Program stuck in while loop - c

Question

I was trying to use semaphores in a while loop and my program is running in an infinite loop. Basically I fork off children and use execv to start new user processes. The processes use semaphore in shared memory to communicate.
struct shared
{
int count;
sem_t sem;
} *shmem;
void forking()
{
key_t shmkey = ftok("makefile",777);
if (shmkey == -1) {
perror("Ftok failed");
exit(-1);
}
// Get our shm id
shmid = shmget(shmkey, sizeof(shared), 0666 | IPC_CREAT);
if (shmid == -1) {
perror("Failed to allocate shared memory region");
exit(-1);
}
// Attach to our shared memory
shmem = (shared*)shmat(shmid, (void *)0, 0);
if (shmem == (void *)-1) {
perror("Failed to attach to shared memory region");
exit(-1);
}
sem_init(&(shmem->sem),1,1);
pid_t PID;
int i;
for(i=0;i<4;i++)
{
PID=fork();
if(PID==0)
{
static char *args[]={"./child",NULL};
int status;
if(( status= (execv(args[0], args)))==-1)
perror("failed to execv");
exit(EXIT_FAILURE);
}
if(PID>0)
{
while(1)
{
sem_wait(&(shmem->sem));
{
shmem->count=shmem->count+1;
}
sem_post(&(shmem->sem));
}
}
if(PID<0)
perror("error in fork");
}
}
main()
{
forking();
}
in child.c
struct shared
{
int count;
sem_t sem;
} shmem;
void main()
{
bool alive=true;
do
{
sem_wait(&(shmem->sem));
if(shmem->count==5)
{
shmem->count=0;
alive=false;
}
sem_post(&(shmem->sem));
}while(alive);
}
child.c loops endlessly without allowing parent to increment i till 5. This happens even if I comment out while loop in first file and just let it increment without loops.In the end nothing exits.

Answer 1

You are using fork wrong, and definitely not using shared memory.
Here's some few point
By calling fork, you are making a copy of parent running in different memory space. http://man7.org/linux/man-pages/man2/fork.2.html
By using execve, you essentially delete the old memory and replace it with a new one which are not linked in anyway to your parent process. http://man7.org/linux/man-pages/man2/execve.2.html
To make your code work, you should be using pthread_* instead as it will live in the same memory space and make your global variable volatile as gcc will try to optimize and will have weird behaviors if it does so.
Ok it seems I've missed quite a few points while being not totally wrong on my assumption.
So your child.c is missing quite a few bit of code
It does not contains all the initialization you've made in your parent which is necessary because if you use execve you essentially lost every memory mapping you've mapped previously in your parent process.
Your parents loops infinitely so shmem->count will never really equals to 5, you will have to break at some moment or change your condition in your child code.
Here's what your child should look like.
#include <semaphore.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
typedef struct
{
int count;
sem_t sem;
} shared;
shared *shmem;
int shmid;
key_t shmkey;
int main()
{
int alive=1;
shmkey = ftok("makefile",777);
if (shmkey == -1) {
perror("Ftok failed");
exit(-1);
}
// Get our shm id
shmid = shmget(shmkey, sizeof *shmem, 0);
if (shmid == -1) {
perror("Failed to allocate shared memory region");
exit(-1);
}
// Attach to our shared memory
shmem = (shared*)shmat(shmid, (void *)0, 0);
if (shmem == (shared *)-1) {
perror("Failed to attach to shared memory region");
exit(-1);
}
do
{
sem_wait(&(shmem->sem));
printf("%d\n", shmem->count);
if(shmem->count > 5)
{
shmem->count=0;
alive=0;
}
sem_post(&(shmem->sem));
}while(alive);
printf("I have finished %u\n", getpid());
exit(0);
}