LIBPIPELINE(3) GNU Library Functions Manual LIBPIPELINE(3)NAME
libpipeline — pipeline manipulation library
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
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 han‐
dled while executing it. The calling program may then start the pipe‐
line, read output from it, wait for it to complete, and gather its exit
Strings passed as const char * function arguments will be copied by the
Functions to build individual commands
pipecmd *pipecmd_new(const char *name)
Construct a new command representing execution of a program called
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 NULL.
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 signifi‐
cant 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,
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 "&&" in
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.
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 argu‐
ments at once. Terminate arguments with NULL.
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.
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 com‐
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 envi‐
ronment 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).
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.
void pipecmd_free(pipecmd *cmd)
Destroy a command. Safely does nothing if cmd is NULL.
Functions to build pipelines
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.
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 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 concatenated
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 NULL.
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 descrip‐
tor. 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 descrip‐
tors, and guarantees that the files are opened with the same privi‐
leges 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()
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
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
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
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.
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.
int pipeline_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 simi‐
lar to the exit status that a shell would return, with some modifi‐
cations. 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 sta‐
tus; 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 sta‐
tus, 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
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 pipe‐
lines 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
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, return‐
ing 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
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 poten‐
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.
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 libpipeline's SIGCHLD handler before call‐
ing 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
libpipeline leaves child processes with the default disposition of
SIGPIPE, namely to terminate the process. It ignores SIGPIPE in the par‐
ent 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 pro‐
cesses 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.
In the following examples, function names starting with pipecmd_ or
pipeline_ are real libpipeline functions, while any other function names
The simplest case is simple. To run a single command, such as mv source
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 following
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 certain
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);
SEE ALSOfork(2), execve(2), system(3), popen(3).
Most of libpipeline was written by Colin Watson ⟨firstname.lastname@example.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
⟨email@example.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.
GNU October 11, 2010 GNU