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/* libstdbuf -- a shared lib to preload to setup stdio buffering for a command
Copyright (C) 2009-2023 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Pádraig Brady. LD_PRELOAD idea from Brian Dessent. */
#include <config.h>
#include <stdio.h>
#include <stdint.h>
#include "system.h"
/* Deactivate config.h's "rpl_"-prefixed definitions, since we don't
link gnulib here, and the replacements aren't needed. */
#undef fprintf
#undef free
#undef malloc
#undef strtoumax
/* Note currently for glibc (2.3.5) the following call does not change
the buffer size, and more problematically does not give any indication
that the new size request was ignored:
setvbuf (stdout, nullptr, _IOFBF, 8192);
The ISO C99 standard section 7.19.5.6 on the setvbuf function says:
... If buf is not a null pointer, the array it points to _may_ be used
instead of a buffer allocated by the setvbuf function and the argument
size specifies the size of the array; otherwise, size _may_ determine
the size of a buffer allocated by the setvbuf function. ...
Obviously some interpret the above to mean setvbuf(....,size)
is only a hint from the application which I don't agree with.
FreeBSD's libc seems more sensible in this regard. From the man page:
The size argument may be given as zero to obtain deferred optimal-size
buffer allocation as usual. If it is not zero, then except for
unbuffered files, the buf argument should point to a buffer at least size
bytes long; this buffer will be used instead of the current buffer. (If
the size argument is not zero but buf is null, a buffer of the given size
will be allocated immediately, and released on close. This is an extension
to ANSI C; portable code should use a size of 0 with any null buffer.)
--------------------
Another issue is that on glibc-2.7 the following doesn't buffer
the first write if it's greater than 1 byte.
setvbuf(stdout,buf,_IOFBF,127);
Now the POSIX standard says that "allocating a buffer of size bytes does
not necessarily imply that all of size bytes are used for the buffer area".
However I think it's just a buggy implementation due to the various
inconsistencies with write sizes and subsequent writes. */
static char const *
fileno_to_name (const int fd)
{
char const *ret = nullptr;
switch (fd)
{
case 0:
ret = "stdin";
break;
case 1:
ret = "stdout";
break;
case 2:
ret = "stderr";
break;
default:
ret = "unknown";
break;
}
return ret;
}
static void
apply_mode (FILE *stream, char const *mode)
{
char *buf = nullptr;
int setvbuf_mode;
uintmax_t size = 0;
if (*mode == '0')
setvbuf_mode = _IONBF;
else if (*mode == 'L')
setvbuf_mode = _IOLBF; /* FIXME: should we allow 1ML */
else
{
setvbuf_mode = _IOFBF;
char *mode_end;
size = strtoumax (mode, &mode_end, 10);
if (size == 0 || *mode_end)
{
fprintf (stderr, _("invalid buffering mode %s for %s\n"),
mode, fileno_to_name (fileno (stream)));
return;
}
buf = size <= SIZE_MAX ? malloc (size) : nullptr;
if (!buf)
{
/* We could defer the allocation to libc, however since
glibc currently ignores the combination of null buffer
with non zero size, we'll fail here. */
fprintf (stderr,
_("failed to allocate a %" PRIuMAX
" byte stdio buffer\n"),
size);
return;
}
/* buf will be freed by fclose. */
}
if (setvbuf (stream, buf, setvbuf_mode, size) != 0)
{
fprintf (stderr, _("could not set buffering of %s to mode %s\n"),
fileno_to_name (fileno (stream)), mode);
free (buf);
}
}
/* Use __attribute to avoid elision of __attribute__ on SUNPRO_C etc. */
static void __attribute ((constructor))
stdbuf (void)
{
char *e_mode = getenv ("_STDBUF_E");
char *i_mode = getenv ("_STDBUF_I");
char *o_mode = getenv ("_STDBUF_O");
if (e_mode) /* Do first so can write errors to stderr */
apply_mode (stderr, e_mode);
if (i_mode)
apply_mode (stdin, i_mode);
if (o_mode)
apply_mode (stdout, o_mode);
}
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