lib/pipe-filter.h

   1 /* Filtering of data through a subprocess.  -*- coding: utf-8 -*-
   2    Copyright (C) 2009-2024 Free Software Foundation, Inc.
   3    Written by Bruno Haible <haible@clisp.cons.org>, 2009,
   4    and Paolo Bonzini <bonzini@gnu.org>, 2009.
   5
   6    This program is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation, either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _PIPE_FILTER_H
  20 #define _PIPE_FILTER_H
  21
  22 /* This file uses _GL_ATTRIBUTE_DEALLOC.  */
  23 #if !_GL_CONFIG_H_INCLUDED
  24  #error "Please include config.h first."
  25 #endif
  26
  27 #include <stddef.h>
  28
  29 #ifdef __cplusplus
  30 extern "C" {
  31 #endif
  32
  33
  34 /* Piping data through a subprocess in the naïve way - write data to the
  35    subprocess and read from the subprocess when you expect it to have
  36    produced results - is subject to two kinds of deadlocks:
  37    1) If you write more than PIPE_MAX bytes or, more generally, if you write
  38       more bytes than the subprocess can handle at once, the subprocess
  39       may write its data and wait on you to read it, but you are currently
  40       busy writing.
  41    2) When you don't know ahead of time how many bytes the subprocess
  42       will produce, the usual technique of calling read (fd, buf, BUFSIZ)
  43       with a fixed BUFSIZ will, on Linux 2.2.17 and on BSD systems, cause
  44       the read() call to block until *all* of the buffer has been filled.
  45       But the subprocess cannot produce more data until you gave it more
  46       input.  But you are currently busy reading from it.
  47
  48    This header file declares four set of functions that pipes data through
  49    the subprocess, without risking these deadlocks.
  50
  51    The side that writes data to the subprocess can be seen as a "generator",
  52    that is, as a subroutine that produces and writes a piece of data here and
  53    there, see <https://en.wikipedia.org/wiki/Generator_(computer_science)>.
  54    But often, it can be written in the form of an "iterator", that is, as a
  55    function that, each time it is invoked, produces and writes one more piece
  56    of data.
  57
  58    Similarly, the side that reads data from the subprocess can be seen as
  59    a "generator", that is, as a subroutine that consumes a piece of data here
  60    and there.  Often, it can be written in the form of an "iterator", that
  61    is, as a function that, each time it is invoked, consumes one more piece
  62    of data.
  63
  64    This header file declares four set of functions:
  65
  66                        |   writer   |   reader   |
  67        ----------------+------------+------------+
  68        pipe_filter_ii  |  iterator  |  iterator  |
  69        pipe_filter_ig  |  iterator  |  generator |
  70        pipe_filter_gi  |  generator |  iterator  |
  71        pipe_filter_gg  |  generator |  generator |
  72        ----------------+------------+------------+
  73
  74    The last one uses threads in order to implement two generators running at
  75    the same time.  (For the relation between generators, coroutines, and
  76    threads, see <https://en.wikipedia.org/wiki/Generator_(computer_science)>
  77    and <https://en.wikipedia.org/wiki/Coroutine>.)  It is therefore only
  78    portable to platforms with kernel-based POSIX threads.  */
  79
  80 /* These two functions together describe the side that writes data to the
  81    subprocess when it has the form of an iterator.
  82    - prepare_write (&num_bytes, p) must either return a pointer to data that
  83      is ready to be written and set num_bytes to the number of bytes ready to
  84      be written, or return NULL when no more bytes are to be written.
  85    - done_write (data_written, num_bytes_written) is called after
  86      num_bytes_written bytes were written.  It is guaranteed that
  87      num_bytes_written > 0.
  88    Here p is always the private_data argument passed to the main function.  */
  89 typedef const void * (*prepare_write_fn) (size_t *num_bytes_p,
  90                                           void *private_data);
  91 typedef void (*done_write_fn) (void *data_written, size_t num_bytes_written,
  92                                void *private_data);
  93
  94 /* These two functions together describe the side that reads data from the
  95    subprocess when it has the form of an iterator.
  96    - prepare_read (&num_bytes, p) must return a pointer to a buffer for data
  97      that can be read and set num_bytes to the size of that buffer
  98      (must be > 0).
  99    - done_read (data_read, num_bytes_read, p) is called after num_bytes_read
 100      bytes were read into the buffer.
 101    Here p is always the private_data argument passed to the main function.  */
 102 typedef void * (*prepare_read_fn) (size_t *num_bytes_p,
 103                                    void *private_data);
 104 typedef void (*done_read_fn) (void *data_read, size_t num_bytes_read,
 105                               void *private_data);
 106
 107
 108 /* ============================ pipe_filter_ii ============================ */
 109
 110 /* Create a subprocess and pipe some data through it.
 111    Arguments:
 112    - progname is the program name used in error messages.
 113    - prog_path is the file name of the program to invoke.
 114    - prog_argv is a NULL terminated argument list, starting with prog_path as
 115      first element.
 116    - If null_stderr is true, the subprocess' stderr will be redirected to
 117      /dev/null, and the usual error message to stderr will be omitted.
