ffcall-trampoline_r (3) - Linux Manuals

ffcall-trampoline_r: closures as first-class C functions

NAME

trampoline - closures as first-class C functions

SYNOPSIS

#include <trampoline_r.h>

function = alloc_trampoline_r(address, data0, data1);

free_trampoline_r(function);

is_trampoline_r(function)
trampoline_r_address(function)
trampoline_r_data0(function)
trampoline_r_data1(function)

DESCRIPTION

These functions implement closures as first-class C functions. A closure consists of a regular C function and a piece of data which gets passed to the C function when the closure is called.

Closures as first-class C functions means that they fit into a function pointer and can be called exactly like any other C function. function = alloc_trampoline_r(address, data0, data1) allocates a closure. When function gets called, it stores in a special "lexical chain register" a pointer to a storage area containing data0 in its first word and data1 in its second word and calls the C function at address. The function at address is responsible for fetching data0 and data1 off the pointer. Note that the "lexical chain register" is a call-used register, i.e. is clobbered by function calls.

This is much like gcc's local functions, except that the GNU C local functions have dynamic extent (i.e. are deallocated when the creating function returns), while trampoline provides functions with indefinite extent: function is only deallocated when free_trampoline_r(function) is called.

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

trampoline_r_address(function) returns address,

trampoline_r_data0(function) returns data0,

trampoline_r_data1(function) returns data1.

PORTING

The way gcc builds local functions is described in the gcc source, file gcc-2.6.3/config/cpu/cpu.h.

AUTHOR

Bruno Haible <bruno [at] clisp.org>

ACKNOWLEDGEMENTS

Many ideas were cribbed from the gcc source.