Semaphores, processes and incrementing variables in shared memory

Question

This is a homework question. I have to write a program forking itself 20 times. Each new process is adding +1 to a variable (integer) which is shared between all of them. The thing is, I have to use semaphores (IPC). This piece of code is 'working' - giving the value of 20 in the end.

*buf = 0; for(i=1; i<=20; ++i) { if(fork()!=0) { *buf += 1; exit(0); } }

EDIT: Based on this code I am trying to get output like :
I am child 1...
I am child 2...
.
.
.
I am child 20...

It worked once (first time), and then the order became random. But I did not change any code. What am I doing wrong?

You possibly got your fork() call wrong. It returns non-zero to the parent process; therefore, only your parent increments the value. — zneak
– zneak, Commented Jan 3, 2012 at 2:17

Duck · Accepted Answer · 2012-01-03 02:52:58Z

Well your major problem is this:

 if (fork()!=0) //<-- this

fork() will return -1 on error, the parent pid, OR ZERO for the child. So you are actually doing everything in the parent. Change to (fork() ==0) and it does what you want.

Also you should wait on your children and detach shared memory. (I added some output of the process ids to make it a little clearer.)

printf("I AM THE PARENT pid = %d\n", getpid()); *buf = 0; for(i=1; i<=20; ++i) { if((pid = fork()) == -1) { perror("fork"); exit(1); } if (pid == 0) { v(semid, 0); *buf += 1; p(semid, 0); printf("I am child %d with pid = %d\n", i, getpid()); shmdt(buf); exit(0); } } for (i = 1; i <= 20; ++i) { pid = wait(&status); printf("child pid = %d reaped\n", pid); } printf("buf: %p\n", buf); printf("*buf: %d\n", *buf); shmdt(buf); return 0;

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