/* libstdbuf -- a shared lib to preload to setup stdio buffering for a command Copyright (C) 2009-2017 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 . */ /* Written by Pádraig Brady. LD_PRELOAD idea from Brian Dessent. */ #include #include #include "system.h" /* 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, (char*)NULL, _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 const char * fileno_to_name (const int fd) { const char *ret = NULL; 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, const char *mode) { char *buf = NULL; int setvbuf_mode; size_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; verify (SIZE_MAX <= ULONG_MAX); size = strtoul (mode, NULL, 10); if (size > 0) { if (!(buf = malloc (size))) /* will be freed by fclose() */ { /* 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"), (uintmax_t) size); return; } } else { fprintf (stderr, _("invalid buffering mode %s for %s\n"), mode, fileno_to_name (fileno (stream))); return; } } 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); }