creating array of semaphores

[ravindra koranga]

I am creating an array of semaphores as follows. it compiles correctly but gives" segmentation fault "error on execution.Can anyone tell me why?

sem_t *s;	
	sem_t **sema = mmap(NULL,  sizeof(s)*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
	if (sema == MAP_FAILED) 
	{
		perror("mmap");
		exit(EXIT_FAILURE);
	}


	 // create/initialize semaphore
	for(i=0;i<numofsem;i++)
	{	printf("initializing semaphore %d",i);
		if ( sem_init(sema[i], 1, 0) < 0) //<<<<<---------------------------SEGMENTATION FAULT OCCURS HERE;		{
			perror("sem_init");
			exit(EXIT_FAILURE);
		}
	}

[Stephan van Hulst]

What does sema[i] point to?

[Henry Wong]

ravindra koranga wrote:I am creating an array of semaphores as follows.

Actually, you didn't allocate (mmap) the space for an array of semaphores. You allocated (mmap) the space for an array of semaphore pointers. And of course, you also never initialize those pointers.

Henry

[ravindra koranga]

OK I got it. I should do it like below and it works

sem_t *sema = mmap(NULL,  sizeof(sema)*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (sema == MAP_FAILED) 
	{
		perror("mmap");
		exit(EXIT_FAILURE);
	}


	 // create/initialize semaphore
	for(i=0;i<taskqlines;i++)
	{	if ( sem_init(sema+i, 1, 0) < 0) 
		{
			perror("sem_init");
			exit(EXIT_FAILURE);
		}
	}/*

[Henry Wong]

ravindra koranga wrote:OK I got it. I should do it like below and it works

The mmap size is still wrong. You allocated enough room for 100 semaphore pointers -- which is very different than room for 100 semaphores.

Henry

[ravindra koranga]

how about now

sem_t sema = mmap(NULL,  sizeof(sema)*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (sema == MAP_FAILED) 
    {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
 
 
     // create/initialize semaphore
    for(i=0;i<taskqlines;i++)
    {   if ( sem_init(&sema+i, 1, 0) < 0) 
        {
            perror("sem_init");
            exit(EXIT_FAILURE);
        }
    }/*

[Henry Wong]

Well, since the mmap() function returns a pointer, and a pointer isn't assignable to a sem_t, then the new code shouldn't compile, right?

Also, why do you actually need a declared variable to take the size of? Why can't you just use "sizeof(sem_t)"?

Henry

[ravindra koranga]

Is this right

sem_t *sema = mmap(NULL,  sizeof(sema*)*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (sema == MAP_FAILED) 
    {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
 
 
     // create/initialize semaphore
    for(i=0;i<taskqlines;i++)
    {   if ( sem_init(sema+i, 1, 0) < 0) 
        {
            perror("sem_init");
            exit(EXIT_FAILURE);
        }
    }/*

[ravindra koranga]

sorry, the previous code is not right.
Here is the correct one

sem_t *sema = mmap(NULL,  sizeof(sem_t *)*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (sema == MAP_FAILED) 
    {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
 
 
     // create/initialize semaphore
    for(i=0;i<taskqlines;i++)
    {   if ( sem_init(sema+i, 1, 0) < 0) 
        {
            perror("sem_init");
            exit(EXIT_FAILURE);
        }
    }/*

[Stephan van Hulst]

That is still not correct.

You're creating enough space for 100 semaphore pointers. You need space for 100 semaphores.

[ravindra koranga]

I think I cannot figure it out. Will you please guide me?

[Stephan van Hulst]

Do you want an array of semaphores, or an array of semaphore pointers?

I'm going to assume the first, because the second will introduce a layer of indirection that you probably don't need.

That means that if you want to have 100 semaphores in memory, you need to map memory the size of 100 semaphores. How can you get the size of one semaphore?

After you've done that, you'll have a pointer to a block of memory that has enough space for 100 semaphores. You just need to initialize them in their respective places.

sem_init() requires a pointer to the location where the semaphore will be initialized. You can use pointer arithmetic on the memory block pointer to get a pointer to the semaphore that you want to initialize:
sem_t* semaphores = mmap(...); for (...) {  sem_t* semaphore_location = semaphores+i;  ... }

[ravindra koranga]

Is this code correct?

sem_t *sema = mmap(NULL,  sizeof(sem_t )*100,PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (sema == MAP_FAILED) 
    {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
  
    for(i=0;i<100;i++)
    {   if ( sem_init(sema+i, 1, 0) < 0) 
        {
            perror("sem_init");
            exit(EXIT_FAILURE);
        }
    }/*

[Stephan van Hulst]

Did you test it?