str_upcase.c

#include <stdlib.h>
#include <string.h>

char *
str_upcase(char *source)
{
    int i     = 0;
    int shift = 0;
    char ch   = '\0';

    int buffer_size = strlen(source) + 1;
    char *output    = malloc(buffer_size * sizeof(char));

    for (i = 0; i < buffer_size; i++) {
        ch = *(source + i);

        if (ch >= 97 && ch <= 122) {
            shift = -32;
        } else {
            shift = 0;
        }

        *(output + i) = ch + shift;
    }

    return output;
}