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_chdir(pipecmd *cmd, const char *directory)
135 Change the working directory to directory while running this com‐
137 mand.
138 void pipecmd_fchdir(pipecmd *cmd, int directory_fd)
140 Change the working directory to the directory given by the open
142 file descriptor directory_fd while running this command.
143 void pipecmd_setenv(pipecmd *cmd, const char *name, const char *value)
145 Set environment variable name to value while running this command.
147 void pipecmd_unsetenv(pipecmd *cmd, const char *name)
149 Unset environment variable name while running this command.
151 void pipecmd_clearenv(pipecmd *cmd)
153 Clear the environment while running this command. (Note that envi‐
155 ronment operations work in sequence; pipecmd_clearenv followed by
156 pipecmd_setenv causes the command to have just a single environment
157 variable set.) Beware that this may cause unexpected failures, for
158 example if some of the contents of the environment are necessary to
159 execute programs at all (say, PATH).
160 void pipecmd_pre_exec(pipecmd *cmd, pipecmd_function_type *func,
162 pipecmd_function_free_type *free_func, void *data)
163 Install a pre-exec handler. This will be run immediately before
165 executing the command's payload (process or function). Pass NULL
166 to clear any existing pre-exec handler. The data argument is
167 passed as the function's only argument, and will be freed before
168 returning using free_func (if non-NULL).
169 This is similar to pipeline_install_post_fork, except that is spe‐
171 cific to a single command rather than installing a global handler,
172 and it runs slightly later (immediately before exec rather than
173 immediately after fork).
174 void pipecmd_sequence_command(pipecmd *cmd, pipecmd *child)
176 Add a command to a sequence created using pipecmd_new_sequence().
178 void pipecmd_dump(pipecmd *cmd, FILE *stream)
180 Dump a string representation of a command to stream.
182 char *pipecmd_tostring(pipecmd *cmd)
184 Return a string representation of a command. The caller should
186 free the result.
187 void pipecmd_exec(pipecmd *cmd)
189 Execute a single command, replacing the current process. Never
191 returns, instead exiting non-zero on failure.
192 void pipecmd_free(pipecmd *cmd)
194 Destroy a command. Safely does nothing if cmd is NULL.
196 Functions to build pipelines
198 pipeline *pipeline_new(void)
199 Construct a new pipeline.
201 pipeline *pipeline_new_commandv(pipecmd *cmd1, va_list cmdv)
203 pipeline *pipeline_new_commands(pipecmd *cmd1, ...)
204 Convenience constructors wrapping pipeline_new() and
206 pipeline_command(). Construct a new pipeline consisting of the
207 given list of commands. Terminate commands with NULL.
208 pipeline *pipeline_new_command_argv(const char *name, va_list argv)
210 pipeline *pipeline_new_command_args(const char *name, ...)
211 Construct a new pipeline and add a single command to it.
213 pipeline *pipeline_join(pipeline *p1, pipeline *p2)
215 Joins two pipelines, neither of which are allowed to be started.
217 Discards want_out, want_outfile, and outfd from p1, and want_in,
218 want_infile, and infd from p2.
219 void pipeline_connect(pipeline *source, pipeline *sink, ...)
221 Connect the input of one or more sink pipelines to the output of a
223 source pipeline. The source pipeline may be started, but in that
224 case pipeline_want_out() must have been called with a negative fd;
225 otherwise, calls pipeline_want_out(source, -1). In any event,
226 calls pipeline_want_in(sink, -1) on all sinks, none of which are
227 allowed to be started. Terminate arguments with NULL.
228 This is an application-level connection; data may be intercepted
230 between the pipelines by the program before calling
231 pipeline_pump(), which sets data flowing from the source to the
232 sinks. It is primarily useful when more than one sink pipeline is
233 involved, in which case the pipelines cannot simply be concatenated
234 into one.
235 The result is similar to tee(1), except that output can be sent to
237 more than two places and can easily be sent to multiple processes.
238 void pipeline_command(pipeline *p, pipecmd *cmd)
240 Add a command to a pipeline.
242 void pipeline_command_argv(pipeline *p, const char *name, va_list argv)
244 void pipeline_command_args(pipeline *p, const char *name, ...)
