areadlink, areadlinkat: rewrite in terms of careadlinkat

[gnulib.git] / lib / careadlinkat.c

diff --git a/lib/careadlinkat.c b/lib/careadlinkat.c
new file mode 100644 (file)
index 0000000..15ffe24
--- /dev/null
+++ b/lib/careadlinkat.c
@@ -0,0 +1,175 @@
+/* Read symbolic links into a buffer without size limitation, relative to fd.
+
+   Copyright (C) 2001, 2003-2004, 2007, 2009-2011 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 Paul Eggert, Bruno Haible, and Jim Meyering.  */
+
+#include <config.h>
+
+#include "careadlinkat.h"
+
+#include "allocator.h"
+
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+/* Use the system functions, not the gnulib overrides, because this
+   module does not depend on GNU or POSIX semantics.  */
+#undef malloc
+#undef realloc
+
+/* Define this independently so that stdint.h is not a prerequisite.  */
+#ifndef SIZE_MAX
+# define SIZE_MAX ((size_t) -1)
+#endif
+
+#ifndef SSIZE_MAX
+# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
+#endif
+
+#if ! HAVE_READLINKAT
+/* Ignore FD.  Get the symbolic link value of FILENAME and put it into
+   BUFFER, with size BUFFER_SIZE.  This function acts like readlink
+   but has readlinkat's signature.  */
+ssize_t
+careadlinkatcwd (int fd, char const *filename, char *buffer,
+                 size_t buffer_size)
+{
+  (void) fd;
+  return readlink (filename, buffer, buffer_size);
+}
+#endif
+
+/* A standard allocator.  For now, only careadlinkat needs this, but
+   perhaps it should be moved to the allocator module.  */
+static struct allocator const standard_allocator =
+  { malloc, realloc, free, NULL };
+
+/* Assuming the current directory is FD, get the symbolic link value
+   of FILENAME as a null-terminated string and put it into a buffer.
+   If FD is AT_FDCWD, FILENAME is interpreted relative to the current
+   working directory, as in openat.
+
+   If the link is small enough to fit into BUFFER put it there.
+   BUFFER's size is BUFFER_SIZE, and BUFFER can be null
+   if BUFFER_SIZE is zero.
+
+   If the link is not small, put it into a dynamically allocated
+   buffer managed by ALLOC.  It is the caller's responsibility to free
+   the returned value if it is nonnull and is not BUFFER.  A null
+   ALLOC stands for the standard allocator.
+
+   The PREADLINKAT function specifies how to read links.
+
+   If successful, return the buffer address; otherwise return NULL and
+   set errno.  */
+
+char *
+careadlinkat (int fd, char const *filename,
+              char *buffer, size_t buffer_size,
+              struct allocator const *alloc,
+              ssize_t (*preadlinkat) (int, char const *, char *, size_t))
+{
+  char *buf;
+  size_t buf_size;
+  size_t buf_size_max =
+    SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
+  char stack_buf[1024];
+
+  if (! alloc)
+    alloc = &standard_allocator;
+
+  if (! buffer_size)
+    {
+      /* 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().  */
+      buffer = stack_buf;
+      buffer_size = sizeof stack_buf;
+    }
+
+  buf = buffer;
+  buf_size = buffer_size;
+
+  do
+    {
+      /* Attempt to read the link into the current buffer.  */
+      ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
+      size_t link_size;
+      if (link_length < 0)
+        {
+          /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
+             with errno == ERANGE if the buffer is too small.  */
+          int readlinkat_errno = errno;
+          if (readlinkat_errno != ERANGE)
+            {
+              if (buf != buffer)
+                {
+                  alloc->free (buf);
+                  errno = readlinkat_errno;
+                }
+              return NULL;
+            }
+        }
+
+      link_size = link_length;
+
+      if (link_size < buf_size)
+        {
+          buf[link_size++] = '\0';
+
+          if (buf == stack_buf)
+            {
+              char *b = (char *) alloc->malloc (link_size);
+              if (! b)
+                break;
+              memcpy (b, buf, link_size);
+              buf = b;
+            }
+          else if (link_size < buf_size && buf != buffer && alloc->realloc)
+            {
+              /* Shrink BUF before returning it.  */
+              char *b = (char *) alloc->realloc (buf, link_size);
+              if (b)
+                buf = b;
+            }
+
+          return buf;
+        }
+
+      if (buf != buffer)
+        alloc->free (buf);
+
+      if (buf_size <= buf_size_max / 2)
+        buf_size *= 2;
+      else if (buf_size < buf_size_max)
+        buf_size = buf_size_max;
+      else
+        break;
+      buf = (char *) alloc->malloc (buf_size);
+    }
+  while (buf);
+
+  if (alloc->die)
+    alloc->die ();
+  errno = ENOMEM;
+  return NULL;
+}