.\" Copyright (C) 1995-2017 Bruno Haible
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.TH TRAMPOLINE 3 "1 January 2017"
.SH NAME
trampoline \- closures as first-class C functions
.SH SYNOPSIS
.B #include <trampoline.h>
.LP
.B function = alloc_trampoline(address, variable, data);
.LP
.B free_trampoline(function);
.LP
.nf
.B is_trampoline(function)
.B trampoline_address(function)
.B trampoline_variable(function)
.B trampoline_data(function)
.fi
.SH DESCRIPTION
.LP
These functions implement
.I 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
.I first-class C functions
means that they fit into a function
pointer and can be called exactly like any other C function.
.IB function " = alloc_trampoline(" address ", " variable ", " data ")"
allocates a closure. When
.I function
gets called, it stores
.I data
in the variable
.I variable
and calls the C function at
.IR address .
The function at
.I address
is responsible for fetching
.I data
out of
.I variable
immediately, before execution of any other function call.

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

.BI "is_trampoline(" function ")"
checks whether the C function
.I function
was produced by a call to
.IR alloc_trampoline .
If this returns true, the arguments given to
.I alloc_trampoline
can be retrieved:
.RS 4
.LP
.BI "trampoline_address(" function ")"
returns
.IR address ,
.LP
.BI "trampoline_variable(" function ")"
returns
.IR variable ,
.LP
.BI "trampoline_data(" function ")"
returns
.IR data .
.RE

.SH SEE ALSO
.BR gcc (1),
.BR stdarg (3),
.BR callback (3)

.SH BUGS
Passing the data through a global variable is not reentrant. Don't call
trampoline functions from within signal handlers. This is fixed in the
.BR callback (3)
package.

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

.SH AUTHOR

Bruno Haible <bruno@clisp.org>

.SH ACKNOWLEDGEMENTS

Many ideas were cribbed from the gcc source.