lib/spawn-pipe.h

   1 /* Creation of subprocesses, communicating via pipes.
   2    Copyright (C) 2001-2003, 2006, 2008-2024 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
  27 #ifdef __cplusplus
  28 extern "C" {
  29 #endif
  30
  31
  32 /* All these functions create a subprocess and don't wait for its termination.
  33    They return the process id of the subprocess.  They also return in fd[]
  34    one or two file descriptors for communication with the subprocess.
  35    If the subprocess creation fails: if exit_on_error is true, the main
  36    process exits with an error message; otherwise, an error message is given
  37    if null_stderr is false, then -1 is returned, with errno set, and fd[]
  38    remain uninitialized.
  39
  40    After finishing communication, the caller should call wait_subprocess()
  41    to get rid of the subprocess in the process table.
  42
  43    progname is the name of the program to be executed by the subprocess, used
  44    for error messages.
  45    prog_path is the file name of the program to be executed by the subprocess.
  46    If it contains no slashes, a search is conducted in $PATH.  An operating
  47    system dependent suffix is added, if necessary.
  48    prog_argv is the array of strings that the subprocess shall receive in
  49    argv[].  It is a NULL-terminated array.  prog_argv[0] should normally be
  50    identical to prog_path.
  51
  52    If directory is not NULL, the subprocess is started in that directory.  If
  53    prog_path is a relative file name, it resolved before changing to that
  54    directory.  The current directory of the current process remains unchanged.
  55
  56    If slave_process is true, the child process will be terminated when its
  57    creator receives a catchable fatal signal or exits normally.  If
  58    slave_process is false, the child process will continue running in this
  59    case, until it is lucky enough to attempt to communicate with its creator
  60    and thus get a SIGPIPE signal.
  61
  62    If exit_on_error is false, a child process id of -1 should be treated the
  63    same way as a subprocess which accepts no input, produces no output and
  64    terminates with exit code 127.  Why?  Some errors during posix_spawnp()
  65    cause the function posix_spawnp() to return an error code; some other
  66    errors cause the subprocess to exit with return code 127.  It is
  67    implementation dependent which error is reported which way.  The caller
  68    must treat both cases as equivalent.
  69
  70    It is recommended that no signal is blocked or ignored (i.e. have a
  71    signal handler with value SIG_IGN) while any of these functions is called.
  72    The reason is that child processes inherit the mask of blocked signals
  73    from their parent (both through posix_spawn() and fork()/exec());
  74    likewise, signals ignored in the parent are also ignored in the child
  75    (except possibly for SIGCHLD).  And POSIX:2001 says [in the description
  76    of exec()]:
  77        "it should be noted that many existing applications wrongly
  78         assume that they start with certain signals set to the default
  79         action and/or unblocked. In particular, applications written
  80         with a simpler signal model that does not include blocking of
  81         signals, such as the one in the ISO C standard, may not behave
  82         properly if invoked with some signals blocked. Therefore, it is
  83         best not to block or ignore signals across execs without explicit
  84         reason to do so, and especially not to block signals across execs
  85         of arbitrary (not closely co-operating) programs."  */
  86
  87 /* Open a pipe for output to a child process.
  88  * The child's stdout goes to a file.
  89  *
  90  *           write       system                read
  91  *    parent  ->   fd[0]   ->   STDIN_FILENO    ->   child
  92  *
  93  * Note: When writing to a child process, it is useful to ignore the SIGPIPE
  94  * signal and the EPIPE error code.
  95  */
  96 extern pid_t create_pipe_out (const char *progname,
  97                               const char *prog_path,
  98                               const char * const *prog_argv,
  99                               const char *directory,
 100                               const char *prog_stdout, bool null_stderr,
 101                               bool slave_process, bool exit_on_error,
 102                               int fd[1]);
 103
 104 /* Open a pipe for input from a child process.
 105  * The child's stdin comes from a file.
 106  *
 107  *           read        system                write
 108  *    parent  <-   fd[0]   <-   STDOUT_FILENO   <-   child
 109  *
 110  */
 111 extern pid_t create_pipe_in (const char *progname,
 112                              const char *prog_path,
 113                              const char * const *prog_argv,
 114                              const char *directory,
 115                              const char *prog_stdin, bool null_stderr,
 116                              bool slave_process, bool exit_on_error,
 117                              int fd[1]);
 118
 119 /* Open a bidirectional pipe.
 120  *
 121  *           write       system                read
 122  *    parent  ->   fd[1]   ->   STDIN_FILENO    ->   child
 123  *    parent  <-   fd[0]   <-   STDOUT_FILENO   <-   child
 124  *           read        system                write
 125  *
 126  * Note: When writing to a child process, it is useful to ignore the SIGPIPE
 127  * signal and the EPIPE error code.
 128  *
 129  * Note: The parent process must be careful to avoid deadlock.
 130  * 1) If you write more than PIPE_MAX bytes or, more generally, if you write
 131  *    more bytes than the subprocess can handle at once, the subprocess
 132  *    may write its data and wait on you to read it, but you are currently
 133  *    busy writing.
 134  * 2) When you don't know ahead of time how many bytes the subprocess
 135  *    will produce, the usual technique of calling read (fd, buf, BUFSIZ)
 136  *    with a fixed BUFSIZ will, on Linux 2.2.17 and on BSD systems, cause
 137  *    the read() call to block until *all* of the buffer has been filled.
 138  *    But the subprocess cannot produce more data until you gave it more
 139  *    input.  But you are currently busy reading from it.
 140  */
 141 extern pid_t create_pipe_bidi (const char *progname,
 142                                const char *prog_path,
 143                                const char * const *prog_argv,
 144                                const char *directory,
 145                                bool null_stderr,
 146                                bool slave_process, bool exit_on_error,
 147                                int fd[2]);
 148
 149 /* The name of the "always silent" device.  */
 150 #if defined _WIN32 && ! defined __CYGWIN__
 151 /* Native Windows API.  */
 152 # define DEV_NULL "NUL"
 153 #else
 154 /* Unix API.  */
 155 # define DEV_NULL "/dev/null"
 156 #endif
 157
 158
 159 #ifdef __cplusplus
 160 }
 161 #endif
 162
 163
 164 #endif /* _SPAWN_PIPE_H */