libpipeline(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ENVIRONMENT | EXAMPLES | SEE ALSO | AUTHORS | BUGS | COLOPHON

LIBPIPELINE(3)        BSD Library Functions Manual        LIBPIPELINE(3)

NAME         top

     libpipeline — pipeline manipulation library

SYNOPSIS         top

     #include <pipeline.h>

DESCRIPTION         top

     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 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 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 NULL.  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 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
           command.

     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.

     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).

     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).

     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
     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 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 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 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 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.

     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 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.

     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
     libpipeline'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.

ENVIRONMENT         top

     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.

EXAMPLES         top

     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 mv
     source 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
     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", 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);
           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, NULL);
           /* Pump data among these pipelines until there's nothing left. */
           pipeline_pump (source, sink1, sink2, NULL);
           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);

SEE ALSO         top

     fork(2), execve(2), system(3), popen(3).

AUTHORS         top

     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.

BUGS         top

     Using this library in a program which runs any other child
     processes and/or installs its own SIGCHLD handler is unlikely to
     work.

COLOPHON         top

     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 2021-04-01.  (At that
     time, the date of the most recent commit that was found in the
     repository was 2020-11-25.)  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                         October 11, 2010                         GNU