convert to uppercase

Write a function upper() that accepts a string and converts all its characters into uppercase characters 
as example upper("c programming")=C PROGRAMMING

	#include <stdio.h>
	#include <string.h>
	#include <stdbool.h>
	bool is_upper(char x)
	{
	if((int)x>=65&&(int)x<=90)
	{
	return true;
	}
	return false;
	}
	char *upper(char *base)//passing array to a function that returns a pointer to char
	{
	while(*base!='\0')
	{
	if(is_upper(*base)||*base==' '){*base++;}
	else{*base-=32;base++;}
	}
	return base;
	}
	int main()
	{
	char array[20]="hE l lO";
	upper(array);
	puts(array);
	return 0;
	}

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