lib/spawn-pipe.h

   1 /* Creation of subprocesses, communicating via pipes.
   2    Copyright (C) 2001-2003, 2006, 2008-2017 Free Software Foundation, Inc.
   3    Written by Bruno Haible <haible@clisp.cons.org>, 2001.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #ifndef _SPAWN_PIPE_H
  19 #define _SPAWN_PIPE_H
  20
  21 /* Get pid_t.  */
  22 #include <stdlib.h>
  23 #include <unistd.h>
  24 #include <sys/types.h>
  25
  26 #include <stdbool.h>
  27
  28
  29 #ifdef __cplusplus
  30 extern "C" {
  31 #endif
  32
  33
  34 /* All these functions create a subprocess and don't wait for its termination.
  35    They return the process id of the subprocess.  They also return in fd[]
  36    one or two file descriptors for communication with the subprocess.
  37    If the subprocess creation fails: if exit_on_error is true, the main
  38    process exits with an error message; otherwise, an error message is given
  39    if null_stderr is false, then -1 is returned, with errno set, and fd[]
  40    remain uninitialized.
  41
  42    After finishing communication, the caller should call wait_subprocess()
  43    to get rid of the subprocess in the process table.
  44
  45    If slave_process is true, the child process will be terminated when its
  46    creator receives a catchable fatal signal or exits normally.  If
  47    slave_process is false, the child process will continue running in this
  48    case, until it is lucky enough to attempt to communicate with its creator
  49    and thus get a SIGPIPE signal.
  50
  51    If exit_on_error is false, a child process id of -1 should be treated the
  52    same way as a subprocess which accepts no input, produces no output and
  53    terminates with exit code 127.  Why?  Some errors during posix_spawnp()
  54    cause the function posix_spawnp() to return an error code; some other
  55    errors cause the subprocess to exit with return code 127.  It is
  56    implementation dependent which error is reported which way.  The caller
  57    must treat both cases as equivalent.
  58
  59    It is recommended that no signal is blocked or ignored (i.e. have a
  60    signal handler with value SIG_IGN) while any of these functions is called.
  61    The reason is that child processes inherit the mask of blocked signals
  62    from their parent (both through posix_spawn() and fork()/exec());
  63    likewise, signals ignored in the parent are also ignored in the child
  64    (except possibly for SIGCHLD).  And POSIX:2001 says [in the description
  65    of exec()]:
  66        "it should be noted that many existing applications wrongly
  67         assume that they start with certain signals set to the default
  68         action and/or unblocked. In particular, applications written
  69         with a simpler signal model that does not include blocking of
  70         signals, such as the one in the ISO C standard, may not behave
  71         properly if invoked with some signals blocked. Therefore, it is
  72         best not to block or ignore signals across execs without explicit
  73         reason to do so, and especially not to block signals across execs
  74         of arbitrary (not closely co-operating) programs."  */
  75
  76 /* Open a pipe for output to a child process.
  77  * The child's stdout goes to a file.
  78  *
  79  *           write       system                read
  80  *    parent  ->   fd[0]   ->   STDIN_FILENO    ->   child
  81  *
  82  * Note: When writing to a child process, it is useful to ignore the SIGPIPE
  83  * signal and the EPIPE error code.
  84  */
  85 extern pid_t create_pipe_out (const char *progname,
  86                               const char *prog_path, char **prog_argv,
  87                               const char *prog_stdout, bool null_stderr,
  88                               bool slave_process, bool exit_on_error,
  89                               int fd[1]);
  90
  91 /* Open a pipe for input from a child process.
  92  * The child's stdin comes from a file.
  93  *
  94  *           read        system                write
  95  *    parent  <-   fd[0]   <-   STDOUT_FILENO   <-   child
  96  *
  97  */
  98 extern pid_t create_pipe_in (const char *progname,
  99                              const char *prog_path, char **prog_argv,
 100                              const char *prog_stdin, bool null_stderr,
 101                              bool slave_process, bool exit_on_error,
 102                              int fd[1]);
 103
 104 /* Open a bidirectional pipe.
 105  *
 106  *           write       system                read
 107  *    parent  ->   fd[1]   ->   STDIN_FILENO    ->   child
 108  *    parent  <-   fd[0]   <-   STDOUT_FILENO   <-   child
 109  *           read        system                write
 110  *
 111  * Note: When writing to a child process, it is useful to ignore the SIGPIPE
 112  * signal and the EPIPE error code.
 113  *
 114  * Note: The parent process must be careful to avoid deadlock.
 115  * 1) If you write more than PIPE_MAX bytes or, more generally, if you write
 116  *    more bytes than the subprocess can handle at once, the subprocess
 117  *    may write its data and wait on you to read it, but you are currently
 118  *    busy writing.
 119  * 2) When you don't know ahead of time how many bytes the subprocess
 120  *    will produce, the usual technique of calling read (fd, buf, BUFSIZ)
 121  *    with a fixed BUFSIZ will, on Linux 2.2.17 and on BSD systems, cause
 122  *    the read() call to block until *all* of the buffer has been filled.
 123  *    But the subprocess cannot produce more data until you gave it more
 124  *    input.  But you are currently busy reading from it.
 125  */
 126 extern pid_t create_pipe_bidi (const char *progname,
 127                                const char *prog_path, char **prog_argv,
 128                                bool null_stderr,
 129                                bool slave_process, bool exit_on_error,
 130                                int fd[2]);
 131
 132 /* The name of the "always silent" device.  */
 133 #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
 134 /* Native Windows API.  */
 135 # define DEV_NULL "NUL"
 136 #else
 137 /* Unix API.  */
 138 # define DEV_NULL "/dev/null"
 139 #endif
 140
 141
 142 #ifdef __cplusplus
 143 }
 144 #endif
 145
 146
 147 #endif /* _SPAWN_PIPE_H */