可变参数打印
使用C语言的同学都知道，C语言的打印函数printf();是可以打印可变参数的，比如：
printf("hello world!My name is %s,I am %d years old\n","Jack",98);
printf("hello world!My name is %s\n","Jack");对于计算机程序来说，总是一行行执行代码，对于C语言来说，可变参数是如何实现的呢？
可变参数实现的知识点
对于可变参数的实现，首先是函数调用栈的入栈顺序，以及基于栈的参数寻址。第二个知识点就是对每个字符使用系统调用进行打印。
【函数调用栈】
不管有多少个可变参数，当我们写完打印语句的时候，参数个数和位置就已经确定了。当我们写下如下语句时候，经过C语言编译后，参数放入栈中。
printf("my name is %s,I am %d years old",
"xiaoxiao",
10);
既然所有的参数已经放入栈的数据结构中，那么我们只需要设计一种算法，对栈中的元素进行某种方式的遍历，从而把我们希望的打印结果按顺序显示即可。
C语言的<stdarg.h>提供3种能够对栈进行遍历的API。
typedef __builtin_va_list va_list;
#define va_start(ap, param) __builtin_va_start(ap, param)
#define va_end(ap) __builtin_va_end(ap)
#define va_arg(ap, type) __builtin_va_arg(ap, type)说明：
一般va_list是char *的指针。
va_start(ap, param)：入参：param，它一般是第一个可变参数的地址；出参：ap，指向第一个可变参数的地址。
va_arg(ap, type)：入参：ap是前一个参数的首地址，该函数使得ap能够加1，指向下一个参数的首地址；type，是ap指向的参数的类型。该函数将解析ap指向的参数
va_end：释放指针，将输入的参数 ap 置为 NULL。通常va_start和va_end是成对出现。
举个例子：
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
/*ANSI标准形式的声明方式，括号内的省略号表示可选参数*/
int demo( char *msg,...)
{
/*定义保存函数参数的结构*/
va_list argp;
char *para;
va_start(argp, msg);
printf("parameter1=%s\n",msg);
para = va_arg( argp, char*);
printf("parameter2=%s\n",para);
va_end(argp);
return 0;
}
int main()
{
demo("DEMO", "demo");
return 0;
}输出结果：
parameter1=DEMO
parameter2=demo【系统调用】
通过某种算法，组合使用va_start、va_arg、va_end这样的API，就能够按格式输出想要的打印结果。但是对于输出而言，程序需要调用系统API进行字符串显示。
printf的内部底层采用putchar()，用来输出一个字符的系统调用，这样来保证整个字符串输出正确。
cprintf可变参数实现源代码
这份源代码实现了完整的printf打印，它包含一个头文件和一个源文件。
error.h
#ifndef __LIBS_ERROR_H__
#define __LIBS_ERROR_H__
/* kernel error codes -- keep in sync with list in lib/printfmt.c */
#define E_UNSPECIFIED 1 // Unspecified or unknown problem
#define E_BAD_PROC 2 // Process doesn't exist or otherwise
#define E_INVAL 3 // Invalid parameter
#define E_NO_MEM 4 // Request failed due to memory shortage
#define E_NO_FREE_PROC 5 // Attempt to create a new process beyond
#define E_FAULT 6 // Memory fault
#define E_SWAP_FAULT 7 // SWAP READ/WRITE fault
#define E_INVAL_ELF 8 // Invalid elf file
#define E_KILLED 9 // Process is killed
#define E_PANIC 10 // Panic Failure
#define E_TIMEOUT 11 // Timeout
#define E_TOO_BIG 12 // Argument is Too Big
#define E_NO_DEV 13 // No such Device
#define E_NA_DEV 14 // Device Not Available
#define E_BUSY 15 // Device/File is Busy
#define E_NOENT 16 // No Such File or Directory
#define E_ISDIR 17 // Is a Directory
#define E_NOTDIR 18 // Not a Directory
#define E_XDEV 19 // Cross Device-Link
#define E_UNIMP 20 // Unimplemented Feature
#define E_SEEK 21 // Illegal Seek
#define E_MAX_OPEN 22 // Too Many Files are Open
#define E_EXISTS 23 // File/Directory Already Exists
#define E_NOTEMPTY 24 // Directory is Not Empty
/* the maximum allowed */
#define MAXERROR 24
#endif /* !__LIBS_ERROR_H__ */
main.c
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <error.h>
static const char * const error_string[MAXERROR + 1] = {
[0] NULL,
[E_UNSPECIFIED] "unspecified error",
[E_BAD_PROC] "bad process",
[E_INVAL] "invalid parameter",
[E_NO_MEM] "out of memory",
[E_NO_FREE_PROC] "out of processes",
[E_FAULT] "segmentation fault",
[E_INVAL_ELF] "invalid elf file",
