varargs.5

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 5 代码 · 共 131 行

.TH varargs 5 RISC
.SH Name
varargs \- handle variable argument list
.SH Syntax
.B #include <varargs.h>
.PP
.B va_alist
.PP
.B va_dcl
.PP
.B void va_start(pvar)
.br
.B va_list pvar;
.PP
.B \fItype\fB va_arg(pvar, \fItype\fB)
.br
.B va_list pvar;
.PP
.B void va_end(pvar)
.br
.B va_list pvar;
.SH Description
This set of macros allows portable procedures that accept variable
argument lists to be written.
Routines that have variable argument lists, such as
.MS printf 3s ,
but that do not use 
.PN varargs
are inherently nonportable, as different
machines use different argument-passing conventions.
.PP
.TP 10
.B va_alist
Is used as the parameter list in a function header.
.TP 10
.B va_dcl
Is a declaration for 
.IR va_alist .
A semicolon should not follow
.IR va_dcl .
.TP 10
.B va_list
Is a type defined for the variable
used to traverse the list.
.TP 10
.B va_start
Is called to initialize
.I pvar
to the beginning of the list.
.TP 10
.B va_arg
Returns the next argument in the list
pointed to by
.IR pvar .
.I 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. This information cannot be determined at run time.
.TP 10
.B va_end
is used to clean up.
.PP
Multiple traversals, each bracketed by
.I va_start
\&...
.IR va_end ,
are possible.
.PP
The calling routine must specify how many arguments
there are, because it is not always possible to determine this from the
stack frame.  For example,
.PN execl
is passed a zero pointer to signal the end of the list.  The
.PN printf
routine can tell how many arguments there are by the format.
.PP
It is nonportable to specify a second argument of
.IR char ,
.IR short ,
or
.IR float
to 
.IR va_arg ,
because arguments seen by the called function are not
.IR char ,
.IR short ,
or
.IR float .
C converts 
.I char
and 
.I short
arguments to 
.I int
and
converts
.I float
arguments to
.IR double 
before passing them to a function.
.SH Examples
The following example presents an implementation of 
.MS execl 2 :
.PP
.nf
	#include <varargs.h>
	#define MAXARGS	100

	/\(**	execl is called by
			execl(file, arg1, arg2, ..., (char \(**)0);
	\(**/
	execl(va_alist)
	va_dcl
	{
		va_list ap;
		char \(**file;
		char \(**args[MAXARGS];
		int argno = 0;

		va_start(ap);
		file = va_arg(ap, char \(**);
		while ((args[argno++] = va_arg(ap, char \(**)) != (char \(**)0)
			;
		va_end(ap);
		return execv(file, args);
	}
.fi
.SH See Also
exec(2), printf(3s), vprintf(3s)