/**
* 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;
}