[E_KILLED] "process is killed",
[E_PANIC] "panic failure",
};
#define do_div(n, base) ({ \
unsigned long __upper, __low, __high, __mod, __base; \
__base = (base); \
asm ("" : "=a" (__low), "=d" (__high) : "A" (n)); \
__upper = __high; \
if (__high != 0) { \
__upper = __high % __base; \
__high = __high / __base; \
} \
asm ("divl %2" : "=a" (__low), "=d" (__mod) \
: "rm" (__base), "0" (__low), "1" (__upper)); \
asm ("" : "=A" (n) : "a" (__low), "d" (__high)); \
__mod; \
})
unsigned long long
getuint(va_list *ap, int lflag);
long long
getint(va_list *ap, int lflag);
void
printnum(void (*putch)(int, void*), void *putdat,
unsigned long long num, unsigned base, int width, int padc);
void
printfmt(void (*putch)(int, void*), void *putdat, const char *fmt, ...);
void
cputch(int c, int *cnt);
void
vprintfmt(void (*putch)(int, void*), void *putdat, const char *fmt, va_list ap);
int
vcprintf(const char *fmt, va_list ap);
int
cprintf(const char *fmt, ...);
int main()
{
cprintf("hello world!My name is %s,I am %d years old\n","Jack",98);
printf("hello world!My name is %s\n","Jack");
return 0;
}
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, width, precision, lflag, altflag;
while (1) {
while ((ch = *(unsigned char *)fmt ++) != '%') {
if (ch == '\0') {
return;
}
putch(ch, putdat);
}
// Process a %-escape sequence
char padc = ' ';
width = precision = -1;
lflag = 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' ... '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':
num = getuint(&ap, lflag);
base = 8;
goto number;
// 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;
}
}
}
int
vcprintf(const char *fmt, va_list ap) {
int cnt = 0;
vprintfmt((void*)cputch, &cnt, fmt, ap);
return cnt;
}
int
cprintf(const char *fmt, ...) {
va_list ap;
va_start(ap, fmt);
int cnt = vcprintf(fmt, ap);
va_end(ap);
return cnt;
}
void
cputch(int c, int *cnt) {
putchar(c);
(*cnt) ++;
}
void
printfmt(void (*putch)(int, void*), void *putdat, const char *fmt, ...) {
va_list ap;
va_start(ap, fmt);
vprintfmt(putch, putdat, fmt, ap);
va_end(ap);
};
unsigned long long
getuint(va_list *ap, int lflag) {
if (lflag >= 2) {
return va_arg(*ap, unsigned long long);
}
else if (lflag) {
return va_arg(*ap, unsigned long);
}
else {
return va_arg(*ap, unsigned int);
}
}
long long
getint(va_list *ap, int lflag) {
if (lflag >= 2) {
return va_arg(*ap, long long);
}
else if (lflag) {
return va_arg(*ap, long);
}
else {
return va_arg(*ap, int);
}
}
void
printnum(void (*putch)(int, void*), void *putdat,
unsigned long long num, unsigned base, int width, int padc) {
unsigned long long result = num;
unsigned mod = do_div(result, base);
// first recursively print all preceding (more significant) digits
if (num >= base) {
printnum(putch, putdat, result, base, width - 1, padc);
} else {
// print any needed pad characters before first digit
while (-- width > 0)
putch(padc, putdat);
}
// then print this (the least significant) digit
putch("0123456789abcdef"[mod], putdat);
};
输出结果：
hello world!My name is Jack,I am 98 years old
hello world!My name is Jack版权声明：本文为u012750235原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接和本声明。