VARARGS(5) VARARGS(5)
NAME
varargs - variable argument list
SYNOPSIS
#include <varargs.h>
function(vaalist)
vadcl
valist pvar;
vastart(pvar);
f = vaarg(pvar, type);
vaend(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(3)) that do
not use varargs are inherently nonportable, since different
machines use different argument passing conventions.
vaalist is used in a function header to declare a variable
argument list.
vadcl is a declaration for vaalist. Note that there is no
semicolon after vadcl.
valist 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.
vastart(pvar) is called to initialize pvar to the beginning
of the list.
vaarg(pvar, type) will return the next argument in the list
pointed to by pvar. Type is the type to which the expected
argument will be converted when passed as an argument. In
standard C, arguments that are char or short should be
accessed as int, unsigned char or unsigned short are
converted to unsigned int, and float arguments are converted
to double. 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. Also, templates cannot
be used with varargs(5).
vaend(pvar) is used to finish up.
Multiple traversals, each bracketed by vastart ... vaend,
are possible.
EXAMPLE
#include <varargs.h>
execl(vaalist)
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VARARGS(5) VARARGS(5)
vadcl
{
valist ap;
char *file;
char *args[100];
int argno = 0;
vastart(ap);
file = vaarg(ap, char *);
while (args[argno++] = vaarg(ap, char *))
;
vaend(ap);
return execv(file, args);
}
BUGS
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, execl passes a 0 to
signal the end of the list. Printf can tell how many
arguments are supposed to be there by the format.
The macros va_start and va_end may be arbitrarily complex;
for example, va_start might contain an opening brace, which
is closed by a matching brace in va_end. Thus, they should
only be used where they could be placed within a single
complex statement.
ORIGIN
MIPS Computer Systems
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