1LIBPIPELINE(3) GNU Library Functions Manual LIBPIPELINE(3)
2
4 libpipeline — pipeline manipulation library
5
7 #include <pipeline.h>
8
10 libpipeline is a C library for setting up and running pipelines of pro‐
11 cesses, without needing to involve shell command-line parsing which is
12 often error-prone and insecure. This relieves programmers of the need to
13 laboriously construct pipelines using lower-level primitives such as fork
14 and execve.
15 The general way to use libpipeline involves constructing a pipeline
17 structure and adding one or more pipecmd structures to it. A pipecmd
18 represents a subprocess (or “command”), while a pipeline represents a
19 sequence of subprocesses each of whose outputs is connected to the next
20 one's input, as in the example ls | grep pattern | less. The calling
21 program may adjust certain properties of each command independently, such
22 as its environment and nice(3) priority, as well as properties of the
23 entire pipeline such as its input and output and the way signals are han‐
24 dled while executing it. The calling program may then start the pipe‐
25 line, read output from it, wait for it to complete, and gather its exit
26 status.
27 Strings passed as const char * function arguments will be copied by the
29 library.
30 Functions to build individual commands
32 pipecmd *pipecmd_new(const char *name)
33 Construct a new command representing execution of a program called
35 name.
36 pipecmd *pipecmd_new_argv(const char *name, va_list argv)
38 pipecmd *pipecmd_new_args(const char *name, ...)
39 Convenience constructors wrapping pipecmd_new() and pipecmd_arg().
41 Construct a new command representing execution of a program called
42 name with arguments. Terminate arguments with NULL.
43 pipecmd *pipecmd_new_argstr(const char *argstr)
45 Split argstr on whitespace to construct a command and arguments,
47 honouring shell-style single-quoting, double-quoting, and back‐
48 slashes, but not other shell evilness like wildcards, semicolons,
49 or backquotes. This is included only to support situations where
50 command arguments are encoded into configuration files and the
51 like. While it is safer than system(3), it still involves signifi‐
52 cant string parsing which is inherently riskier than avoiding it
53 altogether. Please try to avoid using it in new code.
54 typedef void pipecmd_function_type (void *);
56 typedef void pipecmd_function_free_type (void *);
57 pipecmd *pipecmd_new_function(const char *name,
58 pipecmd_function_type *func, pipecmd_function_free_type *free_func,
59 void *data)
60 Construct a new command that calls a given function rather than
62 executing a process.
63 The data argument is passed as the function's only argument, and
65 will be freed before returning using free_func (if non-NULL).
66 pipecmd_* functions that deal with arguments cannot be used with
68 the command returned by this function.
69 pipecmd *pipecmd_new_sequencev(const char *name, va_list cmdv)
71 pipecmd *pipecmd_new_sequence(const char *name, ...)
72 Construct a new command that itself runs a sequence of commands,
74 supplied as command * arguments following name and terminated by
75 NULL. The commands will be executed in forked children; if any
76 exits non-zero then it will terminate the sequence, as with "&&" in
77 shell.
78 pipecmd_* functions that deal with arguments cannot be used with
80 the command returned by this function.
81 pipecmd *pipecmd_new_passthrough(void)
83 Return a new command that just passes data from its input to its
85 output.
86 pipecmd *pipecmd_dup(pipecmd *cmd)
88 Return a duplicate of a command.
90 void pipecmd_arg(pipecmd *cmd, const char *arg)
92 Add an argument to a command.
94 void pipecmd_argf(pipecmd *cmd, const char *format, ...)
96 Convenience function to add an argument with printf substitutions.
98 void pipecmd_argv(pipecmd *cmd, va_list argv)
100 void pipecmd_args(pipecmd *cmd, ...)
101 Convenience functions wrapping pipecmd_arg() to add multiple argu‐
103 ments at once. Terminate arguments with NULL.
104 void pipecmd_argstr(pipecmd *cmd, const char *argstr)
106 Split argstr on whitespace to add a list of arguments, honouring
108 shell-style single-quoting, double-quoting, and backslashes, but
109 not other shell evilness like wildcards, semicolons, or backquotes.
