I am writing a small c program which does thread synchronization - c

Question

I am a student and i encountered a following task using multithreading in c, the problem is I am trying to protect shared variable cnt using P and V operataions. I am using semaphore to protect the shared variable, but it is giving me following errors:
correctcntn.c:15:10: error: expected declaration specifiers or ‘...’ before ‘&’ token
sem_init(&mutex, 0, 1);
^
correctcntn.c:15:18: error: expected declaration specifiers or ‘...’ before numeric constant
sem_init(&mutex, 0, 1);
^
correctcntn.c:15:21: error: expected declaration specifiers or ‘...’ before numeric constant
sem_init(&mutex, 0, 1);
Thank you in advance! Any help is appreciated!
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <signal.h>
#include <sys/wait.h>
#include <unistd.h>
#include <errno.h>
#include<semaphore.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);
void *thread(void *vargp);
volatile long cnt = 0;
sem_t mutex;
sem_init(&mutex, 0, 1);
void P(sem_t *s)
{
sem_wait(&s);
}
void V(sem_t *s)
{
sem_post(&s);
}
int main(int argc, char **argv)
{
long niters;
pthread_t tid1, tid2;
niters = atoi(argv[1]);
pthread_create(&tid1, NULL, thread, &niters);
printf("THREAD 1 with tid= %ld CREATED \n", tid1);
pthread_create(&tid2, NULL, thread, &niters);
printf("THREAD 2 with tid= %ld CREATED \n", tid2);
pthread_join(tid1, NULL);
printf("THREAD 1 with tid= %ld TERMINATED \n", tid1);
pthread_join(tid2, NULL);
printf("THREAD 2 with tid= %ld TERMINATED \n", tid2);
if (cnt != (2 * niters))
printf("BOOM! cnt=%ld\n", cnt);
else
printf("OK cnt=%ld\n", cnt);
exit(0);
}
void *thread(void *vargp)
{
long i, niters = *((long *)vargp);
for (i = 0; i < niters; i++)
P(&mutex);
cnt++;
V(&mutex);
return NULL;
}

Answer 1

Statements to be executed like sem_init(&mutex, 0, 1); must be in function bodies in C.
You should move that inside a function body -- for example, at the beginning of the main() function.
int main(int argc, char **argv)
{
long niters;
pthread_t tid1, tid2;
sem_init(&mutex, 0, 1); /* move to here */
niters = atoi(argv[1]);