add reopen

Signed-off-by: Mattias Andrée <maandree@member.fsf.org>

1 files changed, 79 insertions, 0 deletions

diff --git a/src/fcntl/reopen.c b/src/fcntl/reopen.c
new file mode 100644
index 0000000..e689e58
--- /dev/null
+++ b/src/fcntl/reopen.c
@@ -0,0 +1,79 @@
+/**
+ * slibc — Yet another C library
+ * Copyright © 2015  Mattias Andrée (maandree@member.fsf.org)
+ * 
+ * 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 <fcntl.h>
+#include <errno.h>
+#include <stdio.h>
+#include <unistd.h>
+
+
+
+/**
+ * Reopen a file via its file descriptor. The old file descriptor
+ * is closed and replace with the new open file descriptor.
+ * 
+ * This function call is different from the `dup` function-family
+ * in two ways: (1) you can select news flags, for example, if
+ * `fd` was read-only, the new file descriptor can be write-only,
+ * and (2) the file descriptor is not duplicated, the file is
+ * reopened.
+ * 
+ * This function is useful if you want to reopen a file you do
+ * now know the pathname, you do not want to create a new string
+ * with the pathname, or if the the file no longer has a pathname
+ * because it has been unlinked.
+ * 
+ * Because the file is reopened, the file position is reset and
+ * is not shared with any duplicates of `fd`, unlike a call to
+ * dup(3). it is visible to the filesystem as a new open file
+ * descriptor and is distinguished from the duplocates of `fd`,
+ * and all file locks associated with `fd` are lost, except for
+ * on the duplicates of `fd`, which means that you can create
+ * new locks without removing locks from the duplicates of `fd`.
+ * 
+ * This is a slibc extension.
+ * 
+ * @etymology  (Reopen) file!
+ * 
+ * @param   fd     The file descriptor.
+ * @param   oflag  How the file shall be opened, see open(3).
+ *                 `O_CREAT` is not allowed.
+ * @return         Zero on success, -1 on error.
+ * 
+ * @throws  EINVAL  `oflag` contains `O_CREAT`.
+ * @throws  EINVAL  `fd` is negative.
+ * @throws          Any error specified for open(3).
+ * @throws          Any error specified for dup2(3).
+ * 
+ * @since  Always.
+ */
+int reopen(int fd, int oflag)
+{
+  char path[sizeof("/dev/fd/") + 3 * sizeof(int)];
+  int r, saved_errno;
+
+  if ((oflag & O_CREAT) || (fd < 0))
+    return errno = EINVAL, -1;
+
+  sprintf(path, "/dev/fd/%i", fd);
+  if (r = open(path, oflag), r < 0)
+    return -1;
+  if (dup2(r, fd) == -1)
+    return saved_errno = errno, close(r), errno = saved_errno, -1;
+  return close(fd), 0;
+}
+