GATE CS 2001 Q2.18 — Programming in C

To identify which functions cause pointer-related problems, let us analyze each function individually:

1. Function $P1$:

   int *g(void)
   {
       int x = 10;
       return (&x);
   }

   The variable x is a local automatic variable allocated on the stack. When the function g() returns, its stack frame is deallocated, and x ceases to exist. Returning &x returns the address of a deallocated memory location, resulting in a dangling pointer. Accessing this pointer in the calling function leads to undefined behavior.

2. Function $P2$:

   int *g(void)
   {
       int *px;
       *px = 10;
       return px;
   }

   The pointer px is declared but not initialized, meaning it contains a garbage address (wild pointer). Attempting to dereference it with *px = 10 writes to an arbitrary memory location, which typically causes a segmentation fault (undefined behavior).

3. Function $P3$:

   int *g(void)
   {
       int *px;
       px = (int*) malloc (sizeof(int));
       *px = 10;
       return px;
   }

   This function dynamically allocates memory on the heap using malloc(). Heap-allocated memory persists until it is explicitly deallocated using free(). Therefore, returning px is perfectly safe and does not cause any immediate pointer-related errors (though the caller must eventually free it to avoid a memory leak).

Thus, only $P1$ and $P2$ cause pointer-related problems.