Forking 3 processes, using shared memory - c

Question

I have an assignment, and im beating my head against the wall. It is in C. I have a feeling im close to the solution, however I cant get the program to do whats required. I am changing the numbers and some small details, because most of the class is as stumped as I.
Requirements: Create 3 processes, the first one will increment a shared memory variable "total->value" from 1 to 10000, the second from 10000 to 12000, the third from 12000 to 14000
The process functions are labeled such (process1(), process2(), process3())
and the internals of those functions are as follows
process1()
{
int k = 0;
while (k < 10000)
{
k++;
total->value = total->value + 1;
}
printf("From process 1 = %d/n", total->value);
}
The second would be k < 2000 (because it only needs to increment the shared value 2000 more) and etc.
The main portion of the program is:
main()
{
int shmid;
int pid1;
int pid2;
int pid3;
int ID;
int status;
char *shmadd = (char *)0;
/* Create and connect to a shared memory segmentt */
if ((shmid = shmget(SHMKEY, sizeof (int), IPC_CREAT | 0666)) < 0)
{
perror("shmget");
exit(1);
}
if ((total = (shared_mem *)shmat(shmid, shmadd, 0)) == (shared_mem *)-1)
{
perror("shmat");
exit(0);
}
total->value = 0;
if ((pid1 = fork()) == 0)
process1();
if ((pid1 != 0) && (pid2 = fork()) == 0)
process2();
if ((pid1 != 0) && (pid2 != 0) && (pid3 = fork()) == 0)
process3();
if ((pid1 != 0) && (pid2 != 0) && (pid3 != 0))
{
if ((shmctl(shmid, IPC_RMID, (struct shmid_ds *)0)) == -1)
{
perror("shmctl");
exit(-1);
}
printf("\t\t End of Program.\n");
}
}
What I need is for the first process to finish, before the 2nd starts. I tried inserting a wait(&status) after the process1() (or 2 or 3) calls and am at a loss. Any pointers? (no pun intended) =) there is more to implement, but I believe once I have this part I can handle the rest on my own. I have been intentionally vague in some regards, but I would like to finish this project and more importantly understand it and there are others who want a free lunch. I will provide anything else in the code that is required. Thank you in advance for your help
The output should appear
From process 1 = 10000
From process 2 = 12000
From process 3 = 14000

Answer 1

I believe that Celada's comment/guess on the requirements is correct. However, barring that, and at the risk of doing too much work, the following code fulfills your spec. The use of the gcc built-in __sync_fetch_and_add() is perhaps unnecessary.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <sys/wait.h>
static struct {
int value;
} *total;
static void process1(void) {
int k = 0;
while (k < 10000) {
k++;
__sync_fetch_and_add(&total->value, 1);
}
printf("From process 1 = %d\n", total->value); //<-- not quite right: could be >10000
}
static void process2(void) {
int k = 0;
while (__sync_fetch_and_add(&total->value, 0) != 10000)
;
while (k < 2000) {
k++;
__sync_fetch_and_add(&total->value, 1);
}
printf("From process 2 = %d\n", total->value);
}
static void process3(void) {
int k = 0;
while (__sync_fetch_and_add(&total->value, 0) != 12000)
;
while (k < 2000) {
k++;
__sync_fetch_and_add(&total->value, 1);
}
printf("From process 3 = %d\n", total->value);
}
int main(void) {
int shmid;
int pid1;
int pid2;
int pid3;
int status;
/* Create and connect to a shared memory segment */
if ((shmid = shmget(1234, sizeof *total, IPC_CREAT|0666)) < 0) {
perror ("shmget");
exit (1);
}
if ((total = shmat(shmid, 0, 0)) == (void *)-1) {
perror("shmat");
exit (0);
}
total->value = 0; // not necessary in Linux if IPC_CREAT invoked
if (!(pid1 = fork()))
process1();
else if (!(pid2 = fork()))
process2();
else if (!(pid3 = fork()))
process3();
else {
wait(&status);
wait(&status);
wait(&status);
if ((shmctl(shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1) {
perror("shmctl");
exit (-1);
}
printf("\t\t End of Program.\n");
}
return 0;
}