/* Rename two files atomically, swapping one for the other.
 * Copyright (C) 2015 Peter Wu <peter@lekensteyn.nl>
 */

#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/fs.h>       /* for RENAME_* */
#include <fcntl.h>          /* for AT_* */
#include <stdio.h>
#include <errno.h>

int renameat2(int olddirfd, const char *oldpath,
              int newdirfd, const char *newpath, unsigned int flags)
{
    long r;
    r = syscall(SYS_renameat2, olddirfd, oldpath, newdirfd, newpath, flags);
    return (int)r;
}

int main(int argc, char **argv)
{
    int olddirfd, newdirfd;
    const char *oldpath, *newpath;
    unsigned flags;
    int r;

    if (argc - 1 < 2) {
        fprintf(stderr, "Usage: %s file1 file2\n", argv[0]);
        fprintf(stderr, "Atomically swaps file1 and file2.\n");
        return 1;
    }

    olddirfd = AT_FDCWD;
    newdirfd = AT_FDCWD;
    oldpath = argv[1];
    newpath = argv[2];
    flags = RENAME_EXCHANGE;

    r = renameat2(olddirfd, oldpath, newdirfd, newpath, flags);
    if (r < 0) {
        if (errno == ENOSYS) {
            fprintf(stderr, "renameat2 syscall not available!"
                            " You need Linux 3.15 or newer.\n");
        } else {
            perror("renameat2 failed");
        }
    }

    return r < 0 ? 1 : 0;
}