110 This is included only to support situations where command arguments
111 are encoded into configuration files and the like. While it is
112 safer than system(3), it still involves significant string parsing
113 which is inherently riskier than avoiding it altogether. Please
114 try to avoid using it in new code.
115 void pipecmd_get_nargs(pipecmd *cmd)
117 Return the number of arguments to this command. Note that this
119 includes the command name as the first argument, so the command
120 ‘echo foo bar’ is counted as having three arguments.
121 void pipecmd_nice(pipecmd *cmd, int value)
123 Set the nice(3) value for this command. Defaults to 0. Errors
125 while attempting to set the nice value are ignored, aside from
126 emitting a debug message.
127 void pipecmd_discard_err(pipecmd *cmd, int discard_err)
129 If discard_err is non-zero, redirect this command's standard error
131 to /dev/null. Otherwise, and by default, pass it through. This is
132 usually a bad idea.
133 void pipecmd_setenv(pipecmd *cmd, const char *name, const char *value)
135 Set environment variable name to value while running this command.
137 void pipecmd_unsetenv(pipecmd *cmd, const char *name)
139 Unset environment variable name while running this command.
141 void pipecmd_clearenv(pipecmd *cmd)
143 Clear the environment while running this command. (Note that envi‐
145 ronment operations work in sequence; pipecmd_clearenv followed by
146 pipecmd_setenv causes the command to have just a single environment
147 variable set.)
148 void pipecmd_sequence_command(pipecmd *cmd, pipecmd *child)
150 Add a command to a sequence created using pipecmd_new_sequence().
152 void pipecmd_dump(pipecmd *cmd, FILE *stream)
154 Dump a string representation of a command to stream.
156 char *pipecmd_tostring(pipecmd *cmd)
158 Return a string representation of a command. The caller should
160 free the result.
161 void pipecmd_exec(pipecmd *cmd)
163 Execute a single command, replacing the current process. Never
165 returns, instead exiting non-zero on failure.
166 void pipecmd_free(pipecmd *cmd)
168 Destroy a command. Safely does nothing if cmd is NULL.
170 Functions to build pipelines
172 pipeline *pipeline_new(void)
173 Construct a new pipeline.
175 pipeline *pipeline_new_commandv(pipecmd *cmd1, va_list cmdv)
177 pipeline *pipeline_new_commands(pipecmd *cmd1, ...)
178 Convenience constructors wrapping pipeline_new() and
180 pipeline_command(). Construct a new pipeline consisting of the
181 given list of commands. Terminate commands with NULL.
182 pipeline *pipeline_new_command_argv(const char *name, va_list argv)
184 pipeline *pipeline_new_command_args(const char *name, ...)
185 Construct a new pipeline and add a single command to it.
187 pipeline *pipeline_join(pipeline *p1, pipeline *p2)
189 Joins two pipelines, neither of which are allowed to be started.
191 Discards want_out, want_outfile, and outfd from p1, and want_in,
192 want_infile, and infd from p2.
193 void pipeline_connect(pipeline *source, pipeline *sink, ...)
195 Connect the input of one or more sink pipelines to the output of a
197 source pipeline. The source pipeline may be started, but in that
198 case pipeline_want_out() must have been called with a negative fd;
199 otherwise, calls pipeline_want_out(source, -1). In any event,
200 calls pipeline_want_in(sink, -1) on all sinks, none of which are
201 allowed to be started. Terminate arguments with NULL.
202 This is an application-level connection; data may be intercepted
204 between the pipelines by the program before calling
205 pipeline_pump(), which sets data flowing from the source to the
206 sinks. It is primarily useful when more than one sink pipeline is
207 involved, in which case the pipelines cannot simply be concatenated
208 into one.
209 The result is similar to tee(1), except that output can be sent to
211 more than two places and can easily be sent to multiple processes.
