1stdarg(3)                  Library Functions Manual                  stdarg(3)
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NAME

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

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <stdarg.h>
13
14       void va_start(va_list ap, last);
15       type va_arg(va_list ap, type);
16       void va_end(va_list ap);
17       void va_copy(va_list dest, va_list src);
18

DESCRIPTION

20       A  function may be called with a varying number of arguments of varying
21       types.  The include file <stdarg.h> declares a type va_list and defines
22       three  macros for stepping through a list of arguments whose number and
23       types are not known to the called function.
24
25       The called function must declare an object of  type  va_list  which  is
26       used by the macros va_start(), va_arg(), and va_end().
27
28   va_start()
29       The  va_start() macro initializes ap for subsequent use by va_arg() and
30       va_end(), and must be called first.
31
32       The argument last is the name of the last argument before the  variable
33       argument list, that is, the last argument of which the calling function
34       knows the type.
35
36       Because the address of this argument may  be  used  in  the  va_start()
37       macro,  it should not be declared as a register variable, or as a func‐
38       tion or an array type.
39
40   va_arg()
41       The va_arg() macro expands to an expression that has the type and value
42       of  the  next  argument in the call.  The argument ap is the va_list ap
43       initialized by va_start().  Each call to va_arg() modifies ap  so  that
44       the  next  call returns the next argument.  The argument type is a type
45       name specified so that the type of a pointer to an object that has  the
46       specified type can be obtained simply by adding a * to type.
47
48       The  first use of the va_arg() macro after that of the va_start() macro
49       returns the argument after last.   Successive  invocations  return  the
50       values of the remaining arguments.
51
52       If  there  is  no  next argument, or if type is not compatible with the
53       type of the actual next argument (as promoted according to the  default
54       argument promotions), random errors will occur.
55
56       If ap is passed to a function that uses va_arg(ap,type), then the value
57       of ap is undefined after the return of that function.
58
59   va_end()
60       Each invocation of va_start() must be matched by a corresponding  invo‐
61       cation of va_end() in the same function.  After the call va_end(ap) the
62       variable ap is undefined.  Multiple traversals of the list, each brack‐
63       eted  by va_start() and va_end() are possible.  va_end() may be a macro
64       or a function.
65
66   va_copy()
67       The va_copy() macro copies the (previously initialized) variable  argu‐
68       ment  list  src to dest.  The behavior is as if va_start() were applied
69       to dest with the same last argument, followed by  the  same  number  of
70       va_arg() invocations that was used to reach the current state of src.
71
72       An  obvious  implementation  would  have  a va_list be a pointer to the
73       stack frame of the variadic function.  In such a setup (by far the most
74       common) there seems nothing against an assignment
75
76           va_list aq = ap;
77
78       Unfortunately, there are also systems that make it an array of pointers
79       (of length 1), and there one needs
80
81           va_list aq;
82           *aq = *ap;
83
84       Finally, on systems where arguments are passed in registers, it may  be
85       necessary for va_start() to allocate memory, store the arguments there,
86       and also an indication of which argument is next, so that va_arg()  can
87       step  through  the  list.   Now  va_end() can free the allocated memory
88       again.  To accommodate this situation, C99 adds a macro  va_copy(),  so
89       that the above assignment can be replaced by
90
91           va_list aq;
92           va_copy(aq, ap);
93           ...
94           va_end(aq);
95
96       Each invocation of va_copy() must be matched by a corresponding invoca‐
97       tion of va_end() in the same function.  Some systems that do not supply
98       va_copy()  have  __va_copy instead, since that was the name used in the
99       draft proposal.
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ATTRIBUTES

102       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
103       tributes(7).
104
105       ┌────────────────────────────────────┬───────────────┬─────────────────┐
106Interface                           Attribute     Value           
107       ├────────────────────────────────────┼───────────────┼─────────────────┤
108va_start(), va_end(), va_copy()     │ Thread safety │ MT-Safe         │
109       ├────────────────────────────────────┼───────────────┼─────────────────┤
110va_arg()                            │ Thread safety │ MT-Safe race:ap │
111       └────────────────────────────────────┴───────────────┴─────────────────┘
112

STANDARDS

114       C11, POSIX.1-2008.
115

HISTORY

117       va_start()
118       va_arg()
119       va_end()
120              C89, POSIX.1-2001.
121
122       va_copy()
123              C99, POSIX.1-2001.
124

CAVEATS

126       Unlike  the  historical varargs macros, the stdarg macros do not permit
127       programmers to code a function with no fixed arguments.   This  problem
128       generates  work mainly when converting varargs code to stdarg code, but
129       it also creates difficulties for variadic functions that wish  to  pass
130       all  of their arguments on to a function that takes a va_list argument,
131       such as vfprintf(3).
132

EXAMPLES

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
137       #include <stdio.h>
138       #include <stdarg.h>
139
140       void
141       foo(char *fmt, ...)   /* '...' is C syntax for a variadic function */
142
143       {
144           va_list ap;
145           int d;
146           char c;
147           char *s;
148
149           va_start(ap, fmt);
150           while (*fmt)
151               switch (*fmt++) {
152               case 's':              /* string */
153                   s = va_arg(ap, char *);
154                   printf("string %s\n", s);
155                   break;
156               case 'd':              /* int */
157                   d = va_arg(ap, int);
158                   printf("int %d\n", d);
159                   break;
160               case 'c':              /* char */
161                   /* need a cast here since va_arg only
162                      takes fully promoted types */
163                   c = (char) va_arg(ap, int);
164                   printf("char %c\n", c);
165                   break;
166               }
167           va_end(ap);
168       }
169

SEE ALSO

171       vprintf(3), vscanf(3), vsyslog(3)
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174
175Linux man-pages 6.05              2023-07-20                         stdarg(3)
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