Example of writing a program that will mimic chmod behavior. NOTE: This is overly simple, not robust, & hardcoded/derived. Much more would be needed for a useful program.

simple_chmod.c

/*See man -s 2 chmod for more information */
#include <stdio.h> 
#include <sys/stat.h>

int main(void){
  /*mode_t mode = S_IXOTH */
  mode_t mode = 00755; /*Replace this with the actual mode you want. Ideally you'd want to ask the user */
  
  chmod("hi.txt", mode); /*This assumes hi.txt is present. You'd want to prompt the user for a filename in a real scenario */

  return 0;
}

/*gcc simple_chmod.c; ./a.out */