212 void pipeline_command(pipeline *p, pipecmd *cmd)
214 Add a command to a pipeline.
216 void pipeline_command_argv(pipeline *p, const char *name, va_list argv)
218 void pipeline_command_args(pipeline *p, const char *name, ...)
219 Construct a new command and add it to a pipeline in one go.
221 void pipeline_command_argstr(pipeline *p, const char *argstr)
223 Construct a new command from a shell-quoted string and add it to a
225 pipeline in one go. See the comment against pipecmd_new_argstr()
226 above if you're tempted to use this function.
227 void pipeline_commandv(pipeline *p, va_list cmdv)
229 void pipeline_commands(pipeline *p, ...)
230 Convenience functions wrapping pipeline_command() to add multiple
232 commands at once. Terminate arguments with NULL.
233 void pipeline_want_in(pipeline *p, int fd)
235 void pipeline_want_out(pipeline *p, int fd)
236 Set file descriptors to use as the input and output of the whole
238 pipeline. If non-negative, fd is used directly as a file descrip‐
239 tor. If negative, pipeline_start() will create pipes and store the
240 input writing half and the output reading half in the pipeline's
241 infd or outfd field as appropriate. The default is to leave input
242 and output as stdin and stdout unless pipeline_want_infile() or
243 pipeline_want_outfile() respectively has been called.
244 Calling these functions supersedes any previous call to
246 pipeline_want_infile() or pipeline_want_outfile() respectively.
247 void pipeline_want_infile(pipeline *p, const char *file)
249 void pipeline_want_outfile(pipeline *p, const char *file)
250 Set file names to open and use as the input and output of the whole
252 pipeline. This may be more convenient than supplying file descrip‐
253 tors, and guarantees that the files are opened with the same privi‐
254 leges under which the pipeline is run.
255 Calling these functions (even with NULL, which returns to the
257 default of leaving input and output as stdin and stdout) supersedes
258 any previous call to pipeline_want_in() or pipeline_want_outfile()
259 respectively.
260 The given files will be opened when the pipeline is started. If an
262 output file does not already exist, it is created (with mode 0666
263 modified in the usual way by umask); if it does exist, then it is
264 truncated.
265 void pipeline_ignore_signals(pipeline *p, int ignore_signals)
267 If ignore_signals is non-zero, ignore SIGINT and SIGQUIT in the
269 calling process while the pipeline is running, like system(3).
270 Otherwise, and by default, leave their dispositions unchanged.
271 int pipeline_get_ncommands(pipeline *p)
273 Return the number of commands in this pipeline.
275 pipecmd *pipeline_get_command(pipeline *p, int n)
277 Return command number n from this pipeline, counting from zero, or
279 NULL if n is out of range.
280 pipecmd *pipeline_set_command(pipeline *p, int n, pipecmd *cmd)
282 Set command number n in this pipeline, counting from zero, to cmd,
284 and return the previous command in that position. Do nothing and
285 return NULL if n is out of range.
286 pid_t pipeline_get_pid(pipeline *p, int n)
288 Return the process ID of command number n from this pipeline,
290 counting from zero. The pipeline must be started. Return -1 if n
291 is out of range or if the command has already exited and been
292 reaped.
293 FILE *pipeline_get_infile(pipeline *p)
295 FILE *pipeline_get_outfile(pipeline *p)
296 Get streams corresponding to infd and outfd respectively. The
298 pipeline must be started.
299 void pipeline_dump(pipeline *p, FILE *stream)
301 Dump a string representation of p to stream.
303 char *pipeline_tostring(pipeline *p)
305 Return a string representation of p. The caller should free the
307 result.
308 void pipeline_free(pipeline *p)
310 Destroy a pipeline and all its commands. Safely does nothing if p
312 is NULL. May wait for the pipeline to complete if it has not
313 already done so.
314 Functions to run pipelines and handle signals
316 typedef void pipeline_post_fork_fn (void);
317 void pipeline_install_post_fork(pipeline_post_fork_fn *fn)
318 Install a post-fork handler. This will be run in any child process
320 immediately after it is forked. For instance, this may be used for
321 cleaning up application-specific signal handlers. Pass NULL to
322 clear any existing post-fork handler.
