关于stdarg, va_start, va_arg, va_end, va_copy - variable argument lists?

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答案：【SYNOPSIS】 #include [ stdarg.h] void va_start(va_list ap, last); type va_arg(va_list ap, type); void va_end(va_list ap); void va_copy(va_list dest, va_list src); 【DESCRIPTION】 A function may be called with a varying number of arguments of varying types. The include file [ stdarg.h] declares a type va_list and defines three macros for stepping through a list of arguments whose number and types are not known to the called function. The called function must declare an object of type va_list which is used by the macros va_start(), va_arg(), and va_end(). 1）va_start() The va_start() macro initializes ap for subsequent use by va_arg() and va_end(), and must be called first. The argument last is the name of the last argument before the variable argument list, that is, the last argument of which the calling func‐ tion knows the type. Because the address of this argument may be used in the va_start() macro, it should not be declared as a register variable, or as a function or an array type. 2)va_arg() The va_arg() macro expands to an expression that has the type and value of the next argument in the call. The argument ap is the va_list ap initialized by va_start(). Each call to va_arg() modifies ap so that the next call returns the next argument. The argument type is a type name specified so that the type of a pointer to an object that has the specified type can be obtained simply by adding a * to type. The first use of the va_arg() macro after that of the va_start() macro returns the argument after last. Successive invocations return the values of the remaining arguments. If there is no next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur. If ap is passed to a function that uses va_arg(ap,type), then the value of ap is undefined after the return of that function. 3)va_end() Each invocation of va_start() must be matched by a corresponding invocation of va_end() in the same function. After the call va_end(ap) the variable ap is undefined. Multiple traversals of the list, each bracketed by va_start() and va_end() are possible. va_end() may be a macro or a function. 4）va_copy() The va_copy() macro copies the (previously initialized) variable argument list src to dest. The behavior is as if va_start() were applied to dest with the same last argument, followed by the same number of va_arg() invocations that was used to reach the current state of src. An obvious implementation would have a va_list be a pointer to the stack frame of the variadic function. In such a setup (by far the most common) there seems nothing against an assignment va_list aq = ap; Unfortunately, there are also systems that make it an array of pointers (of length 1), and there one needs va_list aq; *aq = *ap; Finally, on systems where arguments are passed in registers, it may be necessary for va_start() to allocate memory, store the arguments there, and also an indication of which argument is next, so that va_arg() can step through the list. Now va_end() can free the allocated memory again. To accommodate this situation, C99 adds a macro va_copy(), so that the above assignment can be replaced by va_list aq; va_copy(aq, ap); ... va_end(aq); Each invocation of va_copy() must be matched by a corresponding invocation of va_end() in the same function. Some systems that do not supply va_copy() have __va_copy instead, since that was the name used in the draft proposal. 【翻译】 va_copy An obvious implementation would have a va_list be a pointer to the stack frame of the variadic function . In such a setup (by far the most common) there seems nothing against an assignment 一种最常见的实现是将 va_list 作为指向变参函数栈帧的指针使用。在这种情况下（最常见情况），va_copy 的功能和下面的赋值操作可以等价 va_list aq = ap; Unfortunately, there are also systems that make it an array of pointers (of length 1), and there one needs 不幸的是，有一些系统将 va_list 实现成了指针数组（长度为 1），故操作必须如下才能保证正确 va_list aq; *aq = *ap; Finally, on systems where parameters are passed in registers, it may be necessary for va_start() to allocate memory, store the parameters there, and also an indication of which parameter is next, so that va_arg() can step through the list. Now va_end() can free the allocated memory again. To accommodate this situation, C99 adds a macro va_copy(), so that the above assignment can be replaced by 在使用寄存器（register）传参的系统上，va_start() 函数会负责所需内存的分配以及相应参数值的存储，并且会指示下一个待处理的参数是谁，以便于 va_arg() 能够遍历变参列表。va_end() 会负责分配内存的释放工作。为了满足上述行为，C99 添加了宏定义 va_copy()，所以上面的赋值操作可以被下面的代码替代 va_list aq; va_copy(aq, ap); ... va_end(aq); Each invocation of va_copy() must be matched by a corresponding invocation of va_end() in the same function . Some systems that do not supply va_copy() have __va_copy instead, since that was the name used in the draft proposal. 每一次对 va_copy() 的调用都必须与 va_end() 进行配对（同一个函数内部）。一些系统上不支持 va_copy() 但是提供了 __va_copy() 也是可以的，该名字的由来是因为在草案中就是这个。 CONFORMING TO The va_start(), va_arg(), and va_end() macros conform to C89. C99 defines the va_copy() macro. va_copy() 是在 C99 中被支持的。【注意】 1）关于三个宏的定义 #define _INTSIZEOF(n) ((sizeof(n)+sizeof(int)-1)&~(sizeof(int) - 1) ) //与4字节的int型对齐 #define va_start(ap,v) ( ap = (va_list)&v + _INTSIZEOF(v) ) //第一个可选参数地址 #define va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) ) //下一个参数地址 #define va_end(ap) ( ap = (va_list)0 ) // 将指针置为无效更便于理解