Automatic cleanup
- C course
- Advanced
- Lesson 41/83
- 5 min
Learn automatic lock cleanup and safety.
Automatic cleanup
Automatic lock cleanup improves safety. For flock, calling close(fd) releases the lock immediately; the same holds for fcntl. So calling close early prevents lock leakage.
If the program terminates unexpectedly, the kernel closes all descriptors and releases locks. Still, registering a cleanup function with atexit(cleanup) is a good habit. This function does close, unlink and other cleanup. So whether the program exits normally or with error, no resources leak.
Exit cleanup with atexit
atexit is declared in <stdlib.h>. When the program exits normally, it calls the registered functions. You define a function with signature void cleanup(void) and register it with atexit(cleanup).
For testing, a simple scenario: open a file, take flock(fd, LOCK_EX), print printf("locked\n"), release the lock with close(fd), print printf("cleaned\n"). atexit is not strictly needed here; but in real apps, when the program returns or exits anywhere, the cleanup function runs.
Practice Demonstrate automatic cleanup
Open the file with int fd = open("/tmp/cb_clean.txt", O_CREAT | O_RDWR, 0644);. Call flock(fd, LOCK_EX); and printf("locked\n");. Release the lock automatically with close(fd);. Print printf("cleaned\n");. Call unlink("/tmp/cb_clean.txt");.
Starter code
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/file.h>
// Open file and take lock
// Release the lock with close
// Print cleaned and remove file
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/file.h>
int main(void) {
int fd = open("/tmp/cb_clean.txt", O_CREAT | O_RDWR, 0644);
flock(fd, LOCK_EX);
printf("locked\n");
close(fd);
printf("cleaned\n");
unlink("/tmp/cb_clean.txt");
return 0;
}Expected output
locked cleaned
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match cleanup mechanisms
| Term | Meaning |
|---|---|
close |
Releases lock |
atexit |
Exit function |
unlink |
Removes file |
kernel |
Cleans on crash |
2 What does close(fd) do to a flock lock?
- Releases it Correct answer
- Makes it permanent
- Nothing
- Duplicates it
Explanation: close(fd) releases the flock lock automatically.
3 atexit call
atexit(____);atexit(cleanup);Explanation: atexit runs cleanup at exit.