Add half-hearted support for Clang, because its `blocks' are deficient.

[finally] / FINALLY.3

.\" -*-nroff-*-
.TH control 3 "28 April 2023" "Straylight/Edgeware" "FINALLY macro package"
.SH NAME
FINALLY \- defer execution until scope exit
.SH SYNOPSIS
.nf
.B "#include <finally.h>"

.BI FINALLY( stmt );
.BI FINALLY_TAGGED( tag ", " stmt );
.fi
.
.SH DESCRIPTION
.
.SS "General usage of FINALLY"
The
.B FINALLY
macro arranges that the statement
.I stmt
should be executed when control leaves the immediately enclosing scope,
whether this is by just running off the end, or by explicit control
flow, e.g.,
.BR return ,
.BR goto ,
.BR break,
or
.BR continue .
If a scope contains multiple
.B FINALLY
calls, then the statements are executed in reverse order.
.PP
In particular, if the macro call is placed at top-level within a
function body, then
.I stmt
is executed when the function returns, either as a result of a
.B return
statement, or by running off the end of the function body.
.PP
With GNU-like compilers, you can arrange for the statement to be
executed when control escapes the scope as the result of a C++ exception
by setting the
.B \-fexceptions
compiler flag.
.PP
The statement will
.I not
be executed if control leaves the scope as a result of
.BR longjmp (3)
or similar.  It also won't be executed if control `leaves' as a result
of a call to
.BR exit (3)
or similar; but that's almost certainly what you want.
.PP
The
.B FINALLY
macro's expansion is syntactically one or more declarations.  In C89
code, therefore, it must appear at the head of a block, before any
statements.  In C99 or later, it may appear anywhere in a block.
However, because it may expand to more than one declaration, it is not
suitable for use in a
.B for
statement.
.
.SS "Macros and FINALLY_TAGGED"
At most one use of
.B FINALLY
can occur on any given line of source code.  In particular, this
rule forbids macros which expand into more than one use of
.BR FINALLY ,
since the macro expansions will cause them all to be credited to the
line holding the original expansion site.  To avoid trouble, macro
authors should use
.B FINALLY_TAGGED
instead.  Each use of
.B FINALLY_TAGGED
on the same source line must have a distinct
.I tag
argument, which may be any identifier.
.
.SS "Nonlocal transfers, and thread and process exits"
The statement will
.I not
be executed if control leaves the scope as a result of
.BR longjmp (3)
or similar.  It also won't be executed if control `leaves' as a result
of a call to
.BR exit (3),
.BR pthread_exit (3),
.BR thrd_exit (3),
or similar.
.PP
You didn't want
.B FINALLY
blocks to run at
.BR exit (3)
time.  Almost all uses of
.B FINALLY
are to release process-local resources such as memory, file descriptors
or locks, or something else similar.  The kernel will release these by
itself when the process exits, because leaking resources when processes
crash would just be too awful to consider.  All you'd achieve by
carefully freeing memory prior to
.BR exit (3)
would be to waste time.
.
.SS "Setup and configuration"
The
.B FINALLY
macro uses compiler-specific techniques.  Not only does it need
different implementations on different compilers, but it may require
unusual compiler options and runtime support libraries to work.
.PP
Firstly, the
.B <finally.h>
header can work portably under C++11 or later, using RAII and lambdas:
idiomatic C++ doesn't benefit much from this, but the facility is there
anyway.  If the header detects that such a compiler is in use (the macro
.B __cplusplus
is defined to a value greater than or equal to 201103), then it will use
the C++-specific implementation.
.PP
Otherwise, the header checks the value of
.BR FINALLY_CONFIG_FLAVOUR :
it should be one of the following options.
.TP
.B NIL
No support.  An error is reported (using
.BR #error ).
.TP
.B GCC_NESTED_FUNCTIONS
Use the GNU C Compiler's support for nested functions and the
.B cleanup
function attribute.  Note that this use of nested functions does not
involve the use of trampolines, and does not require an executable
stack.  (This is verified as part of the package tests.)
.TP
.B CLANG_BLOCKS
Use the LLVM Clang compiler's support for `blocks' and the
.B cleanup
function attribute.  Depending on the environment, this may require the
.B \-fblocks
compiler option, and linking against the
.B \-lBlocksRuntime
library.
.IP
Support is only partial.  When the statement is executed on exit from
its scope, it sees the variable values that were current when the
.B FINALLY
declaration was processed, and not any changes since.  As a result of
this bug, the
.B <finally.h>
header will fail unless the macro
.B FINALLY_TOLERATE_BUG_CAPTURE_COPIES
is defined to a nonzero value, and will set
.B FINALLY_BUG_CAPTURE_COPIES
for your program to act on.
.PP
If
.B FINALLY_CONFIG_FLAVOUR
is not set, then
.B <finally.h>
guesses one of these flavours based on the compiler version and other
predefined macros.  This guess may be wrong, and things may not work
anyway because necessary compiler options or libraries aren't available.
.PP
The best way to set everything up correctly is to use the provided
.B FINALLY_CHECK
Autoconf macro.  It will define
.B FINALLY_CONFIG_FLAVOUR
to a suitable value, and also report necessary compiler flags and
libraries in the
.B FINALLY_CFLAGS
and
.B FINALLY_LIBS
makefile variables.  See the
.B finally.m4
file's internal documentation for the details.
.
.SH BUGS
The selection of supported compilers is extremely limited.
.PP
Clang doesn't work properly because its `blocks' capture a copy of the
outer scope's variables, rather than closing over them properly.
.
.SH AUTHOR
Mark Wooding, <mdw@distorted.org.uk>