Shared memory issue using processes in C - c

Question

I am working on a program that stores the variable Bank in shared memory and causes a race condition. My code compiles but it only prints the initial balance statements and not the resulting balance. Essentially, the program should be printing a balance other than 200 with the race condition and when the condition is fixed using semaphores, it should use different numbers to always print a balance of 200. What am I missing that could cause my program to get caught up after the first printf?
#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 <sys/ipc.h>
#include <semaphore.h>
#include <string.h>
//sem_t mutex; // semaphore global variable
struct {
int balance[2];
}
Bank = {{100, 100}};
//struct Bank *bank;
// routine for thread execution
void* MakeTransactions() {
key_t shmkey;
int shmid, seqid;
int seqnum = 5858;
int i, j, tmp1, tmp2, rint;
int *shmint1, *shmint2;
double dummy;
if ((shmkey=ftok(getenv("HOME"), seqnum))==(key_t)-1) {
perror("ERROR: ftok");
}
else if ((shmid=shmget(shmkey,sizeof(int)*2,0600|IPC_CREAT))==-1) {
perror("ERROR: shmget");
}
if ((shmint1=(int *)shmat(shmid,NULL,0600))==(int*)-1) {
perror("ERROR: shmalloc");
}
shmint2 = shmint1 + 1;
// wait on semaphore
//sem_wait(&mutex);
for (i=0; i < 100; i++) {
rint = (rand()%30)-15;
if (((tmp1= *shmint1)+rint) >=0 &&
((tmp2= *shmint2)-rint)>=0) {
//sem_wait(&mutex);
//bank->balance[0] = tmp1 + rint;
*shmint1 = tmp1 + rint;
//sem_post(&mutex);
usleep(5000);
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;
*shmint2 = tmp2 - rint;
//sem_post(&mutex);
}
}
Bank.balance[0]= *shmint1;
Bank.balance[1]= *shmint2;
// increment value of semaphore
//sem_post(&mutex);
return NULL;
}
vint main(int argc, char **argv) {
key_t shmkey;
int shmid, seqid, seqnum;
int *shmint1, *shmint2;
int i;
// 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);
if ((shmkey=ftok(getenv("HOME"), seqnum))==(key_t)-1) {
perror("ERROR: ftok");
}
else if ((shmid=shmget(shmkey,sizeof(int)*2,0600|IPC_CREAT))==-1) {
perror("ERROR: shmget");
}
if ((shmint1=(int *)shmat(shmid,NULL,0600))==(int*)-1) {
perror("ERROR: shmalloc");
}
shmint2 = shmint1 + 1;
memset((int*)shmint1,0,sizeof(int));
memset((int*)shmint2,0,sizeof(int));
/*//check if bank is not NULL
Bank.balance[0] = 100;
Bank.balance[1] = 100;
*/
*shmint1 = Bank.balance[0];
*shmint2 = Bank.balance[1];
pid_t pid;
srand(getpid());
printf("\nInit balances A:%d + B:%d ==> %d!\n",*shmint1,*shmint2,*shmint1 + *shmint2);
pid=fork();
if (pid < 0) {
fprintf(stderr, "Fork failed");
return 1;
}
if (pid == 0) {
MakeTransactions();
return 0;
}
else {
MakeTransactions();
wait(NULL);
return 0;
}
printf("Let's check the balances A:%d + B:%d ==> %d ?= 200\n\n",*shmint1,*shmint2,*shmint1 + *shmint2);
//sem_destroy(&mutex);
// deattach shared memory pointer
shmdt(shmint1);
shmctl(shmid, IPC_RMID, NULL);
// memory unmap struct
//munmap(bank, sizeof(struct Bank));
return 0;
}

Answer 1

The wait() method you're using, as it is described here, looks like it may result in undefined behaviour when parsing NULL; it uses the integer pointer as an indicator of process completion.