MirBSD manpage: stdarg(3), varargs(3), va_arg(3), va_copy(3), va_end(3), va_start(3)

STDARG(3)                  BSD Programmer's Manual                   STDARG(3)


     va_start, va_arg, va_copy, va_end - variable argument lists


     #include <stdarg.h>

     va_start(va_list ap, last);

     va_arg(va_list ap, type);

     va_copy(va_list dst, va_list src);

     va_end(va_list ap);


     A function may be called with a varying number of arguments of varying
     types. The include file <stdarg.h> declares a type (va_list) and defines
     three macros for stepping through a list of arguments whose number and
     types are not known to the called function.

     The called function must declare an object of type va_list which is used
     by the macros va_start(), va_arg(), va_end(), and, optionally, va_copy().

     The va_start() macro initializes ap for subsequent use by va_arg(),
     va_copy() and va_end(), and must be called first.

     The parameter last is the name of the last parameter before the variable
     argument list, i.e., the last parameter of which the calling function
     knows the type.

     Because the address of this parameter is used in the va_start() macro, it
     should not be declared as a register variable, nor as a function, nor an
     array type.

     The va_start() macro returns no value.

     The va_arg() macro expands to an expression that has the type and value
     of the next argument in the call. The parameter ap is the va_list ap ini-
     tialized by va_start(). Each call to va_arg() modifies ap so that the
     next call returns the next argument. The parameter type is a type name
     specified so that the type of a pointer to an object that has the speci-
     fied type can be obtained simply by adding a '*' to type.

     If there is no next argument, or if type is not compatible with the type
     of the actual next argument (as promoted according to the default argu-
     ment promotions, see below), random errors will occur.

     If the type in question is one that would normally be promoted, the pro-
     moted type should be used as the argument to va_arg(). The following
     describes which types should be promoted (and to what):
     -   short is promoted to int
     -   float is promoted to double
     -   char is promoted to int

     The same rules apply to unsigned versions of the above types, as well as
     their bit-type equivalents (e.g. int8_t and int16_t).

     The first use of the va_arg() macro after that of the va_start() macro
     returns the argument after last. Successive invocations return the values
     of the remaining arguments.

     The va_copy() macro makes dst a copy of src as if the va_start() macro
     had been applied to it followed by the same sequence of uses of the
     va_arg() macro as had previously been used to reach the present state of

     The va_copy() macro returns no value.

     The va_end() macro handles a normal return from the function whose vari-
     able argument list was initialized by va_start() or va_copy().

     The va_end() macro returns no value.


     The function foo() takes a string of format characters and prints out the
     argument associated with each format character based on the type.

           foo(char *fmt, ...)
                   va_list ap;
                   int d, c;
                   char *s;
                   double f;

                   va_start(ap, fmt);
                   while (*fmt)
                           switch (*fmt++) {
                           case 's':                       /* string */
                                   s = va_arg(ap, char *);
                                   printf("string %s\n", s);
                           case 'd':                       /* int */
                                   d = va_arg(ap, int);
                                   printf("int %d\n", d);
                           case 'c':                       /* char */
                                   c = va_arg(ap, int);    /* promoted */
                                   printf("char %c\n", c);
                           case 'f':                       /* float */
                                   f = va_arg(ap, double); /* promoted */
                                   printf("float %f\n", f);


     These macros are not compatible with the historic macros they replace. A
     backward compatible version can be found in the include file <varargs.h>.

     The va_start(), va_arg() and va_end() macros conform to ISO/IEC 9899:1999
     ("ISO C99").


     The va_start(), va_arg() and va_end() macros were introduced in ANSI
     X3.159-1989 ("ANSI C89"). The va_copy() macro was introduced in ISO/IEC
     9899:1999 ("ISO C99").


     Unlike the varargs macros, the stdarg macros do not permit programmers to
     code a function with no fixed arguments. This problem generates work
     mainly when converting varargs code to stdarg code, but it also creates
     difficulties for variadic functions that wish to pass all of their argu-
     ments on to a function that takes a va_list argument, such as

MirBSD #10-current             October 24, 2002                              2

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