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NAME    [Toc]    [Back]

     va_start, va_arg, va_copy, va_end - variable argument lists

SYNOPSIS    [Toc]    [Back]

     #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);

DESCRIPTION    [Toc]    [Back]

     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_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
     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
     initialized 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 specified
 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 argument
 promotions, see below), random errors will occur.

     If  the  type in question is one that would normally be promoted, the promoted
 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 variable
  argument  list  was  initialized  by   va_start()   or

     The va_end() macro returns no value.

EXAMPLES    [Toc]    [Back]

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

           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 %s0, s);
                           case 'd':                       /* int
                                   d = va_arg(ap, int);
                                   printf("int %d0, d);
                           case   'c':                         /*
char */
                                   c  =  va_arg(ap,  int);     /*
promoted */
                                   printf("char %c0, c);
                           case   'f':                         /*
float */
                                   f  =  va_arg(ap,  double);  /*
promoted */
                                   printf("float %f0, f);

STANDARDS    [Toc]    [Back]

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

     The  va_start(), va_arg() and va_end() macros conform to ANSI/ISO/IEC
     9899-1999 (``ANSI C99'').

HISTORY    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     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 arguments
 on to a function that takes a va_list  argument,  such

OpenBSD      3.6                         October     24,     2002
[ Back ]
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