src/libstdbuf.c

   1 /* libstdbuf -- a shared lib to preload to setup stdio buffering for a command
   2    Copyright (C) 2009-2016 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 "system.h"
  22
  23 /* Note currently for glibc (2.3.5) the following call does not change
  24    the buffer size, and more problematically does not give any indication
  25    that the new size request was ignored:
  26
  27        setvbuf (stdout, (char*)NULL, _IOFBF, 8192);
  28
  29    The ISO C99 standard section 7.19.5.6 on the setvbuf function says:
  30
  31    ... If buf is not a null pointer, the array it points to _may_ be used
  32    instead of a buffer allocated by the setvbuf function and the argument
  33    size specifies the size of the array; otherwise, size _may_ determine
  34    the size of a buffer allocated by the setvbuf function. ...
  35
  36    Obviously some interpret the above to mean setvbuf(....,size)
  37    is only a hint from the application which I don't agree with.
  38
  39    FreeBSD's libc seems more sensible in this regard. From the man page:
  40
  41    The size argument may be given as zero to obtain deferred optimal-size
  42    buffer allocation as usual.  If it is not zero, then except for
  43    unbuffered files, the buf argument should point to a buffer at least size
  44    bytes long; this buffer will be used instead of the current buffer.  (If
  45    the size argument is not zero but buf is NULL, a buffer of the given size
  46    will be allocated immediately, and released on close.  This is an extension
  47    to ANSI C; portable code should use a size of 0 with any NULL buffer.)
  48    --------------------
  49    Another issue is that on glibc-2.7 the following doesn't buffer
  50    the first write if it's greater than 1 byte.
  51
  52        setvbuf(stdout,buf,_IOFBF,127);
  53
  54    Now the POSIX standard says that "allocating a buffer of size bytes does
  55    not necessarily imply that all of size bytes are used for the buffer area".
  56    However I think it's just a buggy implementation due to the various
  57    inconsistencies with write sizes and subsequent writes.  */
  58
  59 static const char *
  60 fileno_to_name (const int fd)
  61 {
  62   const char *ret = NULL;
  63
  64   switch (fd)
  65     {
  66     case 0:
  67       ret = "stdin";
  68       break;
  69     case 1:
  70       ret = "stdout";
  71       break;
  72     case 2:
  73       ret = "stderr";
  74       break;
  75     default:
  76       ret = "unknown";
  77       break;
  78     }
  79
  80   return ret;
  81 }
  82
  83 static void
  84 apply_mode (FILE *stream, const char *mode)
  85 {
  86   char *buf = NULL;
  87   int setvbuf_mode;
  88   size_t size = 0;
  89
  90   if (*mode == '0')
  91     setvbuf_mode = _IONBF;
  92   else if (*mode == 'L')
  93     setvbuf_mode = _IOLBF;      /* FIXME: should we allow 1ML  */
  94   else
  95     {
  96       setvbuf_mode = _IOFBF;
  97       verify (SIZE_MAX <= ULONG_MAX);
  98       size = strtoul (mode, NULL, 10);
  99       if (size > 0)
 100         {
 101           if (!(buf = malloc (size)))   /* will be freed by fclose()  */
 102             {
 103               /* We could defer the allocation to libc, however since
 104                  glibc currently ignores the combination of NULL buffer
 105                  with non zero size, we'll fail here.  */
 106               fprintf (stderr,
 107                        _("failed to allocate a %" PRIuMAX
 108                          " byte stdio buffer\n"), (uintmax_t) size);
 109               return;
 110             }
 111         }
 112       else
 113         {
 114           fprintf (stderr, _("invalid buffering mode %s for %s\n"),
 115                    mode, fileno_to_name (fileno (stream)));
 116           return;
 117         }
 118     }
 119
 120   if (setvbuf (stream, buf, setvbuf_mode, size) != 0)
 121     {
 122       fprintf (stderr, _("could not set buffering of %s to mode %s\n"),
 123                fileno_to_name (fileno (stream)), mode);
 124       free (buf);
 125     }
 126 }
 127
 128 /* Use __attribute to avoid elision of __attribute__ on SUNPRO_C etc.  */
 129 static void __attribute ((constructor))
 130 stdbuf (void)
 131 {
 132   char *e_mode = getenv ("_STDBUF_E");
 133   char *i_mode = getenv ("_STDBUF_I");
 134   char *o_mode = getenv ("_STDBUF_O");
 135   if (e_mode) /* Do first so can write errors to stderr  */
 136     apply_mode (stderr, e_mode);
 137   if (i_mode)
 138     apply_mode (stdin, i_mode);
 139   if (o_mode)
 140     apply_mode (stdout, o_mode);
 141 }