#define _bnd(X, bnd)            (((sizeof (X)) + (bnd)) & (~(bnd)))
#define va_arg(ap, T)           (*(T *)(((ap) += (_bnd (T, _AUPBND))) - (_bnd (T,_ADNBND))))
#define va_end(ap)              (ap = (va_list) NULL)
#define va_start(ap, A)         (void) ((ap) = (((char *) &(A)) + (_bnd (A,_AUPBND))))
 

va_arg(ap, T)：ap地址+T型別的位元組數，獲得新地址
va_start(ap, A)：ap地址+字串A的位元組數，獲得新地址

int printf(const char *fmt, ...)
{
    char printf_buf[1024];
    va_list args;
    int printed;

    va_start(args, fmt);
    printed = vsprintf(printf_buf, fmt, args);//搜尋字串fmt中需要特定模式的字元(比如%s),args為後面引數的首地址
    va_end(args);

    puts(printf_buf);

    return
 
 printed;
}

…表示不讓編譯器做型別檢查，可以新增多個變數

下面的程式碼很容易懂，多看看

int vsprintf(char *buf, const char *fmt, va_list args)
{
    int len;
    unsigned long num;
    int i, base;
    char *str;
    const char *s;

    int flags;      /* flags to number() */

    int field_width;    /* width of output field */
    int
 
 precision;      /* min. # of digits for integers; max
                   number of chars for from string */
    int qualifier;      /* 'h', 'l', or 'L' for integer fields */

    for (str = buf; *fmt; ++fmt) {
        if (*fmt != '%') {
            *str++ = *fmt;
            continue;
        }

        /* process flags */
        flags = 0;
          repeat:
        ++fmt;      /* this also skips first '%' */
        switch (*fmt) {
        case '-':
            flags |= LEFT;
            goto repeat;
        case '+':
            flags |= PLUS;
            goto repeat;
        case ' ':
            flags |= SPACE;
            goto repeat;
        case '#':
            flags |= SPECIAL;
            goto repeat;
        case '0':
            flags |= ZEROPAD;
            goto repeat;
        }

        /* get field width */
        field_width = -1;
        if (isdigit(*fmt))
            field_width = skip_atoi(&fmt);
        else if (*fmt == '*') {
            ++fmt;
            /* it's the next argument */
            field_width = va_arg(args, int);
            if (field_width < 0) {
                field_width = -field_width;
                flags |= LEFT;
            }
        }

        /* get the precision */
        precision = -1;
        if (*fmt == '.') {
            ++fmt;
            if (isdigit(*fmt))
                precision = skip_atoi(&fmt);
            else if (*fmt == '*') {
                ++fmt;
                /* it's the next argument */
                precision = va_arg(args, int);
            }
            if (precision < 0)
                precision = 0;
        }

        /* get the conversion qualifier */
        qualifier = -1;
        if (*fmt == 'h' || *fmt == 'l' || *fmt == 'L') {
            qualifier = *fmt;
            ++fmt;
        }

        /* default base */
        base = 10;

        switch (*fmt) {
        case 'c':
            if (!(flags & LEFT))
                while (--field_width > 0)
                    *str++ = ' ';
            *str++ = (unsigned char)va_arg(args, int);
            while (--field_width > 0)
                *str++ = ' ';
            continue;

        case 's':
            s = va_arg(args, char *);
            len = strnlen(s, precision);

            if (!(flags & LEFT))
                while (len < field_width--)
                    *str++ = ' ';
            for (i = 0; i < len; ++i)
                *str++ = *s++;
            while (len < field_width--)
                *str++ = ' ';
            continue;

        case 'p':
            if (field_width == -1) {
                field_width = 2 * sizeof(void *);
                flags |= ZEROPAD;
            }
            str = number(str,
                     (unsigned long)va_arg(args, void *), 16,
                     field_width, precision, flags);
            continue;

        case 'n':
            if (qualifier == 'l') {
                long *ip = va_arg(args, long *);
                *ip = (str - buf);
            } else {
                int *ip = va_arg(args, int *);
                *ip = (str - buf);
            }
            continue;

        case '%':
            *str++ = '%';
            continue;

            /* integer number formats - set up the flags and "break" */
        case 'o':
            base = 8;
            break;

        case 'x':
            flags |= SMALL;
        case 'X':
            base = 16;
            break;

        case 'd':
        case 'i':
            flags |= SIGN;
        case 'u':
            break;

        default:
            *str++ = '%';
            if (*fmt)
                *str++ = *fmt;
            else
                --fmt;
            continue;
        }
        if (qualifier == 'l')
            num = va_arg(args, unsigned long);
        else if (qualifier == 'h') {
            num = (unsigned short)va_arg(args, int);
            if (flags & SIGN)
                num = (short)num;
        } else if (flags & SIGN)
            num = va_arg(args, int);
        else
            num = va_arg(args, unsigned int);
        str = number(str, num, base, field_width, precision, flags);
    }
    *str = '\0';
    return str - buf;
}