Question

I am struggling really hard to understand this behavior so maybe someone can shed some light on the situation.
I simply can’t figure out why I can’t return a pointer to a struct from a method and expect to be able to still re-use it afterwards.

As you can see the generateSmallMatrix() method creates an int[] array and sets it inside the ysmf struct that I then return to main. Main then takes the ysmf* and calls printArray (again). And on the third try the array cannot be retrieved any more..
It’s driving me crazy..

I have checked with my Eclipse debugger that on all calls the location of matrix->A is identical (0x7fffffffe180 – so for all I know about C pointers any form of accessing that int should return the correct value – be it *(ax++) or ax[i]) .. but neither do..

Very frustrating to say the least, so here is the code:

#include <stdio.h>
#include <stdlib.h>

typedef struct sparseMatrix
{
    int* A;
} ysmf;

void printArray(int* ax, int length) {
    int i = 0;
    for (i = 0; i < length; i++) {
        printf("%i,", ax[i]);
    }
    printf("\n");
}

ysmf* generateSmallMatrix()
{
    ysmf *matrix = malloc(sizeof(ysmf));

    int a[] = {1,2,3,9,1,4};
    printArray(a, 6);                   // returns 1,2,3,9,1,4,
    matrix->A = a;
    printArray(matrix->A, 6);           //returns 1,2,3,9,1,4,
    //printArray(matrix->A, 6);

    return matrix;
}


int main(void) {
    ysmf* matrix = generateSmallMatrix();
    printArray(matrix->A, 6);           //returns 1,6,-7856,32767,1,4,
    return EXIT_SUCCESS;
}

You can see the problem occuring where I have inserted the comments.

I know this is probably something totally basic I shouldn’t have missed..

Answer 1

Because a is a local array, whose lifetime ends when the generateSmallMatrix() function ends. Accessing it after that results in undefined behaviour.