1. C語言中可變参數(va_list)的用法: va_start、va_arg、va_end参數定義
2019/5/5
C語言中可變参數(va_list)的用法: va_start、va_arg、va_end参數定義
Date: 2016/11/15
Department of Computer Science and Information Engineering
National Cheng Kung University, Taiwan R.O.C.
CSIE CIAL Lab 1

2. C語言可變参數簡介
我們在C語言編程中會遇到一些参數個數可變的函數,例如printf()這個函數,它的定義是: 　    　int printf( const char* format, ...);
它除了有一個参數format固定以外,後面跟的参數的個數和類型是可變的,例如:  printf("%d",i);  printf("%s",s);  printf("the number is %d ,string is:%s", i, s);
National Cheng Kung University CSIE Computer & Internet Architecture Lab

3. 寫一個簡單的可變参數的C函數 
void va_start( va_list arg_ptr, prev_param);  type va_arg( va_list arg_ptr, type);  void va_end( va_list arg_ptr);
va在這裏是variable-argument(可變参數)的意思這些macro定義在stdarg.h中,所以用到可變参數的程序應該包含這個頭文件.下面我們寫一個簡單的可變参數的函數,改函數至少有一個整數参數,第二個参數也是整數,是可選的.函數只是打印這兩個参數的值.
National Cheng Kung University CSIE Computer & Internet Architecture Lab

4. 簡單CODE void simple_va_fun(int i, ...) { va_list arg_ptr; int j=0;
　  　va_start(arg_ptr, i);  　  　j=va_arg(arg_ptr, int);  　  　va_end(arg_ptr); 
　  　printf("%d %d\n", i, j);  　  　return;  　　}
National Cheng Kung University CSIE Computer & Internet Architecture Lab

5. extern void simple_va_fun(int i, ...); 我們在程序中可以這样調用:
可以在上述的文件中這樣宣告: 
   　extern void simple_va_fun(int i, ...); 　   我們在程序中可以這样調用: 
　   simple_va_fun(100); 
      simple_va_fun(100,200);
National Cheng Kung University CSIE Computer & Internet Architecture Lab

6. 範例 :CODE
#include "stdio.h" #include "stdarg.h" void minprintf(char *fmt, ...); void errmsg(int code, ...); int errorf(const char *format, ...); int sum(int num, ...); int sum1(int num, ...); double double_sum(double num, ...); int FILENAME=1, LINENUMBER=2, WARN=4;
National Cheng Kung University CSIE Computer & Internet Architecture Lab

7. main(int argc, char *argv[])
{ char *s = "HELLO"; int x = 10; double f = 10.0; fprintf(stdout, "\n===== Test errmsg() =====\n"); errmsg(1, "<FILE>", "cannot open %s\n", "a_file"); errmsg(3,"<FILE>",7,"cannot open %s\n","a_file"); errmsg(2, 7, "cannot open %s\n", "a_file"); errmsg(4, "<FILE> %s, x=%d", "a_file", 10); printf("\n");
National Cheng Kung University CSIE Computer & Internet Architecture Lab

8. main(int argc, char *argv[])
fprintf(stdout, "\n===== Test minprintf() =====\n"); minprintf("x = %d, f = %f, s = %s\n", x, f, s); minprintf("s = %s, x = %d, f = %f, s = %s\n", s, x, f, s); minprintf("s = %s, x = %d, f = %f, s = %x\n", s, x, f, s); fprintf(stdout, "\n===== Test errorf() =====\n"); errorf("x = %d, f = %f, s = %s\n", x, f, s);
National Cheng Kung University CSIE Computer & Internet Architecture Lab

9. main(int argc, char *argv[])
fprintf(stdout, "\n===== Test sum, sum1, double_sum() =====\n"); printf("sum(4, 1, 2, 3, 4)=%d\n",sum(4, 1, 2, 3, 4)); printf("sum1(4, 1, 2, 3, 4, 0) = %d\n", sum1(4, 1, 2, 3, 4, 0)); printf("double_sum(4.0, 1.0, 2.0, 3.4, 0.0) = %f\n", double_sum(4.0, 1.0, 2.0, 3.4, 0.0)); }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

10. int errorf(const char *format, ...)
{static int num_errors = 0; int n; va_list ap; num_errors++; fprintf(stderr, "** Error %d: ", num_errors); va_start(ap, format); n = vfprintf(stderr, format, ap); va_end(ap); fprintf(stderr, "\n"); return n; }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

11. void errmsg(int code, ...)
va_list ap; char *fmt; va_start(ap, code); if (code&FILENAME) //int FILENAME=1 (void)fprintf(stderr, "\"%s\":", va_arg(ap, char *)); if (code&LINENUMBER) // LINENUMBER=2, (void)fprintf(stderr, "%d:", va_arg(ap, int)); //WARN=4; if (code&WARN) (void)fputs("Warning:", stderr); fmt = va_arg(ap, char *); (void) vfprintf(stderr, fmt, ap); va_end(ap); }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

12. int sum(int num, ...)
{ va_list ap; int i, n = 0; va_start(ap, num); for (i=0; i<num; i++) n += va_arg(ap, int); va_end(ap); return n; }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

13. int sum1(int num, ...)
{ va_list ap; int i=0, t, n = 0; va_start(ap, num); n = num; while (1) { t = va_arg(ap, int); if (t<=0) break; n += t; } va_end(ap); return n; }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

14. double double_sum(double num, ...)
va_list ap; double t, f = 0.0; va_start(ap, num); f = num; if(num==0.0) return 0.0; cnt=1; while (1) { t = va_arg(ap, double); if (t==0.0) break; f += t; } va_end(ap); return f;
National Cheng Kung University CSIE Computer & Internet Architecture Lab

15. void minprintf(char *fmt, ...)
{ va_list ap; char *p, *sval; int ival; double dval; va_start(ap, fmt); for(p=fmt; *p; p++){ if (*p != '%'){ putchar(*p); continue; } switch (*++p){ case 'd': ival = va_arg(ap, int); printf("%d", ival); break; case 'f': dval = va_arg(ap, double); printf("%f", dval); break;
National Cheng Kung University CSIE Computer & Internet Architecture Lab

16. void minprintf(char *fmt, ...)
case 's': for (sval=va_arg(ap, char *); *sval; sval++) putchar(*sval); break; default: printf("'undefined'"); break; }//end switch } // end for va_end(ap); }
National Cheng Kung University CSIE Computer & Internet Architecture Lab

17. Source http://fanli7.net/a/bianchengyuyan/C__/20130113/289956.html
National Cheng Kung University CSIE Computer & Internet Architecture Lab