Passing around int pointers in structs - c

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.

Answer 2

This line
ysmf *matrix = malloc(sizeof(ysmf));
Allocates only enough space for your ysmf struct, which in this case, is just an int pointer. So you then go and point that at 'a'.
matrix->A = a;
The problem is that a is on the stack. So it is now pointing at this memory address but when you exit the function that memory is no longer reserved for the array.
If you instead malloced memory for 'a' and pointed your struct at that, then you would be OK.

Answer 3

Try this...
ysmf* generateSmallMatrix()
{
ysmf *matrix = malloc(sizeof(ysmf));
int a[] = {1,2,3,9,1,4};
printArray(a, 6);
matrix->A = malloc(sizeof(int) * 6);
memcpy(matrix->A, a, sizeof(int) * 6);
printArray(matrix->A, 6);
return matrix;
}
Now outside of the generateSmallMatrix function you should be able to print the correct values. However, be sure to free what was malloc'd.
int main(void) {
ysmf* matrix = generateSmallMatrix();
printArray(matrix->A, 6);
free(matrix->A);
free(matrix);
return EXIT_SUCCESS;
}
Hope this helps.