libpipeline is a C library for setting up and running pipelines of pro‐
cesses, 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
Strings passed as const char * function arguments will be copied by the
Functions to build individual commandspipecmd *pipecmd_new(const char *name)
Construct a new command representing execution of a program
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
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 back‐
slashes, 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 sig‐
nificant 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_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
NULL. The commands will be executed in forked children; if any
exits non-zero then it will terminate the sequence, as with "&&"
pipecmd_* functions that deal with arguments cannot be used with
the command returned by this function.
Return a new command that just passes data from its input to its
pipecmd *pipecmd_dup(pipecmd *cmd)
Return a duplicate of a command.
voidpipecmd_arg(pipecmd *cmd, const char *arg)
Add an argument to a command.
voidpipecmd_argf(pipecmd *cmd, const char *format, ...)
Convenience function to add an argument with printf substitu‐
voidpipecmd_argv(pipecmd *cmd, va_list argv)
voidpipecmd_args(pipecmd *cmd, ...)
Convenience functions wrapping pipecmd_arg() to add multiple
arguments at once. Terminate arguments with NULL.
voidpipecmd_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 back‐
quotes. This is included only to support situations where com‐
mand 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 alto‐
gether. Please try to avoid using it in new code.
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.
voidpipecmd_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.
voidpipecmd_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.
voidpipecmd_chdir(pipecmd *cmd, const char *directory)
Change the working directory to directory while running this com‐
voidpipecmd_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.
voidpipecmd_setenv(pipecmd *cmd, const char *name, const char *value)
Set environment variable name to value while running this com‐
voidpipecmd_unsetenv(pipecmd *cmd, const char *name)
Unset environment variable name while running this command.
Clear the environment while running this command. (Note that
environment operations work in sequence; pipecmd_clearenv fol‐
lowed 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).
voidpipecmd_sequence_command(pipecmd *cmd, pipecmd *child)
Add a command to a sequence created using pipecmd_new_sequence().
voidpipecmd_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.
Execute a single command, replacing the current process. Never
returns, instead exiting non-zero on failure.
Destroy a command. Safely does nothing if cmd is NULL.
Functions to build pipelinespipeline *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 NULL.
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.
voidpipeline_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 nega‐
tive 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 NULL.
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 con‐
catenated 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 pro‐
voidpipeline_command(pipeline *p, pipecmd *cmd)
Add a command to a pipeline.
voidpipeline_command_argv(pipeline *p, const char *name, va_list argv)
voidpipeline_command_args(pipeline *p, const char *name, ...)
Construct a new command and add it to a pipeline in one go.
voidpipeline_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 func‐
voidpipeline_commandv(pipeline *p, va_list cmdv)
voidpipeline_commands(pipeline *p, ...)
Convenience functions wrapping pipeline_command() to add multiple
commands at once. Terminate arguments with NULL.
voidpipeline_want_in(pipeline *p, int fd)
voidpipeline_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.
voidpipeline_want_infile(pipeline *p, const char *file)
voidpipeline_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) super‐
sedes any previous call to pipeline_want_in() or
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.
voidpipeline_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.
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 noth‐
ing and return NULL if n is out of range.
pid_tpipeline_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
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.
voidpipeline_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
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 signalstypedef void pipeline_post_fork_fn (void);
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.
Start the processes in a pipeline. Installs this library's
SIGCHLD handler if not already installed. Calls error (FATAL) on
intpipeline_wait_all(pipeline *p, int **statuses, int *n_statuses)
Wait for a pipeline to complete. Set *statuses to a newly-allo‐
cated 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
Wait for a pipeline to complete and return its combined exit sta‐
tus, calculated as for pipeline_wait_all().
Start a pipeline, wait for it to complete, and free it, all in
voidpipeline_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 NULL.
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.
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 poten‐
voidpipeline_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 handlinglibpipeline 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 libpipeline's SIGCHLD han‐
dler 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
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 processeslibpipeline 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
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 pro‐
vide a way to return foreign statuses to the application. Please con‐
tact 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 subpro‐
cess is terminated by a signal.
In the following examples, function names starting with pipecmd_ or
pipeline_ are real libpipeline functions, while any other function
names are pseudocode.
The simplest case is simple. To run a single command, such as mvsource dest:
pipeline *p = pipeline_new_command_args ("mv", source, dest, NULL);
int status = pipeline_run (p);
libpipeline is often used to mimic shell pipelines, such as the follow‐
zsoelim < input-file | tbl | nroff -mandoc -Tutf8
The code to construct this would be:
p = pipeline_new ();
pipeline_want_infile (p, "input-file");
pipeline_command_args (p, "zsoelim", NULL);
pipeline_command_args (p, "tbl", NULL);
pipeline_command_args (p, "nroff", "-mandoc", "-Tutf8", NULL);
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", NULL);
pipecmd_arg (manconv, "-q");
pipeline_command (p, manconv);
Perhaps you want an environment variable set only while running a cer‐
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, NULL);
/* Pump data among these pipelines until there's nothing left. */
pipeline_pump (source, sink1, sink2, NULL);
Maybe one of your commands is actually an in-process function, rather
than an external program:
pipecmd *inproc = pipecmd_new_function ("in-process", &func,
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);
line = pipeline_peekline (p);
if (!strstr (line, "coding: UTF-8"))
printf ("Unicode text follows:0);
while (line = pipeline_readline (p))
printf (" %s", line);
Most of libpipeline was written by Colin Watson <email@example.com>,
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 <firstname.lastname@example.org>. 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.
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 man‐
ual page, see http://savannah.nongnu.org/bugs/?group=libpipeline. This
page was obtained from the project's upstream Git repository
git://git.savannah.nongnu.org/libpipeline.git on 2017-03-13. 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
GNU October 11, 2010 GNU