1 /* Read symbolic links into a buffer without size limitation, relative to fd.
3 Copyright (C) 2001, 2003-2004, 2007, 2009-2011 Free Software Foundation,
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>. */
19 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
23 #include "careadlinkat.h"
25 #include "allocator.h"
33 /* Define this independently so that stdint.h is not a prerequisite. */
35 # define SIZE_MAX ((size_t) -1)
39 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
43 /* Ignore FD. Get the symbolic link value of FILENAME and put it into
44 BUFFER, with size BUFFER_SIZE. This function acts like readlink
45 but has readlinkat's signature. */
47 careadlinkatcwd (int fd, char const *filename, char *buffer,
51 return readlink (filename, buffer, buffer_size);
55 /* Forward declaration. We want to #undef malloc before initializing
56 this struct, but cannot do so until after all code that uses named
57 fields from "allocator.h" has been compiled. */
58 static struct allocator const standard_allocator;
60 /* Assuming the current directory is FD, get the symbolic link value
61 of FILENAME as a null-terminated string and put it into a buffer.
62 If FD is AT_FDCWD, FILENAME is interpreted relative to the current
63 working directory, as in openat.
65 If the link is small enough to fit into BUFFER put it there.
66 BUFFER's size is BUFFER_SIZE, and BUFFER can be null
67 if BUFFER_SIZE is zero.
69 If the link is not small, put it into a dynamically allocated
70 buffer managed by ALLOC. It is the caller's responsibility to free
71 the returned value if it is nonnull and is not BUFFER. A null
72 ALLOC stands for the standard allocator.
74 The PREADLINKAT function specifies how to read links.
76 If successful, return the buffer address; otherwise return NULL and
80 careadlinkat (int fd, char const *filename,
81 char *buffer, size_t buffer_size,
82 struct allocator const *alloc,
83 ssize_t (*preadlinkat) (int, char const *, char *, size_t))
88 SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
92 alloc = &standard_allocator;
96 /* Allocate the initial buffer on the stack. This way, in the
97 common case of a symlink of small size, we get away with a
98 single small malloc() instead of a big malloc() followed by a
99 shrinking realloc(). */
101 buffer_size = sizeof stack_buf;
105 buf_size = buffer_size;
109 /* Attempt to read the link into the current buffer. */
110 ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
114 /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
115 with errno == ERANGE if the buffer is too small. */
116 int readlinkat_errno = errno;
117 if (readlinkat_errno != ERANGE)
122 errno = readlinkat_errno;
128 link_size = link_length;
130 if (link_size < buf_size)
132 buf[link_size++] = '\0';
134 if (buf == stack_buf)
136 char *b = (char *) alloc->malloc (link_size);
139 memcpy (b, buf, link_size);
142 else if (link_size < buf_size && buf != buffer && alloc->realloc)
144 /* Shrink BUF before returning it. */
145 char *b = (char *) alloc->realloc (buf, link_size);
156 if (buf_size <= buf_size_max / 2)
158 else if (buf_size < buf_size_max)
159 buf_size = buf_size_max;
162 buf = (char *) alloc->malloc (buf_size);
172 /* Use the system functions, not the gnulib overrides, because this
173 module does not depend on GNU or POSIX semantics. See comments
174 above why this must occur here. */
178 /* A standard allocator. For now, only careadlinkat needs this, but
179 perhaps it should be moved to the allocator module. */
180 static struct allocator const standard_allocator =
181 { malloc, realloc, free, NULL };