libpipeline man page

libpipeline — pipeline manipulation library

Synopsis

#include <pipeline.h>

Description

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_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.

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-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

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

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

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

Authors

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

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

Info

October 11, 2010