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/* Read symbolic links into a buffer without size limitation, relative to fd.
Copyright (C) 2001, 2003-2004, 2007, 2009-2020 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 Paul Eggert, Bruno Haible, and Jim Meyering. */
#include <config.h>
#include "careadlinkat.h"
#include <errno.h>
#include <limits.h>
#include <string.h>
#include <unistd.h>
/* Define this independently so that stdint.h is not a prerequisite. */
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif
#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif
#include "allocator.h"
/* Assuming the current directory is FD, get the symbolic link value
of FILENAME as a null-terminated string and put it into a buffer.
If FD is AT_FDCWD, FILENAME is interpreted relative to the current
working directory, as in openat.
If the link is small enough to fit into BUFFER put it there.
BUFFER's size is BUFFER_SIZE, and BUFFER can be null
if BUFFER_SIZE is zero.
If the link is not small, put it into a dynamically allocated
buffer managed by ALLOC. It is the caller's responsibility to free
the returned value if it is nonnull and is not BUFFER. A null
ALLOC stands for the standard allocator.
The PREADLINKAT function specifies how to read links. It operates
like POSIX readlinkat()
<https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
but can assume that its first argument is the same as FD.
If successful, return the buffer address; otherwise return NULL and
set errno. */
char *
careadlinkat (int fd, char const *filename,
char *buffer, size_t buffer_size,
struct allocator const *alloc,
ssize_t (*preadlinkat) (int, char const *, char *, size_t))
{
char *buf;
size_t buf_size;
size_t buf_size_max =
SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
char stack_buf[1024];
if (! alloc)
alloc = &stdlib_allocator;
if (! buffer_size)
{
/* Allocate the initial buffer on the stack. This way, in the
common case of a symlink of small size, we get away with a
single small malloc() instead of a big malloc() followed by a
shrinking realloc(). */
buffer = stack_buf;
buffer_size = sizeof stack_buf;
}
buf = buffer;
buf_size = buffer_size;
do
{
/* Attempt to read the link into the current buffer. */
ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
size_t link_size;
if (link_length < 0)
{
/* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
with errno == ERANGE if the buffer is too small. */
int readlinkat_errno = errno;
if (readlinkat_errno != ERANGE)
{
if (buf != buffer)
{
alloc->free (buf);
errno = readlinkat_errno;
}
return NULL;
}
}
link_size = link_length;
if (link_size < buf_size)
{
buf[link_size++] = '\0';
if (buf == stack_buf)
{
char *b = (char *) alloc->allocate (link_size);
buf_size = link_size;
if (! b)
break;
memcpy (b, buf, link_size);
buf = b;
}
else if (link_size < buf_size && buf != buffer && alloc->reallocate)
{
/* Shrink BUF before returning it. */
char *b = (char *) alloc->reallocate (buf, link_size);
if (b)
buf = b;
}
return buf;
}
if (buf != buffer)
alloc->free (buf);
if (buf_size <= buf_size_max / 2)
buf_size *= 2;
else if (buf_size < buf_size_max)
buf_size = buf_size_max;
else if (buf_size_max < SIZE_MAX)
{
errno = ENAMETOOLONG;
return NULL;
}
else
break;
buf = (char *) alloc->allocate (buf_size);
}
while (buf);
if (alloc->die)
alloc->die (buf_size);
errno = ENOMEM;
return NULL;
}
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