Using semaphore for multithreading in C - 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. Everything compiles and I don't get an error exit, however I can't understand why running the program numerous times: 1)It doesn't print 100 lines to the file, infact it's far less 2) The number of lines printed to the file vary each time.
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#define READ "r"
#define NEW "w"
#define ADD "a"
#define NL "\n"
#define TAB "\t"
#define FNAME "PROCTAB.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, ADD);
fprintf(fp, "Thread %d is running.\n", tid);
fclose(fp);
sem_post(&mutex);
}
}
int threads(){
const char *fName = "PROCTAB.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);
}
}
return 0;
}
My main function is contained in a separate file.

Answer 1

You need to wait for all thread to finish before you exit the program.
if you add trace you will see which thread is finish.
void *print_message(void *tid){
int i;
for (i = 0; i < 10; i++){
sem_wait(&mutex);
fp = fopen(FNAME, ADD);
fprintf(fp, "Thread %d is running.\n", tid);
fclose(fp);
sem_post(&mutex);
printf ( "Thread %d has finished.\n", tid);
}
}
This is how you wait for all threads to finish
/* Wait for Threads to Finish */
for (i=0; i<NUMTHREADS; i++) {
pthread_join(thread[i], NULL);
}