src/libstdbuf.c

   1 /* libstdbuf -- a shared lib to preload to setup stdio buffering for a command
   2    Copyright (C) 2009-2013 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Pádraig Brady.  LD_PRELOAD idea from Brian Dessent.  */
  18
  19 #include <config.h>
  20 #include <stdio.h>
  21 #include <stdlib.h>
  22 #include "system.h"
  23 #include "verify.h"
  24
  25 /* Note currently for glibc (2.3.5) the following call does not change
  26    the buffer size, and more problematically does not give any indication
  27    that the new size request was ignored:
  28
  29        setvbuf (stdout, (char*)NULL, _IOFBF, 8192);
  30
  31    The ISO C99 standard section 7.19.5.6 on the setvbuf function says:
  32
  33    ... If buf is not a null pointer, the array it points to _may_ be used
  34    instead of a buffer allocated by the setvbuf function and the argument
  35    size specifies the size of the array; otherwise, size _may_ determine
  36    the size of a buffer allocated by the setvbuf function. ...
  37
  38    Obviously some interpret the above to mean setvbuf(....,size)
  39    is only a hint from the application which I don't agree with.
  40
  41    FreeBSD's libc seems more sensible in this regard. From the man page:
  42
  43    The size argument may be given as zero to obtain deferred optimal-size
  44    buffer allocation as usual.  If it is not zero, then except for
  45    unbuffered files, the buf argument should point to a buffer at least size
  46    bytes long; this buffer will be used instead of the current buffer.  (If
  47    the size argument is not zero but buf is NULL, a buffer of the given size
  48    will be allocated immediately, and released on close.  This is an extension
  49    to ANSI C; portable code should use a size of 0 with any NULL buffer.)
  50    --------------------
  51    Another issue is that on glibc-2.7 the following doesn't buffer
  52    the first write if it's greater than 1 byte.
  53
  54        setvbuf(stdout,buf,_IOFBF,127);
  55
  56    Now the POSIX standard says that "allocating a buffer of size bytes does
  57    not necessarily imply that all of size bytes are used for the buffer area".
  58    However I think it's just a buggy implementation due to the various
  59    inconsistencies with write sizes and subsequent writes.  */
  60
  61 static const char *
  62 fileno_to_name (const int fd)
  63 {
  64   const char *ret = NULL;
  65
  66   switch (fd)
  67     {
  68     case 0:
  69       ret = "stdin";
  70       break;
  71     case 1:
  72       ret = "stdout";
  73       break;
  74     case 2:
  75       ret = "stderr";
  76       break;
  77     default:
  78       ret = "unknown";
  79       break;
  80     }
  81
  82   return ret;
  83 }
  84
  85 static void
  86 apply_mode (FILE *stream, const char *mode)
  87 {
  88   char *buf = NULL;
  89   int setvbuf_mode;
  90   size_t size = 0;
  91
  92   if (*mode == '0')
  93     setvbuf_mode = _IONBF;
  94   else if (*mode == 'L')
  95     setvbuf_mode = _IOLBF;      /* FIXME: should we allow 1ML  */
  96   else
  97     {
  98       setvbuf_mode = _IOFBF;
  99       verify (SIZE_MAX <= ULONG_MAX);
 100       size = strtoul (mode, NULL, 10);
 101       if (size > 0)
 102         {
 103           if (!(buf = malloc (size)))   /* will be freed by fclose()  */
 104             {
 105               /* We could defer the allocation to libc, however since
 106                  glibc currently ignores the combination of NULL buffer
 107                  with non zero size, we'll fail here.  */
 108               fprintf (stderr,
 109                        _("failed to allocate a %" PRIuMAX
 110                          " byte stdio buffer\n"), (uintmax_t) size);
 111               return;
 112             }
 113         }
 114       else
 115         {
 116           fprintf (stderr, _("invalid buffering mode %s for %s\n"),
 117                    mode, fileno_to_name (fileno (stream)));
 118           return;
 119         }
 120     }
 121
 122   if (setvbuf (stream, buf, setvbuf_mode, size) != 0)
 123     {
 124       fprintf (stderr, _("could not set buffering of %s to mode %s\n"),
 125                fileno_to_name (fileno (stream)), mode);
 126       free (buf);
 127     }
 128 }
 129
 130 __attribute__ ((constructor)) static void
 131 stdbuf (void)
 132 {
 133   char *e_mode = getenv ("_STDBUF_E");
 134   char *i_mode = getenv ("_STDBUF_I");
 135   char *o_mode = getenv ("_STDBUF_O");
 136   if (e_mode) /* Do first so can write errors to stderr  */
 137     apply_mode (stderr, e_mode);
 138   if (i_mode)
 139     apply_mode (stdin, i_mode);
 140   if (o_mode)
 141     apply_mode (stdout, o_mode);
 142 }