pipeline_peek (3)

Leading comments

Copyright (C) 2010-2017 Colin Watson.

This file is part of libpipeline.

libpipeline is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at
your option) any later version.

libpipeline is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See...

(The comments found at the beginning of the groff file "man3/pipeline_peek.3".)

NAME

libpipeline - pipeline manipulation library

SYNOPSIS

In pipeline.h

DESCRIPTION

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 involves constructing a Vt pipeline structure and adding one or more Vt pipecmd structures to it. A Vt pipecmd represents a subprocess (or ``command )'' while a Vt 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 Vt const char * function arguments will be copied by the library.

Functions to build individual commands

Ft pipecmd * Fn pipecmd_new const char *name

Construct a new command representing execution of a program called name

Ft pipecmd * Fn pipecmd_new_argv const char *name va_list argv

Ft pipecmd * Fn pipecmd_new_args const char *name ...

Convenience constructors wrapping Fn pipecmd_new and Fn pipecmd_arg . Construct a new command representing execution of a program called name with arguments. Terminate arguments with NULL

Ft pipecmd * Fn 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.

Vt typedef void pipecmd_function_type (void *)

Vt typedef void pipecmd_function_free_type (void *)

Ft pipecmd * Fo pipecmd_new_function Fa const char *name Fa pipecmd_function_type *func Fa pipecmd_function_free_type *free_func Fa void *data Fc

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.

Ft pipecmd * Fn pipecmd_new_sequencev const char *name va_list cmdv

Ft pipecmd * Fn pipecmd_new_sequence const char *name ...

Construct a new command that itself runs a sequence of commands, supplied as Vt 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.

Ft pipecmd * Fn pipecmd_new_passthrough void

Return a new command that just passes data from its input to its output.

Ft pipecmd * Fn pipecmd_dup pipecmd *cmd

Return a duplicate of a command.

Ft void Fn pipecmd_arg pipecmd *cmd const char *arg

Add an argument to a command.

Ft void Fn pipecmd_argf pipecmd *cmd const char *format ...

Convenience function to add an argument with printf substitutions.

Ft void Fn pipecmd_argv pipecmd *cmd va_list argv

Ft void Fn pipecmd_args pipecmd *cmd ...

Convenience functions wrapping Fn pipecmd_arg to add multiple arguments at once. Terminate arguments with NULL

Ft void Fn 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.

Ft void Fn 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.

Ft void Fn 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.

Ft void Fn 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.

Ft void Fn pipecmd_chdir pipecmd *cmd const char *directory

Change the working directory to directory while running this command.

Ft void Fn 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.

Ft void Fn pipecmd_setenv pipecmd *cmd const char *name const char *value

Set environment variable name to value while running this command.

Ft void Fn pipecmd_unsetenv pipecmd *cmd const char *name

Unset environment variable name while running this command.

Ft void Fn 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 )

Ft void Fo pipecmd_pre_exec Fa pipecmd *cmd Fa pipecmd_function_type *func Fa pipecmd_function_free_type *free_func Fa void *data Fc

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).

Ft void Fn pipecmd_sequence_command pipecmd *cmd pipecmd *child

Add a command to a sequence created using Fn pipecmd_new_sequence .

Ft void Fn pipecmd_dump pipecmd *cmd FILE *stream

Dump a string representation of a command to stream.

Ft char * Fn pipecmd_tostring pipecmd *cmd

Return a string representation of a command. The caller should free the result.

Ft void Fn pipecmd_exec pipecmd *cmd

Execute a single command, replacing the current process. Never returns, instead exiting non-zero on failure.

Ft void Fn pipecmd_free pipecmd *cmd

Destroy a command. Safely does nothing if cmd is NULL

Functions to build pipelines

Ft pipeline * Fn pipeline_new void

Construct a new pipeline.

Ft pipeline * Fn pipeline_new_commandv pipecmd *cmd1 va_list cmdv

Ft pipeline * Fn pipeline_new_commands pipecmd *cmd1 ...

Convenience constructors wrapping Fn pipeline_new and Fn pipeline_command . Construct a new pipeline consisting of the given list of commands. Terminate commands with NULL

Ft pipeline * Fn pipeline_new_command_argv const char *name va_list argv

Ft pipeline * Fn pipeline_new_command_args const char *name ...

Construct a new pipeline and add a single command to it.

