Programming exercise 6-3 in chapter 6 of Pointers on C.

del_str.c

#include <stdio.h>
#include <stdlib.h>
#include <String.h>

/* 
 * ===  FUNCTION  ======================================================================
 *         Name:  del_str
 *  Description:  Delete substring in the source string
 * =====================================================================================
 */
 int
del_str (char str[], char const *substr )
{
  int index_start = 0;
  int index_end = 0;
  char *iterator;

  // Find index of first char
  for (iterator = str; *iterator != *substr; iterator++) {
    index_start++;
  }

  for (index_end = index_start; *substr != '\0'; substr++) {
    if (*iterator++ != *substr) {
      break;
    }
    
    index_end++;
  }

  if (*substr != '\0') return 0;

  int length = strlen(str);

  // Add characters from index_end to the start position of substr
  for (int i = index_end; i < length; i++) {
    str[index_start++] = *(str + index_end++);
  }

  str[index_start] = '\0';

  // Match rest of characters
  return 1;
}		/* -----  end of function del_str  ----- */


/* 
 * ===  FUNCTION  ======================================================================
 *         Name:  main
 *  Description:  
 * =====================================================================================
 */
  int
main ( int argc, char *argv[] )
{
  char string1[] = "ABCDEF GXYZ";
  char *sub_str = "DEF";

  int final = del_str(string1, sub_str);
  printf("Result: %d\nFinal String: %s\n", final, string1);

  return EXIT_SUCCESS;
}				/* ----------  end of function main  ---------- */