(xreadlink): Add cast to avoid a warning.

[gnulib.git] / lib / xreadlink.c

diff --git a/lib/xreadlink.c b/lib/xreadlink.c
index 333551f..54b5e9e 100644 (file)
--- a/lib/xreadlink.c
+++ b/lib/xreadlink.c
@@ -1,4 +1,4 @@
-/* exclude.c -- exclude file names
+/* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
 
    Copyright 2001 Free Software Foundation, Inc.
 
@@ -24,6 +24,14 @@
 #endif
 
 #include <stdio.h>
+#include <errno.h>
+#ifndef errno
+extern int errno;
+#endif
+
+#if HAVE_LIMITS_H
+# include <limits.h>
+#endif
 #if HAVE_SYS_TYPES_H
 # include <sys/types.h>
 #endif
@@ -34,39 +42,51 @@
 # include <unistd.h>
 #endif
 
+#ifndef SIZE_MAX
+# define SIZE_MAX ((size_t) -1)
+#endif
+#ifndef SSIZE_MAX
+# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
+#endif
+
 #include "xalloc.h"
 #include "xreadlink.h"
 
 /* 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 (use errno to diagnose).
+   If readlink fails, return NULL (caller may use errno to diagnose).
    If realloc fails, or if the link value is longer than SIZE_MAX :-),
    give a diagnostic and exit.  */
 
 char *
-xreadlink (char const *filename, size_t *link_length_arg)
+xreadlink (char const *filename)
 {
-  size_t buf_size = 128;  /* must be a power of 2 */
-  char *buffer = NULL;
+  /* The initial buffer size for the link value.  A power of 2
+     detects arithmetic overflow earlier, but is not required.  */
+  size_t buf_size = 128;
 
   while (1)
     {
-      int link_length;
-      buffer = (char *) xrealloc (buffer, buf_size);
-      link_length = readlink (filename, buffer, buf_size);
+      char *buffer = xmalloc (buf_size);
+      ssize_t link_length = readlink (filename, buffer, buf_size);
+
       if (link_length < 0)
        {
+         int saved_errno = errno;
          free (buffer);
+         errno = saved_errno;
          return NULL;
        }
-      if (link_length < buf_size)
+
+      if ((size_t) link_length < buf_size)
        {
-         *link_length_arg = link_length;
          buffer[link_length] = 0;
          return buffer;
        }
+
+      free (buffer);
       buf_size *= 2;
-      if (buf_size == 0)
+      if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
        xalloc_die ();
     }
 }