New module pwrite.

[gnulib.git] / lib / pwrite.c

diff --git a/lib/pwrite.c b/lib/pwrite.c
new file mode 100644 (file)
index 0000000..acf6acf
--- /dev/null
+++ b/lib/pwrite.c
@@ -0,0 +1,64 @@
+/* Write block to given position in file without changing file pointer.
+   POSIX version.
+   Copyright (C) 1997, 1998, 1999, 2002 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
+
+   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/>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <unistd.h>
+
+#include <errno.h>
+
+#define __libc_lseek(f,o,w) lseek (f, o, w)
+#define __set_errno(Val) errno = (Val)
+#define __libc_write(f,b,n) write (f, b, n)
+
+/* Note: This implementation of pwrite is not multithread-safe.  */
+
+ssize_t
+pwrite (int fd, const void *buf, size_t nbyte, off_t offset)
+{
+  /* Since we must not change the file pointer preserve the value so that
+     we can restore it later.  */
+  int save_errno;
+  ssize_t result;
+  off_t old_offset = __libc_lseek (fd, 0, SEEK_CUR);
+  if (old_offset == (off_t) -1)
+    return -1;
+
+  /* Set to wanted position.  */
+  if (__libc_lseek (fd, offset, SEEK_SET) == (off_t) -1)
+    return -1;
+
+  /* Write out the data.  */
+  result = __libc_write (fd, buf, nbyte);
+
+  /* Now we have to restore the position.  If this fails we have to
+     return this as an error.  But if the writing also failed we
+     return this error.  */
+  save_errno = errno;
+  if (__libc_lseek (fd, old_offset, SEEK_SET) == (off_t) -1)
+    {
+      if (result == -1)
+       __set_errno (save_errno);
+      return -1;
+    }
+  __set_errno (save_errno);
+
+  return result;
+}