245 Construct a new command and add it to a pipeline in one go.
247 void pipeline_command_argstr(pipeline *p, const char *argstr)
249 Construct a new command from a shell-quoted string and add it to a
251 pipeline in one go. See the comment against pipecmd_new_argstr()
252 above if you're tempted to use this function.
253 void pipeline_commandv(pipeline *p, va_list cmdv)
255 void pipeline_commands(pipeline *p, ...)
256 Convenience functions wrapping pipeline_command() to add multiple
258 commands at once. Terminate arguments with NULL.
259 void pipeline_want_in(pipeline *p, int fd)
261 void pipeline_want_out(pipeline *p, int fd)
262 Set file descriptors to use as the input and output of the whole
264 pipeline. If non-negative, fd is used directly as a file descrip‐
265 tor. If negative, pipeline_start() will create pipes and store the
266 input writing half and the output reading half in the pipeline's
267 infd or outfd field as appropriate. The default is to leave input
268 and output as stdin and stdout unless pipeline_want_infile() or
269 pipeline_want_outfile() respectively has been called.
270 Calling these functions supersedes any previous call to
272 pipeline_want_infile() or pipeline_want_outfile() respectively.
273 void pipeline_want_infile(pipeline *p, const char *file)
275 void pipeline_want_outfile(pipeline *p, const char *file)
276 Set file names to open and use as the input and output of the whole
278 pipeline. This may be more convenient than supplying file descrip‐
279 tors, and guarantees that the files are opened with the same privi‐
280 leges under which the pipeline is run.
281 Calling these functions (even with NULL, which returns to the
283 default of leaving input and output as stdin and stdout) supersedes
284 any previous call to pipeline_want_in() or pipeline_want_outfile()
285 respectively.
286 The given files will be opened when the pipeline is started. If an
288 output file does not already exist, it is created (with mode 0666
289 modified in the usual way by umask); if it does exist, then it is
290 truncated.
291 void pipeline_ignore_signals(pipeline *p, int ignore_signals)
293 If ignore_signals is non-zero, ignore SIGINT and SIGQUIT in the
295 calling process while the pipeline is running, like system(3).
296 Otherwise, and by default, leave their dispositions unchanged.
297 int pipeline_get_ncommands(pipeline *p)
299 Return the number of commands in this pipeline.
301 pipecmd *pipeline_get_command(pipeline *p, int n)
303 Return command number n from this pipeline, counting from zero, or
305 NULL if n is out of range.
306 pipecmd *pipeline_set_command(pipeline *p, int n, pipecmd *cmd)
308 Set command number n in this pipeline, counting from zero, to cmd,
310 and return the previous command in that position. Do nothing and
311 return NULL if n is out of range.
312 pid_t pipeline_get_pid(pipeline *p, int n)
314 Return the process ID of command number n from this pipeline,
316 counting from zero. The pipeline must be started. Return -1 if n
317 is out of range or if the command has already exited and been
318 reaped.
319 FILE *pipeline_get_infile(pipeline *p)
321 FILE *pipeline_get_outfile(pipeline *p)
322 Get streams corresponding to infd and outfd respectively. The
324 pipeline must be started.
325 void pipeline_dump(pipeline *p, FILE *stream)
327 Dump a string representation of p to stream.
329 char *pipeline_tostring(pipeline *p)
331 Return a string representation of p. The caller should free the
333 result.
334 void pipeline_free(pipeline *p)
336 Destroy a pipeline and all its commands. Safely does nothing if p
338 is NULL. May wait for the pipeline to complete if it has not
339 already done so.
340 Functions to run pipelines and handle signals
342 typedef void pipeline_post_fork_fn (void);
343 void pipeline_install_post_fork(pipeline_post_fork_fn *fn)
344 Install a post-fork handler. This will be run in any child process
346 immediately after it is forked. For instance, this may be used for
347 cleaning up application-specific signal handlers. Pass NULL to
348 clear any existing post-fork handler.
