varargs - variable argument list
f = va_arg(pvar, type);
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.
va_alist is used in a function
header to declare a variable
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
va_start(pvar) is called to
initialize pvar to the beginning of the
va_arg(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.
va_end(pvar) is used to finish up.
Multiple traversals, each
bracketed by va_start ... va_end, are
int argno = 0;
file = va_arg(ap, char *);
while (args[argno++] = va_arg(ap, char *))
return execv(file, args);
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.