Rotate a String in C

str_rot.c

char *rotateString(char *str, int count) {
    size_t len = strlen(str);
    int counter;
    for(counter=0;counter < len;counter++) {
        int value = str[counter];
        if((char)value != ' ') {
            value = value + count;
            str[counter] = value;
        }
    }
    return str;
}