swap: atomically renames a file

Uses the renameat2 syscall from Linux 3.15.

2 files changed, 55 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index 83f81a7..8fbf00d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,4 +9,8 @@ hex2bin
 slice
 nntp-get
 revb
+wipe-sectors
+xor
+timeadd
 *.swp
+swap
diff --git a/swap.c b/swap.c
new file mode 100644
index 0000000..3fc5777
--- /dev/null
+++ b/swap.c
@@ -0,0 +1,51 @@
+/* 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;
+}