stdarg(3) [minix man page]

STDARG(3)						     Library Functions Manual							 STDARG(3)

NAME

       stdarg - variable argument list

SYNOPSIS

       #include <stdarg.h>

       void va_start(va_list ap, argtypeN parmN)
       type va_arg(va_list ap, type)
       void va_end(va_list ap)

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 stdarg are inherently nonportable, since different machines use different argument  passing  con-
       ventions.

       A function that accepts a variable argument list is declared with "..." at the end of its parameter list.  It must have at least one normal
       argument before the "...".  For example:

	      int printf(const char *format, ...) { /* code */ }
	      int fprintf(FILE *stream, const char *format, ...) { /* code */ }

       va_list is a type which is used for the variable ap within the body of a variable argument function which is used to traverse the list.

       va_start(ap, parmN) is called to initialize ap to the beginning of the list.  The last true parameter of the function, parmN, must be  sup-
       plied to allow va_start to compute the address of the first variable parameter.

       va_arg(ap,  type) will return the next argument in the list pointed to by ap.  Type is the type to which the expected argument will be con-
       verted when passed as an argument.

       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  run-
       time.

       va_end(ap) must be used to finish up.

       Multiple traversals, each bracketed by va_start ...  va_end, are possible.

EXAMPLE

	   #include <stdarg.h>
	   execl(const char *path, ...)
	   {
	       va_list ap;
	       char *args[100];
	       int argno = 0;

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

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, execl passes a null pointer 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.

BUGS

       It is impossible to properly show the macros as C declarations as is done in the synopsis.  They can never be coded as C functions, because
       all three macros use their arguments by address, and the type field is certainly impossible.  Just look at them as being part of the C lan-
       guage, like sizeof.

7th Edition							   May 15, 1986 							 STDARG(3)

Check Out this Related Man Page

varargs(3EXT)						    Extended Library Functions						     varargs(3EXT)

NAME

       varargs - handle variable argument list

SYNOPSIS

       #include <varargs.h>
       va_alist
       va_dcl
       va_list pvar;

       void va_start(va_listpvar);

       type va_arg(va_list pvar, type);

       void va_end(va_list pvar);

DESCRIPTION

       This set of macros allows portable procedures that accept variable argument lists to be written. Routines that have variable argument lists
       (such as printf(3C)) but do not use varargs are inherently non-portable, as different machines use different argument-passing conventions.

       va_alist is used as the parameter list in a function header.

       va_dcl is a declaration for va_alist. No semicolon should follow va_dcl.

       va_list is a type defined for the variable used to traverse the list.

       va_start is called to initialize pvar to the beginning of the list.

       va_arg 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, as it cannot be determined at runtime.

       va_end is used to clean up.

       Multiple traversals, each bracketed by va_start and va_end, are possible.

EXAMPLES

       Example 1: A sample program.

       This example is a possible implementation of execl (see exec(2) ).

	      #include <unistd.h>
	      #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]; 	 /* assumed big enough*/
		   int argno = 0;

		   va_start(ap);
		   file = va_arg(ap, char *);
		   while ((args[argno++] = va_arg(ap, char *)) != 0)
			;
		   va_end(ap);
		   return execv(file, args);
	      }

SEE ALSO

       exec(2), printf(3C), vprintf(3C), stdarg(3EXT)

NOTES

       It  is  up  to the calling routine to specify in some manner how many arguments there are, since it is not always possible to determine the
       number of arguments from the stack frame. For example, execl is passed a zero pointer to signal the end of the list. printf  can  tell  how
       many arguments are there by the format.

       It  is  non-portable  to  specify a second argument of char, short, or float to va_arg, since arguments seen by the called function are not
       char, short, or float. C converts char and short arguments to int and converts float arguments to double before passing them to a function.

       stdarg is the preferred interface.

SunOS 5.10							    10 May 2002 						     varargs(3EXT)

Linux and UNIX Man Pages

stdarg(3) [minix man page]

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