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);
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 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 common) there seems nothing against an assignment
72 va_list aq = ap;
74 Unfortunately, there are also systems that make it an array of pointers
76 (of length 1), and there one needs
77 va_list aq;
79 *aq = *ap;
80 Finally, on systems where arguments are passed in registers, it may be
82 necessary for va_start() to allocate memory, store the arguments there,
83 and also an indication of which argument is next, so that va_arg() can
84 step through the list. Now va_end() can free the allocated memory
85 again. To accommodate this situation, C99 adds a macro va_copy(), so
86 that the above assignment can be replaced by
87 va_list aq;
89 va_copy(aq, ap);
90 ...
91 va_end(aq);
92 Each invocation of va_copy() must be matched by a corresponding invoca‐
94 tion of va_end() in the same function. Some systems that do not supply
95 va_copy() have __va_copy instead, since that was the name used in the
96 draft proposal.
97
99 For an explanation of the terms used in this section, see
100 attributes(7).
101 ┌──────────────────────┬───────────────┬─────────────────┐
103 │Interface │ Attribute │ Value │
104 ├──────────────────────┼───────────────┼─────────────────┤
105 │va_start(), va_end(), │ Thread safety │ MT-Safe │
106 │va_copy() │ │ │
107 ├──────────────────────┼───────────────┼─────────────────┤
108 │va_arg() │ Thread safety │ MT-Safe race:ap │
109 └──────────────────────┴───────────────┴─────────────────┘
111 The va_start(), va_arg(), and va_end() macros conform to C89. C99
112 defines the va_copy() macro.
113
115 These macros are not compatible with the historic macros they replace.
116 A backward-compatible version can be found in the include file
117 <varargs.h>.
118 The historic setup is:
120 #include <varargs.h>
122 void
124 foo(va_alist)
125 va_dcl
126 {
127 va_list ap;
128 va_start(ap);
130 while (...) {
131 ...
132 x = va_arg(ap, type);
133 ...
134 }
135 va_end(ap);
136 }
137 On some systems, va_end contains a closing '}' matching a '{' in
139 va_start, so that both macros must occur in the same function, and in a
140 way that allows this.
141
143 Unlike the varargs macros, the stdarg macros do not permit programmers
144 to code a function with no fixed arguments. This problem generates
145 work mainly when converting varargs code to stdarg code, but it also
146 creates difficulties for variadic functions that wish to pass all of
147 their arguments on to a function that takes a va_list argument, such as
148 vfprintf(3).
149
151 The function foo takes a string of format characters and prints out the
152 argument associated with each format character based on the type.
153 #include <stdio.h>
155 #include <stdarg.h>
156 void
158 foo(char *fmt, ...)
159 {
160 va_list ap;
161 int d;
162 char c, *s;
163 va_start(ap, fmt);
165 while (*fmt)
166 switch (*fmt++) {
167 case 's': /* string */
168 s = va_arg(ap, char *);
169 printf("string %s\n", s);
170 break;
171 case 'd': /* int */
172 d = va_arg(ap, int);
173 printf("int %d\n", d);
174 break;
175 case 'c': /* char */
176 /* need a cast here since va_arg only
177 takes fully promoted types */
178 c = (char) va_arg(ap, int);
179 printf("char %c\n", c);
180 break;
181 }
182 va_end(ap);
183 }
184
186 This page is part of release 4.16 of the Linux man-pages project. A
187 description of the project, information about reporting bugs, and the
188 latest version of this page, can be found at
189 https://www.kernel.org/doc/man-pages/.
190 2017-09-15 STDARG(3)