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varargs(3) [bsd man page]

VARARGS(3)						     Library Functions 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 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 con- ventions. 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 converted to unsigned int, and float arguments are converted to double. Different types can be mixed, but it is up to the rou- tine 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 { 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 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. 7th Edition May 15, 1986 VARARGS(3)

Check Out this Related Man Page

varargs(5)							File Formats Manual							varargs(5)

Name
       varargs - handle variable argument list

Syntax
       #include <varargs.h>

       va_alist

       va_dcl

       void va_start(pvar)
       va_list pvar;

       type va_arg(pvar, type)
       va_list pvar;

       void va_end(pvar)
       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 but that do not use are inherently nonportable, 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.  A semicolon should not 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	 Returns 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. This information cannot be determined at run
		 time.

       va_end	 is used to clean up.

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

       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, is passed a zero pointer to signal the end of the list.  The routine can tell how many arguments there are by the format.

       It  is  nonportable  to specify a second argument of char, short, or float to va_arg, because 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  func-
       tion.

Examples
       The following example presents an implementation of

	    #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);
	    }

See Also
       exec(2), printf(3s), vprintf(3s)

								       RISC								varargs(5)

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