Segmentation fault after returning a pointer to a struct

Question

There is program for reading from file and return a struct.

struct ion_bin
{
    int freq;
    long height;
    int amplitude;
};

//Dynamic auto allocating array
typedef struct {
  struct ion_bin *array;
  size_t used;
  size_t size;
} Ionogram;

void freeArray(Ionogram *a); //free memory
void insertArray(Ionogram *a, struct ion_bin element); //realloc memory
void initArray(Ionogram *a, size_t initialSize); //malloc memory

Ionogram* read(int argn, char* argv[])
{
    FILE* stream;
    Ionogram ionogramObj;

    //fill ionogram from file by initArray and insertArray  
    //.....

    return &ionogramObj;
}

int main(int argn, char* argv[])
{
    Ionogram* r = read(argn, argv);

    fprintf(stderr,"Array size: %d Used %d\n",r->size, r->used); //SEGMENTATION FAULT ERROR
    //int second = (*(r->array + 2)).amplitude; //YET SEGMENTATION FAULT ERROR TOO

    //fprintf(stderr, "%d", second);
    return 0;
}

This program compile successfully, but in runtime and debug fires segmentation fault error (SIGSEGV) by attempt getting fields of returned struct (in main method) How to fix this error?

Answer 1

You make a beginners mistake, and return a pointer to a local variable. You got to remember that local variables goes out of scope once the function returns, and the pointers to it will then become invalid. Dereferencing this invalid pointer leads to undefined behavior.

Two possible solutions:

1. Actually return a structure, by value, and not a pointer.
2. Allocate memory for the structure using malloc, and return a pointer to this dynamically allocated memory.

Method one works well for smaller structures, like yours, but become inefficient for larger structures as the whole structure must be copied. (It's a shallow copy though, not a deep copy. So if you have pointers in the structure only the pointers are copied and not what they point to.)

Answer 2

You are returning a pointer to a variable that goes out of scope at the end of the function.

  Ionogram ionogramObj;
  return &ionogramObj;

That's undefined behaviour in C.

As an alternative, malloc the memory for your structure in the function and return the pointer to that. Don't forget to free the pointer at some point.