Thread: how to read current semaphore value

Hi all. I'm using semaphore for processes.
Here a simple code:

Code:

...
//include declarations
...

union semun {
  int val;
  struct semid_ds* buf;
  unsigned short * array;
  struct seminfo* __buf;
};


int main() {
  //allocating&init semaphore
  int sem_id;
  union semun argument;
  unsigned short int values[1];
  values[0] = 12;
  argument.array = values;
  semget(IPC_PRIVATE,1,IPC_CREAT|IPC_EXCL|S_IRUSR|S_IWUSR);
  semctl(sem_id,0,SETALL,argument);

  //at this point the sem value is 12
  //now i want read this value
  ...HOW?...
  return 0;
}

In man semctl page I read:

GETALL Return semval (i.e., the current value) for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have read permission on the semaphore set.

So I write some code like this:

Code:

...
union senum arg;
//debug line: printf("%d",arg.array[0]);
semctl(sem_id,0,GETALL,arg);
printf("%d",arg.array[0]);

but the function semctl seems to have no effects: in fact the debug line printf prints the same value of the second printf...
what's the problem?

thank you!

i response to myself.....
i don't init the sem_id value, just call the semget loosing the return value. so:

Code:

sem_id=semget(IPC_PRIVATE,1,IPC_CREAT|IPC_EXCL|S_IRUSR|S_IWUSR);

damn me!

yes...and then?
I use semun array...have a look to my code

first of all i must set arg.array if i use SETALL...with GETALL cmd i want the information into senum argument...

then -reapet- I can retrieve the current value of the sem with these lines:

Code:

union semun arg;
semctl(sem_id,0,GETALL,arg);
printf("<<<<<<%d>>>>>>>\n",arg.array[0]); //RIGHT VALUE

but if i use my ontop get_semvalue function the program doesn't work...

Originally Posted by np2k

first of all i must set arg.array if i use SETALL...with GETALL cmd i want the information into senum argument...

then -reapet- I can retrieve the current value of the sem with these lines:

Code:

union semun arg;
semctl(sem_id,0,GETALL,arg);
printf("<<<<<<%d>>>>>>>\n",arg.array[0]); //RIGHT VALUE

but if i use my ontop get_semvalue function the program doesn't work...

You may, by being very unlucky, get the answer you want here. That doesn't mean this snippet is correct.

Code:

union semun arg;
arg.array =malloc(some_number_of_bytes); /*how nice to actually have somewhere to store the answer*/
semctl(sem_id,0,GETALL,arg);

Originally Posted by np2k

1. I don't understand....why should be my work allocating the space for argument.array? It should be responsibility of the semctl function allocate this space...

That's pretty much against the whole spirit of nearly all *nix system functions ever created. You give them the memory, and they put the information where it belongs.

Originally Posted by tabstop

That's pretty much against the whole spirit of nearly all *nix system functions ever created. You give them the memory, and they put the information where it belongs.

ok, but...there is the question 2...without malloc the things work....

I'm not on a linux system so I can't check. What if you do arg.array = 2 before the call?

If you are (un)lucky that the random value in arg.array is large enough to be reasonably interpreted as a pointer, then your just overwriting some other chunk of memory somewhere else.

Originally Posted by tabstop

I'm not on a linux system so I can't check. What if you do arg.array = 2 before the call?

seg fault

If you are (un)lucky that the random value in arg.array is large enough to be reasonably interpreted as a pointer, then your just overwriting some other chunk of memory somewhere else.

:S it seems very strange...

Originally Posted by np2k

:S it seems very strange...

Only if somehow you've gotten to this point in your programming experience without learning/wondering that data has to be stored somewhere in order to be used. An int* is not room to store an array, it merely indicates an address of where some memory can be found.