Why I have this semaphores problem in a C-Posix code? - c

Question

The program has to do it:
A process P0 creates P1 and P2;
sizeof(buffer) = N (inserts with command line);
P1 inserts random values in the first N/2 elements of the buffer (N insert by user)
P2 inserts random values in the second part of the buffer
After that: P1 inverts the second part of the buffer and then the process P0 print all elements of it
If the user presses CTRL+C ---> print the buffer elements and kill all process;
I don't know why , but process P1 remains in pause. I called in the concurrent process P2 the increase of the semaphore's value ("semaphore_inv") and the wait has to decrease it to 0. For this reason the program doesn't work correctly.
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <semaphore.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>
int N;
int buff[1024];
void print(int sig) {
int i;
for(i=0; i<N; i++) {
printf("Slot %d of the buffer is %d\n",i, buff[i]);
}
kill(0,SIGKILL);
}
void main (int argc, char* argv[]) {
int p1, p2;
sem_t semaphore_inv;
sem_t semaphore_read;
sem_t semaphore_write;
struct sembuf sembuf;
N=atoi(argv[1]);
if (N<=0 || N>=1024) {
printf("Inserirt a value > 0 and < 1024\n");
exit(-1);
}
if (argc!=2) {
printf("Insert com N\n");
exit(1);
}
int buffer[N];
//I insert this type of semaphore only to try it
int sem_write = semget(IPC_PRIVATE,1,IPC_CREAT|0666);
if (sem_write <0) printf("Error in the semaphore creation\n");
int sem_write_b = sem_init(&semaphore_write,1,1);
if (sem_write_b<0) printf("Error in the semaphore creation\n");
int sem_inv = sem_init(&semaphore_inv, 1, 0);
if (sem_inv<0) printf("Error in the semaphore creation\n");
int sem_read = sem_init(&semaphore_read,1,0);
if (sem_read<0) printf("Error in the semaphore creation\n");
int ret = semctl(sem_write, 0, SETVAL, 1);
if (ret == -1) printf("Error: semctl, with errno %s\n", strerror(errno));
signal(SIGINT, print);
p1 = fork();
p2 = fork();
if (p1 < 0) {
printf("P1: error, fork\n");
exit(-2);
}
if (p2 < 0) {
printf("P2: error, fork\n");
exit(-2);
}
if (p1==0) {
loop:
sembuf.sem_num=0;
sembuf.sem_op= -1;
sembuf.sem_flg=0;
semop(sem_write, &sembuf, 1);
int i;
for (i=0; i<N/2; i++) {
buffer[i] = rand();
printf("P1: the insert value in buffer[%d] is %d\n",i , buffer[i]);
}
sem_wait(&semaphore_inv);
printf("P1: i'm going to invert the second part of the buffer\n");
int j=1;
for (i=N/2; i<N; i++){
int buffer_prev;
buffer_prev=buffer[i];
buffer[i] = buffer[N-j];
buffer[N-j] = buffer_prev;
j++;
}
sem_post(&semaphore_read);
sleep(1);
goto loop;
}
if (p2==0) {
loop_b:
sem_wait(&semaphore_write);
int i;
for (i=N/2; i<N; i++) {
buffer[i] = rand();
printf("P2: the value insert in buffer[%d] is %d\n", i, buffer[i]);
}
sem_post(&semaphore_inv);
sleep(1);
goto loop_b;
}
else{
sem_wait(&semaphore_read);
int k;
for (k=0; k<N; k++) {
buff[k] = buffer[k];
printf(" slot %d of the buffer is %d\n", buffer[k]);
}
sem_post(&semaphore_write);
sembuf.sem_num=0;
sembuf.sem_op= +1;
sembuf.sem_flg=0;
semop(sem_write, &sembuf, 1);
}
}

Answer 1

There are four processes involved. Illustration:
#include <stdio.h>
#include <unistd.h>
int main(void)
{
int pid = -2, pid1 = -2, pid2 = -2;
pid1 = fork();
pid2 = fork();
mypid = getpid();
printf("Pid= {%d, %d %d}\n", mypid, pid1,pid2);
return 0;
}