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va_start(3) [osf1 man page]

varargs(3)						     Library Functions Manual							varargs(3)

NAME
varargs, va_arg, va_start, va_end - Handles a variable-length parameter list LIBRARY
Standard C Library (libc.a) SYNOPSIS
#include <varargs.h> va_alist va_dcl void va_start ( va_alist argp ); type va_arg ( va_alist argp, type ); void va_end ( va_alist argp ); PARAMETERS
argp Specifies a variable that the varargs macros use to keep track of the current location in the parameter list. Do not modify this variable. type Specifies the type to which the expected argument will be converted when passed as an argument. Unsigned char or short arguments 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 because the type cannot be determined at run time. DESCRIPTION
NOTE: When writing new code, it is recommended that you use stdarg instead of varargs. They both perform the same function, but stdarg is standards-compliant and varargs complies only with BSD conventions. The varargs set of macros allows you to write portable functions that accept a variable number of parameters. Subroutines that have vari- able-length parameter lists (such as the printf() function), but that do not use the varargs macros, are inherently nonportable because different systems use different parameter-passing conventions. The varargs macros are as follows: va_alist() Defines the type of the variable used to traverse the list. va_start() Initializes argp to point to the beginning of the list. The va_start() macro will be invoked before any access to the unnamed argu- ments. va_arg() Returns the next parameter in the list pointed to by argp. va_end() Cleans up at the end. Your function can traverse, or scan, the parameter list more than once. Start each traversal with a call to va_start() and end it with va_end(). EXAMPLE
The following example is a possible implementation of the execl() function: #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) ; /* Empty loop body */ va_end(ap); return (execv(file, args)); } NOTES
The calling routine is responsible for specifying the number of parameters because it is not always possible to determine this from the stack frame. For example, the execl() function is passed a null pointer to signal the end of the list. The printf() function determines the number of parameters from its fmt parameter. AES Support Level: Temporary use RELATED INFORMATION
Functions: exec(2), printf(3), stdarg(3), vprintf(3) delim off varargs(3)

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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 5.11 10 May 2002 varargs(3EXT)
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