Sample thread code using mutex

For your education.

#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define THREAD_NUM 5
#define MUTEX_ENABLED 0

struct thread_ctx
{
    int id;
    pthread_t tid;
} ctx_arr[THREAD_NUM];

int fd;
pthread_mutex_t *mtx;
pthread_mutexattr_t *mtxattr;

void * t_worker(void *arg)
{
    int i;
    char buf[10];

    struct thread_ctx *ctx = (struct thread_ctx *)arg;
    sprintf(buf, "%d\n", ctx->id);

    if (MUTEX_ENABLED)
        pthread_mutex_lock(mtx);

    // critical section
    for (i = 0; i < 4; i++)
    {  
        write(fd, buf, strlen(buf));
        usleep(100);
    }

    if (MUTEX_ENABLED)
        pthread_mutex_unlock(mtx);

    return (void *)ctx;
}

int main()
{
    int i;
    void *res;

    // open file
    fd = open("result.dat", O_CREAT|O_RDWR);
    if (fd < 0)
    {  
        printf("[ERROR] file open failure - %s.\n", strerror(fd));
        return -1;
    }

    // init mutex
    mtx = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));
    mtxattr = (pthread_mutexattr_t *)malloc(sizeof(pthread_mutexattr_t));
    pthread_mutexattr_init(mtxattr);
    pthread_mutexattr_settype(mtxattr, PTHREAD_MUTEX_TIMED_NP);
    pthread_mutex_init(mtx, mtxattr);

    for (i = 0; i < THREAD_NUM; i++)
    {  
        ctx_arr[i].id = i;
        if (pthread_create(&ctx_arr[i].tid, NULL, t_worker,
                (void *)(ctx_arr + i)) != 0)
        {  
            printf("[ERROR] pthread_create failure\n");
        }
    }

    // To join all threads.
    for (i = 0; i < THREAD_NUM; i++)
    {  
        pthread_join(ctx_arr[i].tid, &res);
    }
    printf("[MSG] All threads are joined.\n");

    close(fd);
    free(mtx);
    free(mtxattr);

    return 0;
}

result :

cat result.dat # mutex off
2
1
0
1
0
1
3
0
1
2
3
4
4
4
4
3
4
4
4
4
cat result.dat # mutex on
1
1
1
1
0
0
0
0
2
2
2
2
3
3
3
3
4
4
4
4