Why is this malfunctioning?-CodePudding

I have this program in C:

#include <stdio.h>
#include <string.h>
#include <stdint.h>

typedef struct sl{
  int32_t length;
  int32_t* arr;
} Selector;

void somefunction(Selector* temp){
  temp->length = 10;
  temp->arr = (int32_t*)malloc(temp->length * sizeof(int32_t));

  for(int i=0; i<temp->length; i  ){
    temp->arr[i] = i*i;
  }
}

int main () {
  Selector* sel;

  // Make changes to struct from other function
  somefunction(sel);

  // Print each element
  for(int i=0; i<sel->length; i  ){
    printf("Content of index %d: %d\n",i,sel->arr[i]);
  }
  printf("\n");

  return(0);
}

I run it in PowerShell with: gcc .\stest.c; .\a.exe, and it works fine:

Content of index 0: 0
Content of index 1: 1
Content of index 2: 4
Content of index 3: 9
Content of index 4: 16
Content of index 5: 25
Content of index 6: 36
Content of index 7: 49
Content of index 8: 64
Content of index 9: 81

But if I change int main() to this:

int main () {
  Selector* sel;

  // Make changes to struct from other function
  somefunction(sel);

  // Print each element
  for(int i=0; i<sel->length; i  ){
    printf("Content of index %d: %d\n",i,sel->arr[i]);
  }
  printf("\n");

  // ============= ADDED CODE BELOW ============= //
  // Change each element a bit
  for(int i=0; i<sel->length; i  ){
    sel->arr[i] = sel->arr[i]   10;
  }

  // Print each element again
  for(int i=0; i<sel->length; i  ){
    printf("Content of index %d after change: %d\n",i,sel->arr[i]);
  }
  printf("\n");
  // ============= ADDED CODE ABOVE ============= //
  return(0);
}

Suddenly it just gets a segmentation fault? Why? I didn't use the stack and overload it, I used malloc for the small arrays, it's not passing around references again like into the function to make memory go missing or something. Why doesn't this work? And how am I supposed to do it otherwise?

CodePudding user response：

Credit goes to the guys in the comments who gave the solution.

The problem was caused by undefined behavior, because the pointer was not set to anything.

  int main() {

  Selector sel;

  // Make changes to struct from other function
  somefunction(&sel);

  // Print each element
  for(int i=0; i<sel.length; i  ){
    printf("Content of index %d: %d\n",i,sel.arr[i]);
  }
  printf("\n");

  // ============= ADDED CODE BELOW ============= //
  // Change each element a bit
  for(int i=0; i<sel.length; i  ){
    sel.arr[i] = sel.arr[i]   10;
  }

  // Print each element again
  for(int i=0; i<sel.length; i  ){
    printf("Content of index %d after change: %d\n",i,sel.arr[i]);
  }
  printf("\n");
  // ============= ADDED CODE ABOVE ============= //
  return(0);
}

To summarize it, for posterity and others running into the same problem:

Don't make "sel" a pointer.
Send "sel" as a reference.
Change member-variable access accordingly.