1STDARG(3) Linux Programmer's Manual STDARG(3)
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6 stdarg - 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 parameter last is the name of the last parameter before the vari‐
30 able argument list, i.e., the last parameter of which the calling func‐
31 tion knows the type.
32 Because the address of this parameter 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 parameter 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 parameter 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 transversals of the list, each
60 bracketed by va_start() and va_end() are possible. va_end() may be a
61 macro 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;
68 Unfortunately, there are also systems that make it an array of pointers
69 (of length 1), and there one needs
70 va_list aq;
71 *aq = *ap;
72 Finally, on systems where parameters are passed in registers, it may be
73 necessary for va_start() to allocate memory, store the parameters
74 there, and also an indication of which parameter is next, so that
75 va_arg() can step through the list. Now va_end() can free the allocated
76 memory again. To accommodate this situation, C99 adds a macro
77 va_copy(), so that the above assignment can be replaced by
78 va_list aq;
79 va_copy(aq, ap);
80 ...
81 va_end(aq);
82 Each invocation of va_copy() must be matched by a corresponding invoca‐
83 tion of va_end() in the same function. Some systems that do not supply
84 va_copy() have __va_copy instead, since that was the name used in the
85 draft proposal.
86
88 The function foo takes a string of format characters and prints out the
89 argument associated with each format character based on the type.
90 #include <stdio.h>
91 #include <stdarg.h>
92 void foo(char *fmt, ...) {
94 va_list ap;
95 int d;
96 char c, *s;
97 va_start(ap, fmt);
99 while (*fmt)
100 switch(*fmt++) {
101 case 's': /* string */
102 s = va_arg(ap, char *);
103 printf("string %s\n", s);
104 break;
105 case 'd': /* int */
106 d = va_arg(ap, int);
107 printf("int %d\n", d);
108 break;
109 case 'c': /* char */
110 /* need a cast here since va_arg only
111 takes fully promoted types */
112 c = (char) va_arg(ap, int);
113 printf("char %c\n", c);
114 break;
115 }
116 va_end(ap);
117 }
118
120 The va_start(), va_arg(), and va_end() macros conform to C89. C99
121 defines the va_copy() macro.
122
124 These macros are not compatible with the historic macros they replace.
125 A backward compatible version can be found in the include file
126 varargs.h.
127
129 The historic setup is:
130 #include <varargs.h>
131 void foo(va_alist) va_dcl {
133 va_list ap;
134 va_start(ap);
136 while(...) {
137 ...
138 x = va_arg(ap, type);
139 ...
140 }
141 va_end(ap);
142 }
143 On some systems, va_end contains a closing '}' matching a '{' in
144 va_start, so that both macros must occur in the same function, and in a
145 way that allows this.
146
148 Unlike the varargs macros, the stdarg macros do not permit programmers
149 to code a function with no fixed arguments. This problem generates
150 work mainly when converting varargs code to stdarg code, but it also
151 creates difficulties for variadic functions that wish to pass all of
152 their arguments on to a function that takes a va_list argument, such as
153 vfprintf(3).
154 2001-10-14 STDARG(3)