CALLBACK manual page

• Name
• Synopsis
• Description
• VACALL macros
• Notes
• See also
• Bugs
• Non-Bugs
• Porting
• Author
• Acknowledgements

#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)

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.

va_start_type(alist[, return_type]);

arg = va_arg_type(alist[, arg_type]);

va_return_type(alist[[, return_type], 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 registers 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)

struct { type1 id1; type2 id2; type3 id3; }

1. 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.

2. 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.

3. 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.

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

Last modified: 14 January 2001.