up.c

#include <stdio.h>

char *up(const char *);

int main(){
 const char *string;
 string = "hello world";
 printf("%s", up(string));
 return(0);
}

char *up(const char *s){
 char *p, *start;
 void *malloc();
 start = p = (char *)malloc(sizeof s);
 while(*s != 0){
  if(*s > 96 && *s < 123)
   *p = *s - 32;
  else
   *p = *s;
  p++; s++;
 }
 *++p = 0; /* end *p */
 p = start;
 return(p);
}
 

output:
HELLO WORLD