file locking

exclusive.c

/*

Tested on MacOS and Linux Debian.

USE_LOCK = 1:

❯ gcc exclusive.c -o exclusive && parallel -j 1000 ./exclusive -- {1..1000} | sort | uniq -c
1000 10000

USE_LOCK = 0:

❯ gcc exclusive.c -o exclusive && parallel -j 1000 ./exclusive -- {1..1000} | sort | uniq -c
  91 0
 539 10000
 182 4096
 188 8192

*/

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

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define USE_LOCK 1

void write_data(FILE *file) {
    if (file) {
        for(int i = 0; i < 10000; i++) {
        fprintf(file, "x");
        }
        fclose(file);  // closes the underlying file descriptor too
    }
}

void write_with_lock() {
    int fd = open("example.txt", O_WRONLY | O_CREAT);
    if (fd == -1) {
        perror("open");
        return;
    }

    struct flock lock;
    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    if (fcntl(fd, F_SETLKW, &lock) == -1) {
        perror("fcntl");
        close(fd);
        return;
    }

    FILE *file = fdopen(fd, "w");
    write_data(file);

    lock.l_type = F_UNLCK;
    fcntl(fd, F_SETLK, &lock);
    close(fd);
}

void read_data(FILE *file) {
    long bytes = 0;
    int ch;
    while ((ch = fgetc(file)) != EOF) {
        bytes++;
    }
    fclose(file);
    printf("%ld\n", bytes);
}

void read_with_lock() {
    int fd = open("example.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return;
    }

    struct flock lock;
    lock.l_type = F_RDLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    if (fcntl(fd, F_SETLKW, &lock) == -1) {
        perror("fcntl");
        close(fd);
        return;
    }

    FILE *file = fdopen(fd, "r");
    read_data(file);

    lock.l_type = F_UNLCK;
    fcntl(fd, F_SETLK, &lock);
    close(fd);
}

int main() {

#if USE_LOCK
    write_with_lock();
#else
    FILE *file = fopen("example.txt", "w");
    write_data(file);
#endif

#if USE_LOCK
    read_with_lock();
#else
    file = fopen("example.txt", "r");
    read_data(file);
#endif

    return 0;
}