1STDARG(3) Linux Programmer's Manual STDARG(3)
2
6 stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
7
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);
15
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 An obvious implementation would have a va_list be a pointer to the
65 stack frame of the variadic function. In such a setup (by far the most
66 common) there seems nothing against an assignment
67 va_list aq = ap;
69 Unfortunately, there are also systems that make it an array of pointers
71 (of length 1), and there one needs
72 va_list aq;
74 *aq = *ap;
75 Finally, on systems where arguments are passed in registers, it may be
77 necessary for va_start() to allocate memory, store the arguments there,
78 and also an indication of which argument is next, so that va_arg() can
79 step through the list. Now va_end() can free the allocated memory
80 again. To accommodate this situation, C99 adds a macro va_copy(), so
81 that the above assignment can be replaced by
82 va_list aq;
84 va_copy(aq, ap);
85 ...
86 va_end(aq);
87 Each invocation of va_copy() must be matched by a corresponding invoca‐
89 tion of va_end() in the same function. Some systems that do not supply
90 va_copy() have __va_copy instead, since that was the name used in the
91 draft proposal.
92
94 The va_start(), va_arg(), and va_end() macros conform to C89. C99
95 defines the va_copy() macro.
96
98 These macros are not compatible with the historic macros they replace.
99 A backward compatible version can be found in the include file
100 <varargs.h>.
101 The historic setup is:
103 #include <varargs.h>
105 void
107 foo(va_alist)
108 va_dcl
109 {
110 va_list ap;
111 va_start(ap);
113 while (...) {
114 ...
115 x = va_arg(ap, type);
116 ...
117 }
118 va_end(ap);
119 }
120 On some systems, va_end contains a closing '}' matching a '{' in
122 va_start, so that both macros must occur in the same function, and in a
123 way that allows this.
124
126 Unlike the varargs macros, the stdarg macros do not permit programmers
127 to code a function with no fixed arguments. This problem generates
128 work mainly when converting varargs code to stdarg code, but it also
129 creates difficulties for variadic functions that wish to pass all of
130 their arguments on to a function that takes a va_list argument, such as
131 vfprintf(3).
132
134 The function foo takes a string of format characters and prints out the
135 argument associated with each format character based on the type.
136 #include <stdio.h>
138 #include <stdarg.h>
139 void
141 foo(char *fmt, ...)
142 {
143 va_list ap;
144 int d;
145 char c, *s;
146 va_start(ap, fmt);
148 while (*fmt)
149 switch (*fmt++) {
150 case 's': /* string */
151 s = va_arg(ap, char *);
152 printf("string %s\n", s);
153 break;
154 case 'd': /* int */
155 d = va_arg(ap, int);
156 printf("int %d\n", d);
157 break;
158 case 'c': /* char */
159 /* need a cast here since va_arg only
160 takes fully promoted types */
161 c = (char) va_arg(ap, int);
162 printf("char %c\n", c);
163 break;
164 }
165 va_end(ap);
166 }
167
169 This page is part of release 3.22 of the Linux man-pages project. A
170 description of the project, and information about reporting bugs, can
171 be found at http://www.kernel.org/doc/man-pages/.
172 2001-10-14 STDARG(3)