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Diffstat (limited to 'lib/areadlinkat.c')
-rw-r--r-- | lib/areadlinkat.c | 143 |
1 files changed, 143 insertions, 0 deletions
diff --git a/lib/areadlinkat.c b/lib/areadlinkat.c new file mode 100644 index 0000000000..5e8bf9d2ab --- /dev/null +++ b/lib/areadlinkat.c @@ -0,0 +1,143 @@ +/* areadlink.c -- readlink wrapper to return the link name in malloc'd storage + Unlike xreadlinkat, only call exit on failure to change directory. + + Copyright (C) 2001, 2003-2007, 2009 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 <http://www.gnu.org/licenses/>. */ + +/* Written by Jim Meyering <jim@meyering.net>, + and Bruno Haible <bruno@clisp.org>, + and Eric Blake <ebb9@byu.net>. */ + +#include <config.h> + +/* Specification. */ +#include "areadlink.h" + +#include <string.h> +#include <errno.h> +#include <limits.h> +#include <sys/types.h> +#include <stdlib.h> +#include <unistd.h> + +#ifndef SSIZE_MAX +# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2)) +#endif + +#if HAVE_READLINKAT + +/* Call readlinkat to get the symbolic link value of FILENAME relative to FD. + Return a pointer to that NUL-terminated string in malloc'd storage. + If readlinkat fails, return NULL and set errno (although failure to + change directory will issue a diagnostic and exit). + If realloc fails, or if the link value is longer than SIZE_MAX :-), + return NULL and set errno to ENOMEM. */ + +char * +areadlinkat (int fd, char const *filename) +{ + /* The initial buffer size for the link value. A power of 2 + detects arithmetic overflow earlier, but is not required. */ +# define INITIAL_BUF_SIZE 1024 + + /* 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(). */ + char initial_buf[INITIAL_BUF_SIZE]; + + char *buffer = initial_buf; + size_t buf_size = sizeof (initial_buf); + + while (1) + { + /* Attempt to read the link into the current buffer. */ + ssize_t link_length = readlinkat (fd, filename, buffer, buf_size); + + /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1 + with errno == ERANGE if the buffer is too small. */ + if (link_length < 0 && errno != ERANGE) + { + if (buffer != initial_buf) + { + int saved_errno = errno; + free (buffer); + errno = saved_errno; + } + return NULL; + } + + if ((size_t) link_length < buf_size) + { + buffer[link_length++] = '\0'; + + /* Return it in a chunk of memory as small as possible. */ + if (buffer == initial_buf) + { + buffer = (char *) malloc (link_length); + if (buffer == NULL) + /* errno is ENOMEM. */ + return NULL; + memcpy (buffer, initial_buf, link_length); + } + else + { + /* Shrink buffer before returning it. */ + if ((size_t) link_length < buf_size) + { + char *smaller_buffer = (char *) realloc (buffer, link_length); + + if (smaller_buffer != NULL) + buffer = smaller_buffer; + } + } + return buffer; + } + + if (buffer != initial_buf) + free (buffer); + buf_size *= 2; + if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0)) + { + errno = ENOMEM; + return NULL; + } + buffer = (char *) malloc (buf_size); + if (buffer == NULL) + /* errno is ENOMEM. */ + return NULL; + } +} + +#else /* !HAVE_READLINKAT */ + +/* It is more efficient to change directories only once and call + areadlink, rather than repeatedly call the replacement + readlinkat. */ + +# define AT_FUNC_NAME areadlinkat +# define AT_FUNC_F1 areadlink +# define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */ +# define AT_FUNC_POST_FILE_ARGS /* empty */ +# define AT_FUNC_RESULT char * +# define AT_FUNC_FAIL NULL +# include "at-func.c" +# undef AT_FUNC_NAME +# undef AT_FUNC_F1 +# undef AT_FUNC_POST_FILE_PARAM_DECLS +# undef AT_FUNC_POST_FILE_ARGS +# undef AT_FUNC_RESULT +# undef AT_FUNC_FAIL + +#endif /* !HAVE_READLINKAT */ |