Write in file using 20 threads - c

Question

The Idea here is to create a file to be written to. I'm trying to create ten threads and have them print to the file 10 times each. Using a semaphore to stop multiple threads from writing to the file at once. But I have errors. The code is below:
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#define FNAME "fisier.txt"
#define MAX_STRING_LEN 80
#define NUMBER_OF_THREADS 10
FILE *fp;
sem_t mutex;
int counter;
FILE *makeTextFile(char *fname, char mode){
FILE *localFP;
localFP = fopen(fname, &mode);
return (localFP);
}
void *print_message(void *tid){
int i;
for (i = 0; i < 10; i++){
sem_wait(&mutex);
fp = fopen(FNAME, "a");
fprintf(fp, "Thread %d is running.\n", tid);
fclose(fp);
sem_post(&mutex);
printf ( "Thread %d has finished.\n", tid);
}
}
int threads(){
const char *fName = "fisier.txt";
int status;
pthread_t threads[NUMBER_OF_THREADS];
fp = makeTextFile(FNAME, 'w');
fprintf(fp, "Process ID: %ld\n", (long)getpid());
fclose(fp);
int i;
for (i =0; i < NUMBER_OF_THREADS; i++){
status = pthread_create(&threads[i], NULL, &print_message, (void *)i);
if (status != 0){
printf("pthread_create returned error code %d\n", status);
exit(-1);
}
}
}
int main() {
threads();
return 0;
}
Warnings:
probl2.c: In function ‘print_message’:
probl2.c:27:21: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘void *’ [-Wformat=]
fprintf(fp, "Thread %d is running.\n", tid);
^
probl2.c:30:14: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘void *’ [-Wformat=]
printf ( "Thread %d has finished.\n", tid);
^
probl2.c: In function ‘threads’:
probl2.c:44:68: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
status = pthread_create(&threads[i], NULL, &print_message, (void *)i);
Writing in a file just it: Process ID: 10568
I want to write
How to solve it ?

Answer 1

A couple of things I noticed:
You are not initializing your semaphore. Use sem_init to do this (and use sem_destroy when you are finished).
You are not joining on your threads. The program will exit without waiting for the threads to finish. You can use pthread_join in a loop to make sure all threads have finished.
Here is an updated version of your threads function. In production code I would check the return values of the functions I added.
void threads(){
const char *fName = "fisier.txt";
int status;
pthread_t threads[NUMBER_OF_THREADS];
fp = makeTextFile(FNAME, 'w');
fprintf(fp, "Process ID: %ld\n", (long)getpid());
fclose(fp);
sem_init(&mutex,0,1);
int i;
for (i =0; i < NUMBER_OF_THREADS; i++){
status = pthread_create(&threads[i], NULL, &print_message, (void*)i);
if (status != 0){
printf("pthread_create returned error code %d\n", status);
exit(-1);
}
}
void* value = NULL;
for (i = 0; i < NUMBER_OF_THREADS; i++) {
pthread_join(threads[i], &value);
}
sem_destroy(&mutex);
}