Segmentation Fault with array of pointers

Question

For the following code:

#include<stdatomic.h>

int *sp;

int threadFunc()
{
    int *p;
    for(int i = 0; i < 10; i++){
        p = __atomic_load_n(&sp+i, __ATOMIC_SEQ_CST);
        printf("Value loaded = %d from %p", *p, p);
    }
    return 0;
}


int main(int argc, char *argv[])
{
    int a = 0;
    
    sp = malloc(sizeof(int)*10);
    if(sp == NULL){
        printf("Not enough memory\n");
        return -1;
    }
    
    // initialize the contiguous array pointed by sp with zero
    for(int i = 0; i < 10; i++){
        memcpy((void*)sp+i, &a, sizeof(int));
    }
    
    // call the following function on different thread
    threadFunc();
    
    return 0;
}

I am getting a segmentation fault in threadFunc(). The program prints correctly for i=0, but gives a segmentation fault for all i > 0. Where is it that I am going wrong?

You don't need to cast to void*. (and there is no array of pointers present in your program) — wildplasser
– wildplasser, Commented Sep 30, 2021 at 13:40
That int main{ looks wonky - what compiler are you using? If this is supposed to be standard C, please test to compile your code before posting. — Ted Lyngmo
– Ted Lyngmo, Commented Sep 30, 2021 at 13:43
Please put code up that you actually can compile yourself. If this actually compiles for you, please mention what compiler (and options) you are using. — Ted Lyngmo
– Ted Lyngmo, Commented Sep 30, 2021 at 13:45
&sp+i is not what you want. It takes the address of sp and adds i to it. But there are no other pointer after the single one. — the busybee
– the busybee, Commented Sep 30, 2021 at 13:47

Ted Lyngmo · Accepted Answer · 2021-09-30 14:30:51Z

3

You are supposed to supply a pointer to the object here:

_atomic_load_n(&sp+i, __ATOMIC_SEQ_CST);

This is not the correct thing:

&sp + i

The above takes the address of sp and adds i, pointing ... somewhere where you have no int* stored. Also, you want to load from an int, not an int*.

A fix would be:

int threadFunc() {
    int p;                                 // not an int*
    for(int i = 0; i < 10; i++){
        p = atomic_load_explicit(sp + i, memory_order_seq_cst); // not &sp + i
        // or:  p = __atomic_load_n(sp + i, __ATOMIC_SEQ_CST);
        printf("Value loaded = %d\n", p);
    }
    return 0;
}

^{(I just replaced the call with the corresponding standard call)}

Also, use memset to set what you allocated:

memset(sp, 0, sizeof(int) * 10);

Demo

edited Sep 30, 2021 at 14:30

answered Sep 30, 2021 at 14:00

Ted Lyngmo

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2 Comments

Resting Platypus Over a year ago

Thanks a lot for this! I tried a similar approach, but I was setting something like int **p followed by p = &sp and then replacing &sp+i by p[i], and that didn't work.

Ted Lyngmo Over a year ago

@RestingPlatypus You're welcome. I guess you may have made an addition to the pointer erroneously so that it ends up pointing out of bounds. All the **'s makes it tricky.

Ted Lyngmo · Accepted Answer · 2021-09-30 13:55:24Z

2

int main

will not compile. The minimum signature of the main prototype is:

int main(void);

Another problem in the code shown is incorrect use of memcpy(). It does not require a loop use:

Change this:

   for(int i = 0; i < 10; i++){
        memcpy((void*)sp+i, &a, sizeof(int));
   }

to this

memset(sp, 0, 10* sizeof(int));

Finally, int the expression:

int *p;
for(int i = 0; i < 10; i++){
    p = __atomic_load_n(&sp+i, __ATOMIC_SEQ_CST);
    printf("Value loaded = %d from %p", *p, p);
}

p is a pointer, but it points to some location that is not yet owned by your process. Writing to it is undefined behavior, and likely the cause of your seg-fault.

edited Sep 30, 2021 at 13:55

Ted Lyngmo

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answered Sep 30, 2021 at 13:50

ryyker

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