xreadlink.c revision 1.1
1/* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage 2 3 Copyright (C) 2001, 2003, 2004, 2005 Free Software Foundation, Inc. 4 5 This program is free software; you can redistribute it and/or modify 6 it under the terms of the GNU General Public License as published by 7 the Free Software Foundation; either version 2, or (at your option) 8 any later version. 9 10 This program is distributed in the hope that it will be useful, 11 but WITHOUT ANY WARRANTY; without even the implied warranty of 12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13 GNU General Public License for more details. 14 15 You should have received a copy of the GNU General Public License 16 along with this program; see the file COPYING. 17 If not, write to the Free Software Foundation, 18 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 19 20/* Written by Jim Meyering <jim@meyering.net> */ 21 22#ifdef HAVE_CONFIG_H 23# include <config.h> 24#endif 25 26#include "xreadlink.h" 27 28#include <stdio.h> 29#include <errno.h> 30#include <limits.h> 31#include <sys/types.h> 32#include <stdlib.h> 33#if HAVE_UNISTD_H 34# include <unistd.h> 35#endif 36 37#ifndef SIZE_MAX 38# define SIZE_MAX ((size_t) -1) 39#endif 40#ifndef SSIZE_MAX 41# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2)) 42#endif 43 44#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX) 45 46#include "xalloc.h" 47 48/* Call readlink to get the symbolic link value of FILE. 49 SIZE is a hint as to how long the link is expected to be; 50 typically it is taken from st_size. It need not be correct. 51 Return a pointer to that NUL-terminated string in malloc'd storage. 52 If readlink fails, return NULL (caller may use errno to diagnose). 53 If malloc fails, or if the link value is longer than SSIZE_MAX :-), 54 give a diagnostic and exit. */ 55 56char * 57xreadlink (char const *file, size_t size) 58{ 59 /* The initial buffer size for the link value. A power of 2 60 detects arithmetic overflow earlier, but is not required. */ 61 size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE; 62 63 while (1) 64 { 65 char *buffer = xmalloc (buf_size); 66 ssize_t r = readlink (file, buffer, buf_size); 67 size_t link_length = r; 68 69 /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1 70 with errno == ERANGE if the buffer is too small. */ 71 if (r < 0 && errno != ERANGE) 72 { 73 int saved_errno = errno; 74 free (buffer); 75 errno = saved_errno; 76 return NULL; 77 } 78 79 if (link_length < buf_size) 80 { 81 buffer[link_length] = 0; 82 return buffer; 83 } 84 85 free (buffer); 86 if (buf_size <= MAXSIZE / 2) 87 buf_size *= 2; 88 else if (buf_size < MAXSIZE) 89 buf_size = MAXSIZE; 90 else 91 xalloc_die (); 92 } 93} 94