Dynamic array of int pointers causing segmentation fault

Question

I found a segmentation fault in my C code and could not find a good explanation or solution for it after searching.

This first code gives me segmentation fault after printing 0.

#include <stdlib.h>
#include <stdio.h>
int main() {
 int **defs = malloc(16 * sizeof *defs);
 int i;
 for (i = 0; i < 16; i++) {
 printf("%d\n", i);
 *defs[i] = i;
 }
 free(defs);
 return 0;
}

This second code works fine.

#include <stdlib.h>
#include <stdio.h>
int main() {
 int *defs = malloc(16 * sizeof defs);
 int i;
 for (i = 0; i < 16; i++) {
 printf("%d\n", i);
 defs[i] = i;
 }
 free(defs);
 return 0;
}

These are just examples, not my actual code. I also tried doing pointer arithmetic but same result. Could someone please explain this? Thank you.

• n. m. could be an AI: *defs[i]. Before doing that, you need to initializedefs[i] to point somewhere.
• M.M: Pointers must point somewhere before being dereferenced

Answer 1

In first code you have not allocated each of the int* memory blocks. So, before assigning values to defs[i], you have to populate it with memory of type int*.

defs[i] = (int*)malloc(sizeof(int) * number_of_elements);

And then defs[i][some_index] = value.

• Munir: In my case, adding def[i] = malloc(sizeof(int)); in the loop worked. Thanks.
• Parnab Sanyal: your def[i] can only store 1 element....But you can store more than 1 element. But what you should always avoid....segmentation fault can occur.
• Kyle Strand: @Munir ...um. Why are you allocating individual ints on the heap, exactly? Why are you even creating an array of pointers instead of an array of ints, and why are you using malloc instead of new?
• Munir: @KyleStrand I don't know the initial size of the array. So I can't do it on the stack. I need the array of pointers because in my program, the data is read from a file and shared across the program. Also, this is C, not C++. So there is no new or delete, only malloc and free.
• Kyle Strand: @Munir Ah, could have sworn I saw you including iostream--sorry. But I don't understand what the relationship is between (1) keeping pointers rather than raw ints in your array, (2) reading data from a file, and (3) sharing data throughout the program. Those three issues seem entirely unrelated.