2.11BSD#431 - /usr/man/cat3/varargs.0

VARARGS(3)	    UNIX Programmer's Manual	       VARARGS(3)


NAME
     varargs - variable argument list

SYNOPSIS
     #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 pro-
     cedures that accept variable argument lists.  Routines hav-
     ing 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
     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 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 con-
     verted 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 possible.

EXAMPLE
	  #include <varargs.h>
	  execl(va_alist)
	  va_dcl


Printed 11/26/99	  May 15, 1986				1


VARARGS(3)	    UNIX Programmer's Manual	       VARARGS(3)


	  {
	       va_list ap;
	       char *file;
	       char *args[100];
	       int argno = 0;

	       va_start(ap);
	       file = va_arg(ap, char *);
	       while (args[argno++] = va_arg(ap, char *))
		    ;
	       va_end(ap);
	       return execv(file, args);
	  }

BUGS
     It is up to the calling routine to determine how many argu-
     ments there are, since it is not possible to determine this
     from the stack frame.  For example, execl passes a 0 to sig-
     nal 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 com-
     plex statement.


Printed 11/26/99	  May 15, 1986				2