipeline *p, int **statuses, int
*n_statuses)
Wait for a pipeline to complete. Set *statuses to a newly-
allocated array of wait statuses, as returned by waitpid(2),
and *n_statuses to the length of that array. The return
value is similar to the exit status that a shell would
return, with some modifications. If the last command exits
with a signal (other than SIGPIPE, which is considered
equivalent to exiting zero), then the return value is 128
plus the signal number; if the last command exits normally
but non-zero, then the return value is its exit status; if
any other command exits non-zero, then the return value is
127; otherwise, the return value is 0. This means that the
return value is only 0 if all commands in the pipeline exit
successfully.
int pipeline_wait(pipeline *p)
Wait for a pipeline to complete and return its combined exit
status, calculated as for pipeline_wait_all().
int pipeline_run(pipeline *p)
Start a pipeline, wait for it to complete, and free it, all
in one go.
void pipeline_pump(pipeline *p, ...)
Pump data among one or more pipelines connected using
pipeline_connect() until all source pipelines have reached
end-of-file and all data has been written to all sinks (or
failed). All relevant pipelines must be supplied: that is,
no pipeline that has been connected to a source pipeline may
be supplied unless that source pipeline is also supplied.
Automatically starts all pipelines if they are not already
started, but does not wait for them. Terminate arguments
with nullptr (before C23: (void *) 0).
Functions to read output from pipelines
In general, output is returned as a pointer into a buffer owned by
the pipeline, which is automatically freed when pipeline_free() is
called. This saves the caller from having to explicitly free
individual blocks of output data.
const char *pipeline_read(pipeline *p, size_t *len)
Read len bytes of data from the pipeline, returning the data
block. len is updated with the number of bytes read.
const char *pipeline_peek(pipeline *p, size_t *len)
Look ahead in the pipeline's output for len bytes of data,
returning the data block. len is updated with the number of
bytes read. The starting position of the next read or peek
is not affected by this call.
size_t pipeline_peek_size(pipeline *p)
Return the number of bytes of data that can be read using
pipeline_read() or pipeline_peek() solely from the peek
cache, without having to read from the pipeline itself (and
thus potentially block).
void pipeline_peek_skip(pipeline *p, size_t len)
Skip over and discard len bytes of data from the peek cache.
Asserts that enough data is available to skip, so you may
want to check using pipeline_peek_size() first.
const char *pipeline_readline(pipeline *p)
Read a line of data from the pipeline, returning it.
const char *pipeline_peekline(pipeline *p)
Look ahead in the pipeline's output for a line of data,
returning it. The starting position of the next read or
peek is not affected by this call.
Signal handling
libpipeline installs a signal handler for SIGCHLD, and collects
the exit status of child processes in pipeline_wait().
Applications using this library must either refrain from changing
the disposition of SIGCHLD (in other words, must rely on
libpipeline for all child process handling) or else must make sure
to restore 's SIGCHLD handler before calling any of its functions.
If the ignore_signals flag is set in a pipeline (which is the
default), then the SIGINT and SIGQUIT signals will be ignored in
the parent process while child processes are running. This
mirrors the behaviour of system(3).
libpipeline leaves child processes with the default disposition of
SIGPIPE, namely to terminate the process. It ignores SIGPIPE in
the parent process while running pipeline_pump().
Reaping of child processes
libpipeline installs a SIGCHLD handler that will attempt to reap
child processes which have exited. This calls waitpid(2) with -1,
so it will reap any child process, not merely those created by way
of this library. At present, this means that if the calling
program forks other child processes which may exit while a
pipeline is running, the program is not guaranteed to be able to
collect exit statuses of those processes.
You should not rely on this behaviour, and in future it may be
modified either to reap only child processes created by this
library or to provide a way to return foreign statuses to the
application. Please contact the author if you have an example
application and would like to help design such an interface.
If the PIPELINE_DEBUG environment variable is set to “1”, then
libpipeline will emit debugging messages on standard error.
If the PIPELINE_QUIET environment variable is set to any value,
then libpipeline will refrain from printing an error message when
a subprocess is terminated by a signal. Added in libpipeline
1.4.0.
In the following examples, function names starting with pipecmd_
or pipeline_ are real libpipeline functions, while any other
function names are pseudocode. These examples use the C23-style
nullptr: https://en.cppreference.com/w/c/language/nullptr keyword
to terminate variadic argument lists; on earlier versions of C,
use (void *) 0 instead.
The simplest case is simple. To run a single command, such as mv
source dest:
pipeline *p = pipeline_new_command_args ("mv", source, dest, nullptr);
int status = pipeline_run (p);
libpipeline is often used to mimic shell pipelines, such as the
following example:
zsoelim < input-file | tbl | nroff -mandoc -Tutf8
The code to construct this would be:
pipeline *p;
int status;
p = pipeline_new ();
pipeline_want_infile (p, "input-file");
pipeline_command_args (p, "zsoelim", nullptr);
pipeline_command_args (p, "tbl", nullptr);
pipeline_command_args (p, "nroff", "-mandoc", "-Tutf8", nullptr);
status = pipeline_run (p);
You might want to construct a command more dynamically:
pipecmd *manconv = pipecmd_new_args ("manconv", "-f", from_code,
"-t", "UTF-8", nullptr);
if (quiet)
pipecmd_arg (manconv, "-q");
pipeline_command (p, manconv);
Perhaps you want an environment variable set only while running a
certain command:
pipecmd *less = pipecmd_new ("less");
pipecmd_setenv (less, "LESSCHARSET", lesscharset);
You might find yourself needing to pass the output of one pipeline
to several other pipelines, in a “tee” arrangement:
pipeline *source, *sink1, *sink2;
source = make_source ();
sink1 = make_sink1 ();
sink2 = make_sink2 ();
pipeline_connect (source, sink1, sink2, nullptr);
/* Pump data among these pipelines until there's nothing left. */
pipeline_pump (source, sink1, sink2, nullptr);
pipeline_free (sink2);
pipeline_free (sink1);
pipeline_free (source);
Maybe one of your commands is actually an in-process function,
rather than an external program:
pipecmd *inproc = pipecmd_new_function ("in-process", &func,
NULL, NULL);
pipeline_command (p, inproc);
Sometimes your program needs to consume the output of a pipeline,
rather than sending it all to some other subprocess:
pipeline *p = make_pipeline ();
const char *line;
pipeline_want_out (p, -1);
pipeline_start (p);
line = pipeline_peekline (p);
if (!strstr (line, "coding: UTF-8"))
printf ("Unicode text follows:0);
while (line = pipeline_readline (p))
printf (" %s", line);
pipeline_free (p);
fork(2), execve(2), system(3), popen(3).
Most of libpipeline was written by Colin Watson
<cjwatson@debian.org>, originally for use in man-db. The initial
version was based very loosely on the run_pipeline() function in
GNU groff, written by James Clark <jjc@jclark.com>. It also
contains library code by Markus Armbruster, and by various
contributors to Gnulib.
libpipeline is licensed under the GNU General Public License,
version 3 or later. See the README file for full details.
Using this library in a program which runs any other child
processes and/or installs its own SIGCHLD handler is unlikely to
work.
This page is part of the libpipeline (pipeline manipulation
library) project. Information about the project can be found at
http://libpipeline.nongnu.org/. If you have a bug report for this
manual page, see
⟨http://savannah.nongnu.org/bugs/?group=libpipeline⟩. This page
was obtained from the project's upstream Git repository
⟨https://gitlab.com/cjwatson/libpipeline⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2024-08-27.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
GNU April 28, 2022 LIBPIPELINE(3)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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