trim.c

char *trim(char *s) {
    
    size_t length = strlen(s);

    while (length > 0) {
        if (s[0] == ' ')
            memmove(s, s + 1, --length);
        else if (s[length - 1] == ' ')
            s[--length] = '\0';
        else
            --length;
    }

    return s;
}

trim2.c

int iswhitespace(char c) {
    if (c == '\t') return 1;
    if (c == ' ')  return 1;
    return 0;
}

char *trim(char *s) {

    size_t length = strlen(s);

    while (length > 0) {
        if (iswhitespace(*s))
            memmove(s, s + 1, --length);
        else if (iswhitespace(s[length - 1]))
            s[--length] = '\0';
        else
            --length;
    }

    return s;
}