MIT OS 4. printf vsprint fprintf 格式化输出 @ Lab1
关于可变参数这里讲的很明白： http://www.cppblog.com/qiujian5628/archive/2012/02/01/41562.html
主要是C语言函数调用参数入栈顺序和va_list宏。
--> va_start(ap,所有参数中第一个参数) ap指向参数列表，是一个地址。
--->va_arg(ap,type) ap改变，指向下一个参数，type是当前参数的类型
--->va_end 清除ap，就是ap指向NULL
另外还有一个扩展宏 va_cp。
Macro: void __va_copy (va_list dest, va_list src)
-
The __va_copy macro allows copying of objects of type va_list even if this is not an integral type. The argument pointer in dest is initialized to point to the same argument as the pointer in src.
This macro is a GNU extension but it will hopefully also be available in the next update of the ISO C standard.
If you want to use __va_copy you should always be prepared for the possibility that this macro will not be available. On architectures where a simple assignment is invalid, hopefully __va_copy will be available, so one should always write something like this:
{
va_list ap, save;
...
#ifdef __va_copy
__va_copy (save, ap);
#else
save = ap;
#endif
...
} |
关于va_list 不同编译器和体系结构可能会有不同（http://stackoverflow.com/questions/988290/populating-a-va-list），例如JOS里面定义采用编译器自带方案：
typedef __builtin_va_list va_list; #define va_start(ap, last) __builtin_va_start(ap, last) #define va_arg(ap, type) __builtin_va_arg(ap, type) #define va_end(ap) __builtin_va_end(ap)
附上vsprintfmt.c内容：
void vprintfmt(void (*putch)(int, void*), void *putdat, const char *fmt, va_list ap)
{
register const char *p;
register int ch, err;
unsigned long long num;
int base, lflag, width, precision, altflag;
char padc;
while (1) {
while ((ch = *(unsigned char *) fmt++) != '%') {
if (ch == '\0')
return;
putch(ch, putdat);
}
// Process a %-escape sequence
padc = ' ';
width = -1;
precision = -1;
lflag = 0;
altflag = 0;
reswitch:
switch (ch = *(unsigned char *) fmt++) {
// flag to pad on the right
case '-':
padc = '-';
goto reswitch;
// flag to pad with 0's instead of spaces
case '0':
padc = '0';
goto reswitch;
// width field
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
for (precision = 0; ; ++fmt) {
precision = precision * 10 + ch - '0';
ch = *fmt;
if (ch < '0' || ch > '9')
break;
}
goto process_precision;
case '*':
precision = va_arg(ap, int);
goto process_precision;
case '.':
if (width < 0)
width = 0;
goto reswitch;
case '#':
altflag = 1;
goto reswitch;
process_precision:
if (width < 0)
width = precision, precision = -1;
goto reswitch;
// long flag (doubled for long long)
case 'l':
lflag++;
goto reswitch;
// character
case 'c':
putch(va_arg(ap, int), putdat);
break;
// error message
case 'e':
err = va_arg(ap, int);
if (err < 0)
err = -err;
if (err >= MAXERROR || (p = error_string[err]) == NULL)
printfmt(putch, putdat, "error %d", err);
else
printfmt(putch, putdat, "%s", p);
break;
// string
case 's':
if ((p = va_arg(ap, char *)) == NULL)
p = "(null)";
if (width > 0 && padc != '-')
for (width -= strnlen(p, precision); width > 0; width--)
putch(padc, putdat);
for (; (ch = *p++) != '\0' && (precision < 0 || --precision >= 0); width--)
if (altflag && (ch < ' ' || ch > '~'))
putch('?', putdat);
else
putch(ch, putdat);
for (; width > 0; width--)
putch(' ', putdat);
break;
// (signed) decimal
case 'd':
num = getint(&ap, lflag);
if ((long long) num < 0) {
putch('-', putdat);
num = -(long long) num;
}
base = 10;
goto number;
// unsigned decimal
case 'u':
num = getuint(&ap, lflag);
base = 10;
goto number;
// (unsigned) octal
case 'o':
// Replace this with your code.
putch('X', putdat);
putch('X', putdat);
putch('X', putdat);
break;
// pointer
case 'p':
putch('0', putdat);
putch('x', putdat);
num = (unsigned long long)
(uintptr_t) va_arg(ap, void *);
base = 16;
goto number;
// (unsigned) hexadecimal
case 'x':
num = getuint(&ap, lflag);
base = 16;
number:
printnum(putch, putdat, num, base, width, padc);
break;
// escaped '%' character
case '%':
putch(ch, putdat);
break;
// unrecognized escape sequence - just print it literally
default:
putch('%', putdat);
for (fmt--; fmt[-1] != '%'; fmt--)
/* do nothing */;
break;
}
}
}
...