Make it possible to compile strerror.c separately, unconditionally.

[gnulib.git] / lib / xreadlink.c

diff --git a/lib/xreadlink.c b/lib/xreadlink.c
index b8bb3c4..2aba265 100644 (file)
--- a/lib/xreadlink.c
+++ b/lib/xreadlink.c
@@ -1,6 +1,6 @@
 /* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
 
-   Copyright (C) 2001, 2003, 2004, 2005 Free Software Foundation, Inc.
+   Copyright (C) 2001, 2003-2007 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
@@ -17,15 +17,16 @@
    If not, write to the Free Software Foundation,
    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
 
-/* Written by Jim Meyering <jim@meyering.net>  */
+/* Written by Jim Meyering <jim@meyering.net>
+   and Bruno Haible <bruno@clisp.org>.  */
 
-#ifdef HAVE_CONFIG_H
-# include <config.h>
-#endif
+#include <config.h>
 
+/* Specification.  */
 #include "xreadlink.h"
 
 #include <stdio.h>
+#include <string.h>
 #include <errno.h>
 #include <limits.h>
 #include <sys/types.h>
@@ -39,53 +40,92 @@
 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
 #endif
 
-#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
-
-#include "xalloc.h"
+#ifdef NO_XMALLOC
+# define xmalloc malloc
+#else
+# include "xalloc.h"
+#endif
 
-/* Call readlink to get the symbolic link value of FILE.
-   SIZE is a hint as to how long the link is expected to be;
-   typically it is taken from st_size.  It need not be correct.
+/* Call readlink to get the symbolic link value of FILENAME.
    Return a pointer to that NUL-terminated string in malloc'd storage.
    If readlink fails, return NULL (caller may use errno to diagnose).
-   If malloc fails, or if the link value is longer than SSIZE_MAX :-),
+   If realloc fails, or if the link value is longer than SIZE_MAX :-),
    give a diagnostic and exit.  */
 
 char *
-xreadlink (char const *file, size_t size)
+xreadlink (char const *filename)
 {
   /* The initial buffer size for the link value.  A power of 2
      detects arithmetic overflow earlier, but is not required.  */
-  size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE;
+#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)
     {
-      char *buffer = xmalloc (buf_size);
-      ssize_t r = readlink (file, buffer, buf_size);
-      size_t link_length = r;
+      /* Attempt to read the link into the current buffer.  */
+      ssize_t link_length = readlink (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 (r < 0 && errno != ERANGE)
+      if (link_length < 0 && errno != ERANGE)
        {
-         int saved_errno = errno;
-         free (buffer);
-         errno = saved_errno;
+         if (buffer != initial_buf)
+           {
+             int saved_errno = errno;
+             free (buffer);
+             errno = saved_errno;
+           }
          return NULL;
        }
 
-      if (link_length < buf_size)
+      if ((size_t) link_length < buf_size)
        {
-         buffer[link_length] = 0;
+         buffer[link_length++] = '\0';
+
+         /* Return it in a chunk of memory as small as possible.  */
+         if (buffer == initial_buf)
+           {
+             buffer = (char *) xmalloc (link_length);
+#ifdef NO_XMALLOC
+             if (buffer == NULL)
+               return NULL;
+#endif
+             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;
        }
 
-      free (buffer);
-      if (buf_size <= MAXSIZE / 2)
-       buf_size *= 2;
-      else if (buf_size < MAXSIZE)
-       buf_size = MAXSIZE;
-      else
+      if (buffer != initial_buf)
+       free (buffer);
+      buf_size *= 2;
+      if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
+#ifdef NO_XMALLOC
+       return NULL;
+#else
        xalloc_die ();
+#endif
+      buffer = (char *) xmalloc (buf_size);
+#ifdef NO_XMALLOC
+      if (buffer == NULL)
+       return NULL;
+#endif
     }
 }