Returning local variable from a function

Question

In the following code, I return a pointer to a char array that is created locally inside the function. Therefore When the return value is assigned to y, it should all be garbage. This seems to hold true when I print %10s or %100s. But when I print %1000s, I get an output that seems to confound me.

#include <stdio.h>

char* get()
{
  char x[1000] ;

  int i = 0;
  for(; i < 999; ++i)
  {
    x[i] = 'A';
  }

  x[999] = '\0';

  printf("%1000s\n",x);

  return x;
  }

  int main()
  {
    char* y = get();
    printf("Going to print\n");
    printf("%1000s\n",y);
  }

The output is enter image description here

Is it a coincidence that the main() function is accessing the same memory location that was used to create the local function char array or is it something more concrete?

Any assumptions you make upon invoking undefined behavior are meaningless. That is the very nature of undefined behavior; there is *no definition*. Therefore, the very premise you started with "it should all be garbage" is itself grasping a leap of faith. See http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope/6445794#6445794 — WhozCraig, May 29 '15 at 07:31
Why not, `char x[1000] = {0};` then declare `char *get (char *x)` and finally call with `char *y = get (x);` — David C. Rankin, May 29 '15 at 07:41

Sourav Ghosh · Answer 1 · 2015-05-29T07:46:26.327

See, once you use the return value of get(), you're facing undefined behaviour.

After that, nothing is guranteed.

In other words, the output of printf("%1000s\n",y);^## statement cannot be justified. It is, what it is. Undefined.

FWIW, once the get()function has finsihed execution, the stack space allocated for that function is destroyed and available for usage (if required) by any other function. Maybe you're making unauthorized entry into that part of the physical memory, nothing certain, though.

^{## - or, printf("%10s\n",y); or printf("%100s\n",y);, whatever, wherever y is accessed.}

Returning local variable from a function

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