Named Semaphores in child processes confusing me - c

Question

I have this very simple program to test out named semaphores:
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <sys/ipc.h>
#include <unistd.h>
#include <sys/shm.h>
#include <fcntl.h>
sem_t thing;
void processOne() {
sleep(1);
int l;
sem_getvalue(&thing, &l);
printf("processOneThing: %d\n", l);
}
void processTwo(){
sem_wait(&thing);
printf("done");
}
int main(int argc, char **argv) {
int pidOne, pidTwo;
thing = *sem_open("/testest", O_CREAT|O_EXCL,0777,1);
pidTwo=fork();
if(pidTwo==0){
processTwo();
}
else{
pidOne=fork()
if(pidOne==0){
processOne();
}
else{
}
wait(&pidTwo);
wait(&pidOne);
}
sem_unlink("/testest");
sem_close(&thing);
}
The output is:
doneprocessOneThing: 1
Which means that the second process decremented the semaphore however the value of it in the first child process is still 1...
I have no idea what I am doing wrong and I looked at the named semaphore documentation and could not find much that could help me with this issue.
I'm compiling using:
gcc test.c -pthread
Any and all help would be much appreciated.

Answer 1

First, the code is missing a ; at end of this line:
else{
pidOne=fork() <------ here
if(pidOne==0){
processOne();
}
And it fails to #include<sys/wait.h> for wait() prototype. But correcting these issues I get the same output as you. So what is happening? The issue is with how you created the semaphore:
sem_t thing;
...
thing = *sem_open("/testest", O_CREAT|O_EXCL,0777,1);
The return from sem_open is the address of the created semaphore. But you are dereferencing that address and placing a copy of the contents into your variable thing. That means thing isn't actually the semaphore you created, but just a copy of the struct holding state information. The upshot is that interacting with thing in your code isn't actually interacting with what you intended. (In fact, you might get some indication that something isn't right if you check the return values on sem_wait() and sem_getvalue().)
If you instead make thing a pointer to the semaphore and interact with that, you get the following output:
done
processOneThing: 0
which is I think what you expected to see. You can try the corrected code here:
Runnable corrected code