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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)

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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 5.10 10 May 2002 varargs(3EXT)

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