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CALLBACK(3) | Library Functions Manual | CALLBACK(3) |
NAME¶
callback - closures with variable arguments as first-class C functionsSYNOPSIS¶
#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. Moreover, 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.
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 inaccurate: 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/config/cpu/cpu.h.AUTHOR¶
Bruno Haible <bruno@clisp.org>ACKNOWLEDGEMENTS¶
Many ideas were cribbed from the gcc source.14 January 2001 |