flock

Learn file locking with flock.

What is flock?

flock is a simple BSD-origin file locking call. It locks the whole file; it does not support byte ranges. The flock(fd, operation) call is declared in <sys/file.h>. Three main operations exist: LOCK_SH shared, LOCK_EX exclusive, LOCK_UN unlock.

Shared locks are for reading, exclusive locks for writing. flock is advisory: cooperating processes honor the lock, but those that do not cooperate are not stopped. Combined with LOCK_NB, the call does not block if the lock cannot be acquired. Locks are tied to the file descriptor; close(fd) releases them automatically.

Open, lock, unlock, close

The typical flow is: open with open(path, O_CREAT | O_RDWR, 0644), lock with flock(fd, LOCK_EX), do the work, release with flock(fd, LOCK_UN), finally close(fd). Closing also releases the lock.

For testing you can print the state with printf("locked\n") and printf("unlocked\n"). The unlink(path) call removes the file so reruns leave no leftover. Error checking is done with if (flock(fd, LOCK_EX) == -1).

Exercise Lock and unlock a file

Open the file with int fd = open("/tmp/cb_lock.txt", O_CREAT | O_RDWR, 0644);. Acquire the lock with flock(fd, LOCK_EX); and printf("locked\n");. Release with flock(fd, LOCK_UN); and printf("unlocked\n");. close(fd); and unlink("/tmp/cb_lock.txt");.

Show example solution

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/file.h>

int main(void) {
    int fd = open("/tmp/cb_lock.txt", O_CREAT | O_RDWR, 0644);
    flock(fd, LOCK_EX);
    printf("locked\n");
    flock(fd, LOCK_UN);
    printf("unlocked\n");
    close(fd);
    unlink("/tmp/cb_lock.txt");
    return 0;
}

Test yourself

What does flock(fd, LOCK_EX) do?
- Acquires exclusive lock
- Acquires shared lock
- Releases lock
- Deletes file
Show answer

Acquires exclusive lock — LOCK_EX acquires an exclusive lock; used for writing.
flock call
```
flock(fd, ____);
```
Show answer
```
flock(fd, LOCK_EX);
```
Match flock constants

LOCK_SH

Shared
LOCK_EX

Exclusive
LOCK_UN

Unlocks
LOCK_NB

Non-blocking

Next lesson: fcntl

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