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

LIBPIPELINE(3)          GNU 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 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
     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 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_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 substitu‐
           tions.

     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 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.

     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‐
           mand.

     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 com‐
           mand.

     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 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).

     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 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‐
           cesses.

     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 func‐
           tion.

     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) super‐
           sedes 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 noth‐
           ing 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.

     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
           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 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 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 poten‐
           tially 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 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
     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 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.

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 subpro‐
     cess 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 follow‐
     ing 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 cer‐
     tain 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 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 2017-07-05.  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