Ft pipeline * Fn pipeline_join pipeline *p1 pipeline *p2

Joins two pipelines, neither of which are allowed to be started. Discards Fa want_out , Fa want_outfile , and Fa outfd from p1 and Fa want_in , Fa want_infile , and Fa infd from p2

Ft void Fn 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 Fn pipeline_want_out must have been called with a negative Fa fd ; otherwise, calls Fn pipeline_want_out source -1 . In any event, calls Fn 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 Fn 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.

Ft void Fn pipeline_command pipeline *p pipecmd *cmd

Add a command to a pipeline.

Ft void Fn pipeline_command_argv pipeline *p const char *name va_list argv

Ft void Fn pipeline_command_args pipeline *p const char *name ...

Construct a new command and add it to a pipeline in one go.

Ft void Fn 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 Fn pipecmd_new_argstr above if you're tempted to use this function.

Ft void Fn pipeline_commandv pipeline *p va_list cmdv

Ft void Fn pipeline_commands pipeline *p ...

Convenience functions wrapping Fn pipeline_command to add multiple commands at once. Terminate arguments with NULL

Ft void Fn pipeline_want_in pipeline *p int fd

Ft void Fn 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, Fn pipeline_start will create pipes and store the input writing half and the output reading half in the pipeline's Fa infd or Fa outfd field as appropriate. The default is to leave input and output as stdin and stdout unless Fn pipeline_want_infile or Fn pipeline_want_outfile respectively has been called.

Calling these functions supersedes any previous call to Fn pipeline_want_infile or Fn pipeline_want_outfile respectively.

Ft void Fn pipeline_want_infile pipeline *p const char *file

Ft void Fn 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 Fn pipeline_want_in or Fn 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.

Ft void Fn 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.

Ft int Fn pipeline_get_ncommands pipeline *p

Return the number of commands in this pipeline.

Ft pipecmd * Fn 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.

Ft pipecmd * Fn 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.

Ft pid_t Fn 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.

Ft FILE * Fn pipeline_get_infile pipeline *p

Ft FILE * Fn pipeline_get_outfile pipeline *p

Get streams corresponding to Fa infd and Fa outfd respectively. The pipeline must be started.

Ft void Fn pipeline_dump pipeline *p FILE *stream

Dump a string representation of p to stream.

Ft char * Fn pipeline_tostring pipeline *p

Return a string representation of p The caller should free the result.

Ft void Fn 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

Vt typedef void pipeline_post_fork_fn (void)

Ft void Fn 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.

Ft void Fn 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.

Ft int Fn 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.

Ft int Fn pipeline_wait pipeline *p

Wait for a pipeline to complete and return its combined exit status, calculated as for Fn pipeline_wait_all .

Ft int Fn pipeline_run pipeline *p

Start a pipeline, wait for it to complete, and free it, all in one go.

Ft void Fn pipeline_pump pipeline *p ...

Pump data among one or more pipelines connected using Fn 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 Fn pipeline_free is called. This saves the caller from having to explicitly free individual blocks of output data.

Ft const char * Fn 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.

Ft const char * Fn 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.

Ft size_t Fn pipeline_peek_size pipeline *p

Return the number of bytes of data that can be read using Fn pipeline_read or Fn pipeline_peek solely from the peek cache, without having to read from the pipeline itself (and thus potentially block).

Ft void Fn 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 Fn pipeline_peek_size first.

Ft const char * Fn pipeline_readline pipeline *p

Read a line of data from the pipeline, returning it.

Ft const char * Fn 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

installs a signal handler for SIGCHLD and collects the exit status of child processes in Fn pipeline_wait . Applications using this library must either refrain from changing the disposition of SIGCHLD (in other words, must rely on for all child process handling) or else must make sure to restore 's SIGCHLD handler before calling any of its functions.

If the Fa 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).

leaves child processes with the default disposition of SIGPIPE namely to terminate the process. It ignores SIGPIPE in the parent process while running Fn pipeline_pump .

Reaping of child processes

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 will emit debugging messages on standard error.

If the PIPELINE_QUIET environment variable is set to any value, then 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 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);

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:);
while (line = pipeline_readline (p))
        printf ("  %s", line);
pipeline_free (p);

SEE ALSO

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

AUTHORS

An -nosplit Most of was written by An Colin Watson Aq cjwatson@debian.org , originally for use in man-db. The initial version was based very loosely on the Fn run_pipeline function in GNU groff, written by An James Clark Aq jjc@jclark.com . It also contains library code by An Markus Armbruster , and by various contributors to Gnulib.

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.