how to store variable in shared memory - c

Question

I am trying to add the variable bank into shared memory to explore race conditions and process synchronization using semaphores. When I setup the variable in shared memory my program gives me segmentation fault:11. Can someone explain what I am doing wrong here and how I can go about fixing this? Thanks!
RACE
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/shm.h>
#include <semaphore.h>
int shmid;
int shmkey = 585858;
sem_t mutex; // semaphore global variable
struct Bank {
int balance[2];
};
struct Bank *bank;
// routine for thread execution
void* MakeTransactions() {
int i, j, tmp1, tmp2, rint;
double dummy;
// wait on semaphore
//sem_wait(&mutex);
for (i=0; i < 100; i++) {
rint = (rand()%30)-15;
if (((tmp1=bank->balance[0])+rint) >=0 &&
((tmp2=bank->balance[1])-rint)>=0) {
//sem_wait(&mutex);
bank->balance[0] = tmp1 + rint;
//sem_post(&mutex);
for (j=0; j < rint*100; j++) {
dummy=2.345*8.765/1.234; // spend time on purpose
}
//sem_wait(&mutex);
bank->balance[1] = tmp2 - rint;
//sem_post(&mutex);
}
}
// increment value of semaphore
//sem_post(&mutex);
return NULL;
}
int main(int argc, char **argv) {
int i;
void* voidptr = NULL;
// initialize semaphore
sem_init(&mutex, 0, 1);
// shared memory
//bank = mmap(NULL, sizeof(struct Bank), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
// create shared memory
shmid = shmget(shmkey, 1024, IPC_CREAT);
// attach to shared memory
bank = shmat(shmid, NULL, 0);
//check if bank is not NULL
bank->balance[0] = 100;
bank->balance[1] = 100;
pid_t pid;
srand(getpid());
printf("\nInit balances A:%d + B:%d ==> %d!",
bank->balance[0],bank->balance[1],bank->balance[0]+bank->balance[1]);
pid=fork();
if (pid < 0) {
fprintf(stderr, "Fork failed");
return 1;
}
if (pid == 0) {
printf("\nChild computing ...");
MakeTransactions();
printf("\nChild process complete");
printf("\nLet's check the balances A:%d + B:%d ==> %d ?= 200",
bank->balance[0],bank->balance[1],bank->balance[0]+bank->balance[1]);
return 0;
}
else {
printf("\nParent computing...\n");
MakeTransactions();
wait(NULL);
printf("\nParent process complete\n");
printf("Let's check the balances A:%d + B:%d ==> %d ?= 200\n\n",
bank->balance[0],bank->balance[1],bank->balance[0]+bank->balance[1]);
return 0;
}
sem_destroy(&mutex);
// deattach shared memory pointer
shmdt(&bank,NULL);
//munmap(bank, sizeof(struct Bank));
return 0;
}

Answer 1

Okay the reason it gives you a segfault is because you're probably running this program as an unprivileged user. shmat requires privileges. If you are running it under linux you can do this:
sudo setcap cap_ipc_owner=ep ./myprog
to enable it to call shmat etc.
and then run it.
You would've found out about this if you had added a bit of error handling:
shmid = shmget(shmkey, 1024, IPC_CREAT);
if (shmid == -1 ) {
perror("shmget");
exit(errno);
}
// attach to shared memory
bank = shmat(shmid, NULL, 0);
if (bank == (void *)-1) {
perror("bank");
exit(errno);
}