CALLBACK(3)						     Library Functions Manual						       CALLBACK(3)

NAME

       callback - closures with variable arguments as first-class C functions

SYNOPSIS

       #include <callback.h>

       void function (data, alist)
	 void* data;
	 va_alist alist;
       {
	 va_start_type(alist[, return_type]);
	 arg = va_arg_type(alist[, arg_type]);
	 va_return_type(alist[[, return_type], return_value]);
       }

       callback = alloc_callback(&function, data);

       free_callback(callback);

       is_callback(callback)
       callback_address(callback)
       callback_data(callback)

DESCRIPTION

       These functions implement closures with variable arguments as first-class C functions.

       Closures  as first-class C functions means that they fit into a function pointer and can be called exactly like any other C function. More-
       over, they can be called with variable arguments and can return variable return values.

       callback = alloc_callback(&function, data) allocates a callback. When callback gets called, it arranges to call function, passing  data	as
       first argument and, as second argument, the entire sequence of arguments passed to callback.

       Function calling conventions differ considerably on different machines, therefore the arguments are accessed and the result value is stored
       through the same macros as used by the vacall package, see below.

       The callbacks are functions with indefinite extent: callback is only deallocated when free_callback(callback) is called.

       is_callback(callback) checks whether the C function callback was produced by a call to alloc_callback.  If this returns true, the arguments
       given to alloc_callback can be retrieved:

	   callback_address(callback) returns &function,

	   callback_data(callback) returns data.

VACALL MACROS

       Within function, the following macros can be used to walk through the argument list and specify a return value:

       va_start_type(alist[, return_type]);
	      starts the walk through the argument list and specifies the return type.

       arg = va_arg_type(alist[, arg_type]);
	      fetches the next argument from the argument list.

       va_return_type(alist[[, return_type], return_value]);
	      ends the walk through the argument list and specifies the return value.

       The type in va_start_type and va_return_type shall be one of void, int, uint, long, ulong, longlong, ulonglong, double, struct, ptr or (for
       ANSI C calling conventions only) char, schar, uchar, short, ushort, float, depending on the class of return_type.

       The type specifiers in va_start_type and va_return_type must  be  the  same.   The  return_type	specifiers  passed  to	va_start_type  and
       va_return_type must be the same.

       The type in va_arg_type shall be one of int, uint, long, ulong, longlong, ulonglong, double, struct, ptr or (for ANSI C calling conventions
       only) char, schar, uchar, short, ushort, float, depending on the class of arg_type.

       In va_start_struct(alist, return_type, splittable); the splittable flag specifies whether the struct return_type can be returned in  regis-
       ters  such  that  every struct field fits entirely in a single register. This needs to be specified for structs of size 2*sizeof(long). For
       structs of size <= sizeof(long), splittable is ignored and assumed to be 1. For structs of size > 2*sizeof(long), splittable is ignored and
       assumed to be 0. There are some handy macros for this:
       va_word_splittable_1 (type1)
       va_word_splittable_2 (type1, type2)
       va_word_splittable_3 (type1, type2, type3)
       va_word_splittable_4 (type1, type2, type3, type4)
       For a struct with three slots
       struct { type1 id1; type2 id2; type3 id3; }
       you can specify splittable as va_word_splittable_3 (type1, type2, type3) .

NOTES

       Functions  which  want to emulate Kernighan & Ritchie style functions (i.e., in ANSI C, functions without a typed argument list) cannot use
       the type values char, schar, uchar, short, ushort, float.  As prescribed by the default K&R C expression promotions, they have to  use  int
       instead of char, schar, uchar, short, ushort and double instead of float.

       The macros va_start_longlong(), va_start_ulonglong(), va_return_longlong(), va_return_ulonglong(), va_arg_longlong() and va_arg_ulonglong()
       work only if the C compiler has a working long long 64-bit integer type.

       The struct types used in va_start_struct() and va_struct() must only contain (signed or unsigned) int, long, long long or  pointer  fields.
       Struct types containing (signed or unsigned) char, short, float, double or other structs are not supported.

SEE ALSO

       vacall(3), trampoline(3).

BUGS

       The current implementations have been tested on a selection of common cases but there are probably still many bugs.

       There are typically built-in limits on the size of the argument-list, which may also include the size of any structure arguments.

       The  decision  whether a struct is to be returned in registers or in memory considers only the struct's size and alignment. This is inaccu-
       rate: for example, gcc on m68k-next returns struct { char a,b,c; } in registers and struct { char a[3]; } in memory,  although  both  types
       have the same size and the same alignment.

       <callback.h> cannot be included when <varargs.h> or <stdarg.h> is included.  (Name clash for va_alist.)

       The argument list can only be walked once.

NON-BUGS
       All information is passed in CPU registers and the stack. The callback package is therefore multithread-safe.

PORTING

       Porting	callback  consists  in first porting the vacall and trampoline packages, then choosing a CPU register for passing the closure from
       trampoline to vacall.  This register is normally the register designated by STATIC_CHAIN_REGNUM in  the	gcc  source,  file  gcc-2.7.2/con-
       fig/cpu/cpu.h.

AUTHOR

       Bruno Haible <bruno@clisp.org>

ACKNOWLEDGEMENTS

       Many ideas were cribbed from the gcc source.

								  14 January 2001						       CALLBACK(3)