349 See pipecmd_pre_exec for a similar facility limited to a single
351 command rather than global to the calling process.
352 void pipeline_start(pipeline *p)
354 Start the processes in a pipeline. Installs this library's SIGCHLD
356 handler if not already installed. Calls error (FATAL) on error.
357 int pipeline_wait_all(pipeline *p, int **statuses, int *n_statuses)
359 Wait for a pipeline to complete. Set *statuses to a newly-allo‐
361 cated array of wait statuses, as returned by waitpid(2), and
362 *n_statuses to the length of that array. The return value is simi‐
363 lar to the exit status that a shell would return, with some modifi‐
364 cations. If the last command exits with a signal (other than
365 SIGPIPE, which is considered equivalent to exiting zero), then the
366 return value is 128 plus the signal number; if the last command
367 exits normally but non-zero, then the return value is its exit sta‐
368 tus; if any other command exits non-zero, then the return value is
369 127; otherwise, the return value is 0. This means that the return
370 value is only 0 if all commands in the pipeline exit successfully.
371 int pipeline_wait(pipeline *p)
373 Wait for a pipeline to complete and return its combined exit sta‐
375 tus, calculated as for pipeline_wait_all().
376 int pipeline_run(pipeline *p)
378 Start a pipeline, wait for it to complete, and free it, all in one
380 go.
381 void pipeline_pump(pipeline *p, ...)
383 Pump data among one or more pipelines connected using
385 pipeline_connect() until all source pipelines have reached end-of-
386 file and all data has been written to all sinks (or failed). All
387 relevant pipelines must be supplied: that is, no pipeline that has
388 been connected to a source pipeline may be supplied unless that
389 source pipeline is also supplied. Automatically starts all pipe‐
390 lines if they are not already started, but does not wait for them.
391 Terminate arguments with NULL.
392 Functions to read output from pipelines
394 In general, output is returned as a pointer into a buffer owned by the
395 pipeline, which is automatically freed when pipeline_free() is called.
396 This saves the caller from having to explicitly free individual blocks of
397 output data.
398 const char *pipeline_read(pipeline *p, size_t *len)
400 Read len bytes of data from the pipeline, returning the data block.
402 len is updated with the number of bytes read.
403 const char *pipeline_peek(pipeline *p, size_t *len)
405 Look ahead in the pipeline's output for len bytes of data, return‐
407 ing the data block. len is updated with the number of bytes read.
408 The starting position of the next read or peek is not affected by
409 this call.
410 size_t pipeline_peek_size(pipeline *p)
412 Return the number of bytes of data that can be read using
414 pipeline_read() or pipeline_peek() solely from the peek cache,
415 without having to read from the pipeline itself (and thus poten‐
416 tially block).
417 void pipeline_peek_skip(pipeline *p, size_t len)
419 Skip over and discard len bytes of data from the peek cache.
421 Asserts that enough data is available to skip, so you may want to
422 check using pipeline_peek_size() first.
423 const char *pipeline_readline(pipeline *p)
425 Read a line of data from the pipeline, returning it.
427 const char *pipeline_peekline(pipeline *p)
429 Look ahead in the pipeline's output for a line of data, returning
431 it. The starting position of the next read or peek is not affected
432 by this call.
433 Signal handling
435 libpipeline installs a signal handler for SIGCHLD, and collects the exit
436 status of child processes in pipeline_wait(). Applications using this
437 library must either refrain from changing the disposition of SIGCHLD (in
438 other words, must rely on libpipeline for all child process handling) or
439 else must make sure to restore libpipeline's SIGCHLD handler before call‐
440 ing any of its functions.
441 If the ignore_signals flag is set in a pipeline (which is the default),
443 then the SIGINT and SIGQUIT signals will be ignored in the parent process
444 while child processes are running. This mirrors the behaviour of
445 system(3).
446 libpipeline leaves child processes with the default disposition of
448 SIGPIPE, namely to terminate the process. It ignores SIGPIPE in the par‐
449 ent process while running pipeline_pump().
450 Reaping of child processes
452 libpipeline installs a SIGCHLD handler that will attempt to reap child
453 processes which have exited. This calls waitpid(2) with -1, so it will
454 reap any child process, not merely those created by way of this library.