323 void pipeline_start(pipeline *p)
325 Start the processes in a pipeline. Installs this library's SIGCHLD
327 handler if not already installed. Calls error (FATAL) on error.
328 int pipeline_wait_all(pipeline *p, int **statuses, int *n_statuses)
330 Wait for a pipeline to complete. Set *statuses to a newly-allo‐
332 cated array of wait statuses, as returned by waitpid(2), and
333 *n_statuses to the length of that array. The return value is simi‐
334 lar to the exit status that a shell would return, with some modifi‐
335 cations. If the last command exits with a signal (other than
336 SIGPIPE, which is considered equivalent to exiting zero), then the
337 return value is 128 plus the signal number; if the last command
338 exits normally but non-zero, then the return value is its exit sta‐
339 tus; if any other command exits non-zero, then the return value is
340 127; otherwise, the return value is 0. This means that the return
341 value is only 0 if all commands in the pipeline exit successfully.
342 int pipeline_wait(pipeline *p)
344 Wait for a pipeline to complete and return the exit status.
346 int pipeline_run(pipeline *p)
348 Start a pipeline, wait for it to complete, and free it, all in one
350 go.
351 void pipeline_pump(pipeline *p, ...)
353 Pump data among one or more pipelines connected using
355 pipeline_connect() until all source pipelines have reached end-of-
356 file and all data has been written to all sinks (or failed). All
357 relevant pipelines must be supplied: that is, no pipeline that has
358 been connected to a source pipeline may be supplied unless that
359 source pipeline is also supplied. Automatically starts all pipe‐
360 lines if they are not already started, but does not wait for them.
361 Terminate arguments with NULL.
362 Functions to read output from pipelines
364 In general, output is returned as a pointer into a buffer owned by the
365 pipeline, which is automatically freed when pipeline_free() is called.
366 This saves the caller from having to explicitly free individual blocks of
367 output data.
368 const char *pipeline_read(pipeline *p, size_t *len)
370 Read len bytes of data from the pipeline, returning the data block.
372 len is updated with the number of bytes read.
373 const char *pipeline_peek(pipeline *p, size_t *len)
375 Look ahead in the pipeline's output for len bytes of data, return‐
377 ing the data block. len is updated with the number of bytes read.
378 The starting position of the next read or peek is not affected by
379 this call.
380 size_t pipeline_peek_size(pipeline *p)
382 Return the number of bytes of data that can be read using
384 pipeline_read() or pipeline_peek() solely from the peek cache,
385 without having to read from the pipeline itself (and thus poten‐
386 tially block).
387 void pipeline_peek_skip(pipeline *p, size_t len)
389 Skip over and discard len bytes of data from the peek cache.
391 Asserts that enough data is available to skip, so you may want to
392 check using pipeline_peek_size() first.
393 const char *pipeline_readline(pipeline *p)
395 Read a line of data from the pipeline, returning it.
397 const char *pipeline_peekline(pipeline *p)
399 Look ahead in the pipeline's output for a line of data, returning
401 it. The starting position of the next read or peek is not affected
402 by this call.
403 Signal handling
405 libpipeline installs a signal handler for SIGCHLD, and collects the exit
406 status of child processes in pipeline_wait(). Applications using this
407 library must either refrain from changing the disposition of SIGCHLD (in
408 other words, must rely on libpipeline for all child process handling) or
409 else must make sure to restore libpipeline's SIGCHLD handler before call‐
410 ing any of its functions.
411 If the ignore_signals flag is set in a pipeline (which is the default),
413 then the SIGINT and SIGQUIT signals will be ignored in the parent process
414 while child processes are running. This mirrors the behaviour of
415 system(3).
416 libpipeline leaves child processes with the default disposition of
418 SIGPIPE, namely to terminate the process. It ignores SIGPIPE in the par‐
419 ent process while running pipeline_pump().
