stdarg.h头文件源代码分析

谈到C语言中可变参数函数的实现（参见C语言中可变参数函数实现原理），有一个头文件不得不谈，那就是stdarg.h

本文从minix源码中的stdarg.h头文件入手进行分析：

 1 #ifndef _STDARG_H
 2 #define _STDARG_H
 3 
 4 
 5 #ifdef __GNUC__
 6 /* The GNU C-compiler uses its own, but similar varargs mechanism. */
 7 
 8 typedef char *va_list;
 9 
10 /* Amount of space required in an argument list for an arg of type TYPE.
11  * TYPE may alternatively be an expression whose type is used.
12  */
13 
14 #define __va_rounded_size(TYPE)  \
15   (((sizeof (TYPE) + sizeof (int) - 1) / sizeof (int)) * sizeof (int))
16 
17 #if __GNUC__ < 2
18 
19 #ifndef __sparc__
20 #define va_start(AP, LASTARG)                                           \
21  (AP = ((char *) &(LASTARG) + __va_rounded_size (LASTARG)))
22 #else
23 #define va_start(AP, LASTARG)                                           \
24  (__builtin_saveregs (),                                                \
25   AP = ((char *) &(LASTARG) + __va_rounded_size (LASTARG)))
26 #endif
27 
28 void va_end (va_list);          /* Defined in gnulib */
29 #define va_end(AP)
30 
31 #define va_arg(AP, TYPE)                                                \
32  (AP += __va_rounded_size (TYPE),                                       \
33   *((TYPE *) (AP - __va_rounded_size (TYPE))))
34 
35 #else    /* __GNUC__ >= 2 */
36 
37 #ifndef __sparc__
38 #define va_start(AP, LASTARG)                         \
39  (AP = ((char *) __builtin_next_arg ()))
40 #else
41 #define va_start(AP, LASTARG)                    \
42   (__builtin_saveregs (), AP = ((char *) __builtin_next_arg ()))
43 #endif
44 
45 void va_end (va_list);        /* Defined in libgcc.a */
46 #define va_end(AP)
47 
48 #define va_arg(AP, TYPE)                        \
49  (AP = ((char *) (AP)) += __va_rounded_size (TYPE),            \
50   *((TYPE *) ((char *) (AP) - __va_rounded_size (TYPE))))
51 
52 #endif    /* __GNUC__ >= 2 */
53 
54 #else    /* not __GNUC__ */
55 
56 
57 typedef char *va_list;
58 
59 #define __vasz(x)        ((sizeof(x)+sizeof(int)-1) & ~(sizeof(int) -1))
60 
61 #define va_start(ap, parmN)    ((ap) = (va_list)&parmN + __vasz(parmN))
62 #define va_arg(ap, type)      \
63   (*((type *)((va_list)((ap) = (void *)((va_list)(ap) + __vasz(type))) \
64                             - __vasz(type))))
65 #define va_end(ap)
66 
67 
68 #endif /* __GNUC__ */
69 
70 #endif /* _STDARG_H */

stdarg.h源代码