Thread: Segmentation fault; struct and pointer problem

Hello...

I try to create some simple posix shared memory programmed that will take some value... then
I facing with some segmentation fault... I only have basic knowledge about struct and pointer... only learn by theory.. never used it before.. please advice

My structure

Code:

typedef struct {
    unsigned int shm_hour;
    double *shm_adc;
    unsigned int *shm_dio;
/*    pid_t pid; */
} shmem_t;

My idea is to store the data inside the struct.. shm_hour only have one value but shm_adc and shm_dio will have multiple input value...

Here some of my declaration...

Code:

...
shmem_t *data;
double *shm_adc = NULL;
unsigned int *shm_dio = NULL;

int main(int argc, char *argv[])
{
...

    /* attach/map shared memory to our data type */
    data = (shmem_t *) shmat(shmid, NULL, 0);
...
data->shm_hour = 13;        /* data is the pointer to the struct */
data->shm_adc[0] = 0.01;
data->shm_adc[1] = 1.23;
data->shm_adc[3] = 3.33;
data->shm_adc[2] = 1.52;
data->shm_adc[7] = 2.58;
data->shm_dio[1] = 1;
data->shm_dio[3] = 0;
...

And after debugging.. I know the problem came from shm_adc.. maybe I have wrong declaration and wrong concept.. Really appreciate if I could get some little help from all you guys..

Code:

Program received signal SIGSEGV, Segmentation fault.
0x080484e4 in main (argc=1, argv=0xbffff3b4) at shm_data.c:50
50    data->shm_adc[0] = 0.01;

When you write

Code:

double *shm_adc;

you allocate memory for a pointer(actually an int,because the pointer memory block contains the address of where the pointer points to).The shm_adc points to nothing.So you have no permission to access where this pointer points to.You have to allocate memory,with malloc ( check for ref in google),then you can write back to this memory block.When you do not need this memory of block,you must de-allocate it with the function free. Give it a shot,and post back if needed

Please show how you are allocating memory for your pointers. It really looks like the variables in your structure should be arrays, not pointers. After all you should know how many adc and dio channels you have.

Jim

Originally Posted by std10093

When you write

Code:

double *shm_adc;

you allocate memory for a pointer(actually an int,because the pointer memory block contains the address of where the pointer points to).The shm_adc points to nothing.So you have no permission to access where this pointer points to.You have to allocate memory,with malloc ( check for ref in google),then you can write back to this memory block.When you do not need this memory of block,you must de-allocate it with the function free. Give it a shot,and post back if needed

So I point my shm_adc to a "NULL" is totally wrong??
Do I really need malloc? Ok.. I will google about malloc.. but If I could get some more explanation or hint.. It could be great..
Thanks for reply..

As the others pointed out, your problem is because you declared shm_adc as a pointer. Unfortunately, pointers don't work across shared memory, since that shared memory may be mapped to a different logical address in each process that uses it. For example, it may be at 0x4000 in process A and 0x5000 in process B, so if A stores it as 0x4000, then B tries to access data there, it may cause a seg fault in B (if that's an invalid address), or give bogus data. There are workarounds for that (storing an offset from the start of the shared memory segment and calculating when you read it out), but I think Jim's idea is best, just use an array of fixed size, since you probably know how big it will be ahead of time.

Originally Posted by sangelion

So I point my shm_adc to a "NULL" is totally wrong??
Do I really need malloc? Ok.. I will google about malloc.. but If I could get some more explanation or hint.. It could be great..
Thanks for reply..

You set the pointer to NULL,then you allocate memory and then you set the pointer to point into that memory of block.I could explain more,but i agree with the others two.Using jim's way is easier

Then... I will try to use jim's way... thank you for all of your suggestion... If I still have a problem after changing to array I will point it here...

anduril462: I really don't know that I cannot define a pointer across shared memory.. thanks..

Problem solved.. thanks guy