printf的实现，va_list读取参数，vsprintf进行将%s,%d这些参数的值转化为字符串并一起输出到缓存区

void log_printf(const char * fmt, ...) {
    char str_buf[128];
    va_list args;

    kernel_memset(str_buf, '\0', sizeof(str_buf));

    va_start(args, fmt);
    kernel_vsprintf(str_buf, fmt, args);
    va_end(args);

    // 显示，如果发送速度太慢，会造成这里关中断太长时间
    // 所以，这里这样做不是好办法
    mutex_lock(&mutex);
}

void kernel_vsprintf(char * buffer, const char * fmt, va_list args) {
    enum {NORMAL, READ_FMT} state = NORMAL;
    char ch;
    char * curr = buffer;
    while ((ch = *fmt++)) {
        switch (state) {
            // 普通字符
            case NORMAL:
                if (ch == '%') {
                    state = READ_FMT;
                } else {
                    *curr++ = ch;
                }
                break;
            // 格式化控制字符，只支持部分
            case READ_FMT:
                if (ch == 'd') {
                    int num = va_arg(args, int);
                    kernel_itoa(curr, num, 10);
                    curr += kernel_strlen(curr);
                } else if (ch == 'x') {
                    int num = va_arg(args, int);
                    kernel_itoa(curr, num, 16);
                    curr += kernel_strlen(curr);
                } else if (ch == 'c') {
                    char c = va_arg(args, int);
                    *curr++ = c;
                } else if (ch == 's') {
                    const char * str = va_arg(args, char *);
                    int len = kernel_strlen(str);
                    while (len--) {
                        *curr++ = *str++;
                    }
                }
                state = NORMAL;
                break;
        }
    }
}