Segment Fault with shared memory set using mmap

Hi,

My code is segment faulting on line 50 (*ptr = 10)

I can't understand why, could someone explain ?

Code:

(Headers here)

int main()
{

    // Close the shared memory we use just to be safe
    shm_unlink("/shared_mem");
    // Set up some shared memory where we can record the total amount of bytes sent by all processes
    int shmfd;
    
    if( (shmfd = shm_open("/shared_mem", O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR)) == -1)
    {
    	fprintf(stderr, "Error opening shared memory in recordTotalBytes() \n");
		exit(EXIT_FAILURE);
    }
    
    // Set the size of the shared memory to 1 long
    ftruncate(shmfd, sizeof(long) *1 );
    // Map the shared memory into our address space
    long *ptr;
    ptr = mmap(NULL, sizeof(long) *1, PROT_READ | PROT_WRITE, MAP_SHARED, shmfd, 0); 

	// Create a mutex and initalise it
	pthread_mutex_t * muxlock;
	pthread_mutex_init(&muxlock, NULL);
	
	// Lock the mutex some work is going to happen with shared resources
	pthread_mutex_lock(&muxlock);
	
	
	
	
	// Set ptr to 10
	ptr* = 10;
	
	printf("%ld", *ptr);
	// Unlock the mutex
	pthread_mutex_unlock(&muxlock);

	

}

Thanks !

Yep that's going to happen... because ptr is already a pointer so no need for the * and 10 is almost certain to be an OS reserved address.

Oddly enough you had it right in your question above... *ptr = 10;... will set the value at the address in ptr to 10.

Well, for one thing, I guess you should be checking the return value of mmap() to see if it's NULL. My guess it the call is failing. I'm also guessing that "ptr* = 10" is just a copy/paste error.

Hi

Thanks for the replies

I changed it to

*ptr = 10

and it still segfaults


Also I added a check to see if ptr is null and its not

Did you check ptr against MAP_FAILED?

Upon successful completion, the mmap() function shall return the address at which the mapping was placed ( pa); otherwise, it shall return a value of MAP_FAILED and set errno to indicate the error. The symbol MAP_FAILED is defined in the <sys/mman.h> header. No successful return from mmap() shall return the value MAP_FAILED.

Just checked it against MAP_FAILED

And it IS failing.

Any idea why ?


Thanks

Well, I know one way you can find out. Read what I quoted previously again:

otherwise, it shall return a value of MAP_FAILED and set errno to indicate the error.

Looks like you can check errno to find out.

The errno is 0

Is that the first entry under errors on this page?

mmap

[EACCES]
The fildes argument is not open for read, regardless of the protection specified, or fildes is not open for write and PROT_WRITE was specified for a MAP_SHARED type mapping.

errno 0 means "no error," you probably aren't checking it in the right place.

EDIT: Also, doesn't the length parameter have to be a multiple of the page size? You can't map a single long.

I have

Code:

	
if( ptr = MAP_FAILED )
{
	fprintf(stderr, "Error setting shared memory in recordTotalBytes() error is %d \n ", errno);
	exit(EXIT_FAILURE);
}

Is this the right way to do it ?


Thanks

Ugh. You are assigning instead of comparing.

Sorry, stupid mistake.

mmap isn't returning MAP_FAILED

Fixed it !

I wasn't allocating any memory for the mutex lock and was passing in &muxlock when it should have just been muxlock


Not sure why it waited until line 50 to segfault though