VARARGS(S) XENIX System V VARARGS(S)
NAME
varargs - variable argument list
Synposis
#include <varargs.h>
function(va_alist)
va_dcl
va_list pvar;
va_start(pvar);
f = va_arg(pvar, type);
va_end(pvar);
Description
This set of macros provides a means of writing portable
procedures that accept variable argument lists. Routines
having variable argument lists (such as printf(S)) that do
not use varargs are inherently nonportable, since different
machines use different argument passing conventions.
va_alist is used in a function header to denote a variable
argument list.
va_dcl is a declaration for va_alist. Note that there is no
semicolon after va_dcl.
va_list is a type which can be used for the variable pvar,
which is used to traverse the list. One such variable must
always be declared.
va_start(pvar) is called to initialize pvar to the beginning
of the list.
va_arg(pvar, type) will return the next argument in the list
pointed to by pvar. type is the type the argument is
expected to be. Different types can be mixed but it is up
to the routine to know what type of argument is expected
since it cannot be determined at runtime.
va_end(pvar) is used to finish up.
Multiple traversals, each bracketed by va_start ... va_end,
are possible.
Page 1 (printed 8/7/87)
VARARGS(S) XENIX System V VARARGS(S)
Example
#include <stdio.h>
#include <varargs.h>
main()
{
show(2, 3.1, "but", 4.1, "end");
show(1, 5.9, "hello");
show(4, 6.2, "oops", 5.3, "blah", 5.1, "lovely", 2.3, "madrigal");
}
/*
* the first argument is an int which tells how many pairs follow.
* the pairs are doubles and character pointers
*
* remember that when variables are passed to functions
* floats are promoted to doubles and chars to ints.
*/
show(n, va_alist)
int n;
va_dcl
{
va_list ap;
int i;
double f;
char *p;
va_start(ap);
for (i = 0; i < n; ++i) {
f = va_arg(ap, double);
p = va_arg(ap, char *);
printf("%4.1f %s\n", f, p);
}
va_end(ap);
}
Notes
It is up to the calling routine to determine how many
arguments there are, since it is not possible to determine
this from the stack frame. For example, excel passes a 0 to
signal the end of the list. printf can tell how many
arguments are supposed to be there by the format of the
list.
Page 2 (printed 8/7/87)
