1STDARG(3)                  Linux Programmer's Manual                 STDARG(3)
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NAME

6       stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
7

SYNOPSIS

9       #include <stdarg.h>
10
11       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

DESCRIPTION

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
22       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
25   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
29       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
33       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.
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37   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
45       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.
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49       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
53       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
56   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
63   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
69       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
73           va_list aq = ap;
74
75       Unfortunately, there are also systems that make it an array of pointers
76       (of length 1), and there one needs
77
78           va_list aq;
79           *aq = *ap;
80
81       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
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88           va_list aq;
89           va_copy(aq, ap);
90           ...
91           va_end(aq);
92
93       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.
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ATTRIBUTES

99       For  an  explanation  of  the  terms  used   in   this   section,   see
100       attributes(7).
101
102       ┌──────────────────────┬───────────────┬─────────────────┐
103Interface             Attribute     Value           
104       ├──────────────────────┼───────────────┼─────────────────┤
105va_start(), va_end(), │ Thread safety │ MT-Safe         │
106va_copy()             │               │                 │
107       ├──────────────────────┼───────────────┼─────────────────┤
108va_arg()              │ Thread safety │ MT-Safe race:ap │
109       └──────────────────────┴───────────────┴─────────────────┘

CONFORMING TO

111       The va_start(), va_arg(), and va_end()  macros  conform  to  C89.   C99
112       defines the va_copy() macro.
113

NOTES

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
119       The historic setup is:
120
121           #include <varargs.h>
122
123           void
124           foo(va_alist)
125               va_dcl
126           {
127               va_list ap;
128
129               va_start(ap);
130               while (...) {
131                   ...
132                   x = va_arg(ap, type);
133                   ...
134               }
135               va_end(ap);
136           }
137
138       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

BUGS

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

EXAMPLE

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
154       #include <stdio.h>
155       #include <stdarg.h>
156
157       void
158       foo(char *fmt, ...)
159       {
160           va_list ap;
161           int d;
162           char c, *s;
163
164           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

COLOPHON

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/.
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193                                  2017-09-15                         STDARG(3)
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