455 At present, this means that if the calling program forks other child pro‐
456 cesses which may exit while a pipeline is running, the program is not
457 guaranteed to be able to collect exit statuses of those processes.
458 You should not rely on this behaviour, and in future it may be modified
460 either to reap only child processes created by this library or to provide
461 a way to return foreign statuses to the application. Please contact the
462 author if you have an example application and would like to help design
463 such an interface.
464
466 If the PIPELINE_DEBUG environment variable is set to “1”, then
467 libpipeline will emit debugging messages on standard error.
468 If the PIPELINE_QUIET environment variable is set to any value, then
470 libpipeline will refrain from printing an error message when a subprocess
471 is terminated by a signal.
472
474 In the following examples, function names starting with pipecmd_ or
475 pipeline_ are real libpipeline functions, while any other function names
476 are pseudocode.
477 The simplest case is simple. To run a single command, such as mv source
479 dest:
480 pipeline *p = pipeline_new_command_args ("mv", source, dest, NULL);
482 int status = pipeline_run (p);
483 libpipeline is often used to mimic shell pipelines, such as the following
485 example:
486 zsoelim < input-file | tbl | nroff -mandoc -Tutf8
488 The code to construct this would be:
490 pipeline *p;
492 int status;
493 p = pipeline_new ();
495 pipeline_want_infile (p, "input-file");
496 pipeline_command_args (p, "zsoelim", NULL);
497 pipeline_command_args (p, "tbl", NULL);
498 pipeline_command_args (p, "nroff", "-mandoc", "-Tutf8", NULL);
499 status = pipeline_run (p);
500 You might want to construct a command more dynamically:
502 pipecmd *manconv = pipecmd_new_args ("manconv", "-f", from_code,
504 "-t", "UTF-8", NULL);
505 if (quiet)
506 pipecmd_arg (manconv, "-q");
507 pipeline_command (p, manconv);
508 Perhaps you want an environment variable set only while running a certain
510 command:
511 pipecmd *less = pipecmd_new ("less");
513 pipecmd_setenv (less, "LESSCHARSET", lesscharset);
514 You might find yourself needing to pass the output of one pipeline to
516 several other pipelines, in a “tee” arrangement:
517 pipeline *source, *sink1, *sink2;
519 source = make_source ();
521 sink1 = make_sink1 ();
522 sink2 = make_sink2 ();
523 pipeline_connect (source, sink1, sink2, NULL);
524 /* Pump data among these pipelines until there's nothing left. */
525 pipeline_pump (source, sink1, sink2, NULL);
526 pipeline_free (sink2);
527 pipeline_free (sink1);
528 pipeline_free (source);
529 Maybe one of your commands is actually an in-process function, rather
531 than an external program:
532 pipecmd *inproc = pipecmd_new_function ("in-process", &func,
534 NULL, NULL);
535 pipeline_command (p, inproc);
536 Sometimes your program needs to consume the output of a pipeline, rather
538 than sending it all to some other subprocess:
539 pipeline *p = make_pipeline ();
541 const char *line;
542 pipeline_want_out (p, -1);
544 pipeline_start (p);
545 line = pipeline_peekline (p);
546 if (!strstr (line, "coding: UTF-8"))
547 printf ("Unicode text follows:0);
548 while (line = pipeline_readline (p))
549 printf (" %s", line);
550 pipeline_free (p);
551
553 fork(2), execve(2), system(3), popen(3).
554
556 Most of libpipeline was written by Colin Watson <cjwatson@debian.org>,
557 originally for use in man-db. The initial version was based very loosely
558 on the run_pipeline() function in GNU groff, written by James Clark
559 <jjc@jclark.com>. It also contains library code by Markus Armbruster,
560 and by various contributors to Gnulib.
561 libpipeline is licensed under the GNU General Public License, version 3
563 or later. See the README file for full details.
564
566 Using this library in a program which runs any other child processes
567 and/or installs its own SIGCHLD handler is unlikely to work.
568GNU October 11, 2010 GNU