420 Reaping of child processes
422 libpipeline installs a SIGCHLD handler that will attempt to reap child
423 processes which have exited. This calls waitpid(2) with -1, so it will
424 reap any child process, not merely those created by way of this library.
425 At present, this means that if the calling program forks other child pro‐
426 cesses which may exit while a pipeline is running, the program is not
427 guaranteed to be able to collect exit statuses of those processes.
428 You should not rely on this behaviour, and in future it may be modified
430 either to reap only child processes created by this library or to provide
431 a way to return foreign statuses to the application. Please contact the
432 author if you have an example application and would like to help design
433 such an interface.
434
436 If the PIPELINE_DEBUG environment variable is set to “1”, then
437 libpipeline will emit debugging messages on standard error.
438
440 In the following examples, function names starting with pipecmd_ or
441 pipeline_ are real libpipeline functions, while any other function names
442 are pseudocode.
443 The simplest case is simple. To run a single command, such as mv source
445 dest:
446 pipeline *p = pipeline_new_command_args ("mv", source, dest, NULL);
448 int status = pipeline_run (p);
449 libpipeline is often used to mimic shell pipelines, such as the following
451 example:
452 zsoelim < input-file | tbl | nroff -mandoc -Tutf8
454 The code to construct this would be:
456 pipeline *p;
458 int status;
459 p = pipeline_new ();
461 pipeline_want_infile (p, "input-file");
462 pipeline_command_args (p, "zsoelim", NULL);
463 pipeline_command_args (p, "tbl", NULL);
464 pipeline_command_args (p, "nroff", "-mandoc", "-Tutf8", NULL);
465 status = pipeline_run (p);
466 You might want to construct a command more dynamically:
468 pipecmd *manconv = pipecmd_new_args ("manconv", "-f", from_code,
470 "-t", "UTF-8", NULL);
471 if (quiet)
472 pipecmd_arg (manconv, "-q");
473 pipeline_command (p, manconv);
474 Perhaps you want an environment variable set only while running a certain
476 command:
477 pipecmd *less = pipecmd_new ("less");
479 pipecmd_setenv (less, "LESSCHARSET", lesscharset);
480 You might find yourself needing to pass the output of one pipeline to
482 several other pipelines, in a “tee” arrangement:
483 pipeline *source, *sink1, *sink2;
485 source = make_source ();
487 sink1 = make_sink1 ();
488 sink2 = make_sink2 ();
489 pipeline_connect (source, sink1, sink2, NULL);
490 /* Pump data among these pipelines until there's nothing left. */
491 pipeline_pump (source, sink1, sink2, NULL);
492 pipeline_free (sink2);
493 pipeline_free (sink1);
494 pipeline_free (source);
495 Maybe one of your commands is actually an in-process function, rather
497 than an external program:
498 pipecmd *inproc = pipecmd_new_function ("in-process", &func,
500 NULL, NULL);
501 pipeline_command (p, inproc);
502 Sometimes your program needs to consume the output of a pipeline, rather
504 than sending it all to some other subprocess:
505 pipeline *p = make_pipeline ();
507 const char *line;
508 pipeline_want_out (p, -1);
510 pipeline_start (p);
511 line = pipeline_peekline (p);
512 if (!strstr (line, "coding: UTF-8"))
513 printf ("Unicode text follows:0);
514 while (line = pipeline_readline (p))
515 printf (" %s", line);
516 pipeline_free (p);
517
519 fork(2), execve(2), system(3), popen(3).
520
522 Most of libpipeline was written by Colin Watson <cjwatson@debian.org>,
523 originally for use in man-db. The initial version was based very loosely
524 on the run_pipeline() function in GNU groff, written by James Clark
525 <jjc@jclark.com>. It also contains library code by Markus Armbruster,
526 and by various contributors to Gnulib.
527 libpipeline is licensed under the GNU General Public License, version 3
529 or later. See the README file for full details.
530
532 Using this library in a program which runs any other child processes
533 and/or installs its own SIGCHLD handler is unlikely to work.
534GNU October 11, 2010 GNU