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38 .\" @(#)stdarg.3 6.8 (Berkeley) 6/29/91
40 .\" Converted for Linux, Mon Nov 29 15:11:11 1993, faith@cs.unc.edu
41 .\" Additions, 2001-10-14, aeb
43 .TH STDARG 3 2021-03-22 "" "Linux Programmer's Manual"
45 stdarg, va_start, va_arg, va_end, va_copy \- variable argument lists
48 .B #include <stdarg.h>
50 .BI "void va_start(va_list " ap ", " last );
51 .IB type " va_arg(va_list " ap ", " type );
52 .BI "void va_end(va_list " ap );
53 .BI "void va_copy(va_list " dest ", va_list " src );
56 A function may be called with a varying number of arguments of varying
62 and defines three macros for stepping through a list of arguments whose
63 number and types are not known to the called function.
65 The called function must declare an object of type
67 which is used by the macros
81 and must be called first.
85 is the name of the last argument before the variable argument list, that is,
86 the last argument of which the calling function knows the type.
88 Because the address of this argument may be used in the
90 macro, it should not be declared as a register variable,
91 or as a function or an array type.
95 macro expands to an expression that has the type and value of the next
108 so that the next call returns the next argument.
111 is a type name specified so that the type of a pointer to an object that
112 has the specified type can be obtained simply by adding a * to
117 macro after that of the
119 macro returns the argument after
121 Successive invocations return the values of the remaining arguments.
123 If there is no next argument, or if
125 is not compatible with the type of the actual next argument (as promoted
126 according to the default argument promotions), random errors will occur.
130 is passed to a function that uses
131 .BI va_arg( ap , type ),
134 is undefined after the return of that function.
138 must be matched by a corresponding invocation of
140 in the same function.
146 Multiple traversals of the list, each
153 may be a macro or a function.
157 macro copies the (previously initialized) variable argument list
161 The behavior is as if
167 argument, followed by the same number of
169 invocations that was used to reach the current state of
172 .\" Proposal from clive@demon.net, 1997-02-28
173 An obvious implementation would have a
175 be a pointer to the stack frame of the variadic function.
176 In such a setup (by far the most common) there seems
177 nothing against an assignment
185 Unfortunately, there are also systems that make it an
186 array of pointers (of length 1), and there one needs
195 Finally, on systems where arguments are passed in registers,
196 it may be necessary for
198 to allocate memory, store the arguments there, and also
199 an indication of which argument is next, so that
201 can step through the list.
204 can free the allocated memory again.
205 To accommodate this situation, C99 adds a macro
207 so that the above assignment can be replaced by
220 must be matched by a corresponding invocation of
222 in the same function.
223 Some systems that do not supply
227 instead, since that was the name used in the draft proposal.
229 For an explanation of the terms used in this section, see
237 Interface Attribute Value
242 T} Thread safety MT-Safe
245 T} Thread safety MT-Safe race:ap
256 macros conform to C89.
261 Unlike the historical
265 macros do not permit programmers to code a function with no fixed
267 This problem generates work mainly when converting
271 code, but it also creates difficulties for variadic functions that wish to
272 pass all of their arguments on to a function that takes a
279 takes a string of format characters and prints out the argument associated
280 with each format character based on the type.
287 foo(char *fmt, ...) /* \(aq...\(aq is C syntax for a variadic function */
298 case \(aqs\(aq: /* string */
299 s = va_arg(ap, char *);
300 printf("string %s\en", s);
302 case \(aqd\(aq: /* int */
304 printf("int %d\en", d);
306 case \(aqc\(aq: /* char */
307 /* need a cast here since va_arg only
308 takes fully promoted types */
309 c = (char) va_arg(ap, int);
310 printf("char %c\en", c);