allocating array and assigning values from a function C

Question

C question

Hi,

I am passing a double pointer to a function to allocate a double array and initialise the array inside the function with lets say 10.10;

I do the following but get segmentation fault when I access the array in main;

void function(double **array, int size){

    *array = (double*) malloc(size * sizeof(double)); 
    int i;
    for(i=0;i<size;i++){
    *array[i] = 10.10;
    }


}

int main(){

    double *array = NULL;

    function(&array,20);
    printf("array[0] = %lg\n",array[0]);// here is where I get segmentation fault


}

Any help ?

"here is where I get segmentation fault" is not on the line where you get the segfault. — user529758
– user529758, Commented Jan 18, 2013 at 14:48
as a sidenote: the cast to double* is a) unnecessary, and b) wrong, because what you really have there is double** — Andreas Grapentin
– Andreas Grapentin, Commented Jan 18, 2013 at 14:49

Paul R · Accepted Answer · 2013-01-18 14:51:53Z

4

You have fallen foul of operator precedence.

Change:

for(i=0;i<size;i++){
    *array[i] = 10.10;
}

to:

for(i=0;i<size;i++){
    (*array)[i] = 10.10;
}

Note that if you had compiled with warnings enabled (e.g. gcc -Wall ...) your compiler would have caught this for you. Always compile with warnings enabled and always pay heed to, understand and fix any warnings.

edited Jan 18, 2013 at 14:51

answered Jan 18, 2013 at 14:46

Paul R

214k38 gold badges402 silver badges579 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

score 2 · Accepted Answer · 2013-01-18 14:57:16Z

2

*array[i]

doesn't mean what you think it does (look it up using a C operator precedence table).

Instead of unreadable, ugly and confusing (yes, it just confused you) code, use a temporary variable (and do not for the love of God cast the return value of malloc!):

void function(double **array, int size)
{
    if (array == NULL) return;

    double *tmp = malloc(size * sizeof(*tmp)); 
    if (tmp == NULL) {
        *array = NULL;
        return;
    }

    int i;
    for (i = 0; i < size; i++) {
        tmp[i] = 10.10;
    }

    *array = tmp;
}

Also, return 0; from main(). Really.

edited Jan 18, 2013 at 14:57

answered Jan 18, 2013 at 14:50

user529758

4 Comments

Ed Heal Over a year ago

Perhaps a *array = NULL in the event of malloc failure would not go amiss.

user529758 Over a year ago

@EdHeal And a check for the address of the output argument being valid (non-NULL) as well :)

Lundin Over a year ago

return 0 from main() is good style, but not necessary since C99.

user529758 Over a year ago

@Lundin I didn't suggest it because it was necessary, but because it's good practice, as you suggested.

Collectives™ on Stack Overflow

allocating array and assigning values from a function C

C question

2 Answers 2

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

C question

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related