 118      This is suitable when the subprocess does not fulfill an important task.
 119    - If exit_on_error is true, any error will cause the main process to exit
 120      with an error status.
 121    If the subprocess does not terminate correctly, exit if exit_on_error is
 122    true, otherwise return 127.
 123    Callback arguments are as described above.
 124
 125    Data is alternately written to the subprocess, through the functions
 126    prepare_write and done_write, and read from the subprocess, through the
 127    functions prepare_read and done_read.
 128
 129    Note that the prepare_write/done_write functions and the
 130    prepare_read/done_read functions may be called in different threads than
 131    the current thread (depending on the platform).  But they will not be
 132    called after the pipe_filter_ii_execute function has returned.
 133
 134    Return 0 upon success, or (only if exit_on_error is false):
 135    - -1 with errno set upon failure,
 136    - the positive exit code of the subprocess if that failed.  */
 137 extern int
 138        pipe_filter_ii_execute (const char *progname,
 139                                const char *prog_path,
 140                                const char * const *prog_argv,
 141                                bool null_stderr, bool exit_on_error,
 142                                prepare_write_fn prepare_write,
 143                                done_write_fn done_write,
 144                                prepare_read_fn prepare_read,
 145                                done_read_fn done_read,
 146                                void *private_data);
 147
 148
 149 /* ============================ pipe_filter_ig ============================ */
 150
 151 struct pipe_filter_ig;
 152
 153
 154 /* ============================ pipe_filter_gi ============================ */
 155
 156 struct pipe_filter_gi;
 157
 158 /* Finish reading the output via the prepare_read/done_read functions
 159    specified to pipe_filter_gi_create.
 160
 161    Note that the prepare_read/done_read functions may be called in a
 162    different thread than the current thread (depending on the platform).
 163    However, they will always be called before pipe_filter_gi_close has
 164    returned.
 165
 166    The write side of the pipe is closed as soon as pipe_filter_gi_close
 167    starts, while the read side will be closed just before it finishes.
 168
 169    Return 0 upon success, or (only if exit_on_error is false):
 170    - -1 with errno set upon failure,
 171    - the positive exit code of the subprocess if that failed.  */
 172 extern int
 173        pipe_filter_gi_close (struct pipe_filter_gi *filter);
 174
 175 /* Create a subprocess and pipe some data through it.
 176    Arguments:
 177    - progname is the program name used in error messages.
 178    - prog_path is the file name of the program to invoke.
 179    - prog_argv is a NULL terminated argument list, starting with
 180      prog_path as first element.
 181    - If null_stderr is true, the subprocess' stderr will be redirected
 182      to /dev/null, and the usual error message to stderr will be
 183      omitted.  This is suitable when the subprocess does not fulfill an
 184      important task.
 185    - If exit_on_error is true, any error will cause the main process to
 186      exit with an error status.
 187    If the subprocess does not start correctly, exit if exit_on_error is
 188    true, otherwise return NULL and set errno.
 189
 190    The caller will write to the subprocess through pipe_filter_gi_write
 191    and finally call pipe_filter_gi_close.  During such calls, the
 192    prepare_read and done_read function may be called to process any data
 193    that the subprocess has written.
 194
 195    Note that the prepare_read/done_read functions may be called in a
 196    different thread than the current thread (depending on the platform).
 197    But they will not be called after the pipe_filter_gi_close function has
 198    returned.
 199
 200    Return the freshly created 'struct pipe_filter_gi'.  */
 201 extern struct pipe_filter_gi *
 202        pipe_filter_gi_create (const char *progname,
 203                               const char *prog_path,
 204                               const char * const *prog_argv,
 205                               bool null_stderr, bool exit_on_error,
 206                               prepare_read_fn prepare_read,
 207                               done_read_fn done_read,
 208                               void *private_data)
 209   _GL_ATTRIBUTE_DEALLOC (pipe_filter_gi_close, 1);
 210
 211 /* Write size bytes starting at buf into the pipe and in the meanwhile
 212    possibly call the prepare_read and done_read functions specified to
 213    pipe_filter_gi_create.
 214
 215    Note that the prepare_read/done_read functions may be called in a
 216    different thread than the current thread (depending on the platform).
 217    However, they will always be called before pipe_filter_gi_write has
 218    returned, or otherwise not sooner than the next call to
 219    pipe_filter_gi_write or pipe_filter_gi_close.
 220
 221    Return only after all the entire buffer has been written to the pipe or
 222    the subprocess has exited.
 223
 224    Return 0 upon success, or (only if exit_on_error is false):
 225    - -1 with errno set upon failure,
 226    - the positive exit code of the subprocess if that failed.  */
 227 extern int
 228        pipe_filter_gi_write (struct pipe_filter_gi *filter,
 229                              const void *buf, size_t size);
 230
 231
 232 /* ============================ pipe_filter_gg ============================ */
 233
 234
 235 /* ======================================================================== */
 236
 237
 238 #ifdef __cplusplus
 239 }
 240 #endif
 241
 242
 243 #endif /* _PIPE_FILTER_H */