1STDARG(3) Linux Programmer's Manual STDARG(3)
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6 stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
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9 #include <stdarg.h>
10 void va_start(va_list ap, last);
12 type va_arg(va_list ap, type);
13 void va_end(va_list ap);
14 void va_copy(va_list dest, va_list src);
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17 A function may be called with a varying number of arguments of varying
18 types. The include file <stdarg.h> declares a type va_list and defines
19 three macros for stepping through a list of arguments whose number and
20 types are not known to the called function.
21 The called function must declare an object of type va_list which is
23 used by the macros va_start(), va_arg(), and va_end().
24 va_start()
26 The va_start() macro initializes ap for subsequent use by va_arg() and
27 va_end(), and must be called first.
28 The argument last is the name of the last argument before the variable
30 argument list, that is, the last argument of which the calling function
31 knows the type.
32 Because the address of this argument may be used in the va_start()
34 macro, it should not be declared as a register variable, or as a func‐
35 tion or an array type.
36 va_arg()
38 The va_arg() macro expands to an expression that has the type and value
39 of the next argument in the call. The argument ap is the va_list ap
40 initialized by va_start(). Each call to va_arg() modifies ap so that
41 the next call returns the next argument. The argument type is a type
42 name specified so that the type of a pointer to an object that has the
43 specified type can be obtained simply by adding a * to type.
44 The first use of the va_arg() macro after that of the va_start() macro
46 returns the argument after last. Successive invocations return the
47 values of the remaining arguments.
48 If there is no next argument, or if type is not compatible with the
50 type of the actual next argument (as promoted according to the default
51 argument promotions), random errors will occur.
52 If ap is passed to a function that uses va_arg(ap,type), then the value
54 of ap is undefined after the return of that function.
55 va_end()
57 Each invocation of va_start() must be matched by a corresponding invo‐
58 cation of va_end() in the same function. After the call va_end(ap) the
59 variable ap is undefined. Multiple traversals of the list, each brack‐
60 eted by va_start() and va_end() are possible. va_end() may be a macro
61 or a function.
62 va_copy()
64 The va_copy() macro copies the (previously initialized) variable argu‐
65 ment list src to dest. The behavior is as if va_start() were applied
66 to dest with the same last argument, followed by the same number of
67 va_arg() invocations that was used to reach the current state of src.
68 An obvious implementation would have a va_list be a pointer to the
70 stack frame of the variadic function. In such a setup (by far the most
71 com