IPC_RMID not work on linux with C++ - c++

Question

I'm trying to solve my school project in C++. I have to create 15 processes and they have to run in order what means that processes run in this order 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0. It work but when I try to remove semaphore from the memory I am getting error from semctl. On the end I use "semctl(semid, 0, IPC_RMID, 0" but I get error 22 which means EINVAL but it doesn't make sense and I try to remove semaphore from parrent process so I should have privileges to do that.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>
#include <sys/wait.h>
union semun {
int val;
struct semid_ds *buf;
ushort *array;
};
struct sembuf sops[1];
int semid;
int wait_sem(int index, int pid){
fprintf(stderr, "\n------- Proces %d do operation wait (-1) on semaphore %d\n",pid, index);
sops[0].sem_num = index;
sops[0].sem_op = -1;
sops[0].sem_flg = 0 ;
if (semop(semid, sops, 1)<0){
perror("semop fail wait");
return 1;
}
else
return 0;
}
int signal_sem(int index, int pid){
fprintf(stderr, "\n++++++ Proces %d vykonava operaciu signal (1) na semafore %d\n",pid,index);
sops[0].sem_num = index;
sops[0].sem_op = 1;
sops[0].sem_flg = 0;
if (semop(semid, sops, 1)<0){
perror("semop fail signal");
return 1;
}
else
return 0;
}
void createSem(key_t paKey, int paSemFlg, int paNsems)
{
printf ("uid=%d euid=%d\n", (int) getuid (), (int) geteuid ());
(semid = semget(paKey, paNsems, paSemFlg));
for (int i = 0; i < paNsems; ++i) {
semctl(semid, i, SETVAL, 0);
}
}
void kic()
{
printf("\naaaaaaaaaaaaaa\n");
}
int main() {
key_t key = 1234;
int semflg = IPC_CREAT | 0666;
int nsems = 15;
int semid;
fprintf(stderr, "%d=", sops);
createSem(IPC_PRIVATE, semflg, nsems);
if (semid == -1) {
perror("semget: semget failed");
return 1;
}
else
fprintf(stderr, "semget: semget sucess: semid = %d, parrent pid %d\n", semid, getpid());
int PROCESS_ID = 0;
pid_t PID;
for (int i = 1; i < nsems; i++) {
PID = fork();
if(PID == 0)
{
PROCESS_ID = i;
break;
}
}
if(PID == -1)
{
printf("\nPID ERROR");
}
if(PID != 0) //parrent
{
printf("\n\nparrent with ID %d", PROCESS_ID);
signal_sem(PROCESS_ID+1, PROCESS_ID);
wait_sem(PROCESS_ID, PROCESS_ID);
printf ("uid=%d euid=%d\n", (int) getuid (), (int) geteuid ());
printf("\nEND %d\n", getpid());
int s;
wait(&s);
if((semctl(semid, 0, IPC_RMID, 0))==-1)
{
int a = errno;
printf("\nERROR IPC_RMID %d\n", a);
}
}
if(PID == 0)//child
{
if(wait_sem(PROCESS_ID, PROCESS_ID) == 0){
printf("\nI am child with ID %d", PROCESS_ID);
int ID_NEXT_PROCESS = 1+PROCESS_ID;
if(ID_NEXT_PROCESS == nsems)
ID_NEXT_PROCESS = 0;
signal_sem(ID_NEXT_PROCESS, PROCESS_ID);
return 0;
}
}
return 0;
}

Answer 1

You have two semids. One in global scope, another local to main (which shadows global, you should see a warning). createSem only knows about global one, and initializes it. semctl is called directly by main, and is passed the local one, which is garbage.