1 January 2017
trampoline - closures as first-class C functions
function = alloc_trampoline(address, variable, data);
is_trampoline(function) trampoline_address(function) trampoline_variable(function) trampoline_data(function)
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(address, variable, data) allocates a closure. When function gets called, it stores data in the variable variable and calls the C function at address. The function at address is responsible for fetching data out of variable immediately, before execution of any other function call.
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(function) is called.
is_trampoline(function) checks whether the C function function was produced by a call to alloc_trampoline. If this returns true, the arguments given to alloc_trampoline can be retrieved:
trampoline_address(function) returns address,
trampoline_variable(function) returns variable,
trampoline_data(function) returns data.
Passing the data through a global variable is not reentrant. Don’t call trampoline functions from within signal handlers. This is fixed in the callback(3) package.
The way gcc builds local functions is described in the gcc source, file gcc-2.6.3/config/cpu/cpu.h.
Bruno Haible <bruno>
Many ideas were cribbed from the gcc source.