linux内核细节－va_arg、va_end、va_start、printf

#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类型的字节数，获得新地址
va_start(ap, A)：ap地址+字符串A的字节数，获得新地址

int printf(const char *fmt, ...){    char printf_buf[1024];    va_list args;    int printed;    va_start(args, fmt);    printed = vsprintf(printf_buf, fmt, args);//搜索字符串fmt中需要特定模式的字符(比如%s),args为后面参数的首地址    va_end(args);    puts(printf_buf);    return printed;}

…表示不让编译器做类型检查，可以添加多个变量

下面的代码很容易懂，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;}