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Re: scopped lock in plain C (similar to std::lock_guard in C++)

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On 2021-12-13, Kirill Frolov <fk0@fk0.name> wrote:
> Hello...
>
>   I want to discuss the possibility of the implementation of some
> programming technique, which allows to use synchronization primitives,
> especially mutexes, seamlessly, like this can be done in C++ with
> std::lock_guard.

A good way to do this kind of thing is exemplified in the POSIX threads
API with thread cleanup handler management macros:

  pthread_cleanup_push(function, arg);

  /* if thread is cancelled here, function(arg) gets called. */

  pthread_cleanup_pop(1);  /* 1 -> call function(arg); 0 -> don't */

The push and pop calls have to pair.

(The Boolean argument in the pop call is clever; there are scenarios when
you don't want to call the cleanup function, or conditionally call it
based on the value of some expression.)

[snip baroque ideas]

#define LOCK(mutex)    { mutex_lock(&(mutex))

#define UNLOCK(mutex)  mutex_unlock(&(mutex)); }

That's the basic idea. Of course, it's hardly fool proof, e.g:

  LOCK(mutex)

  } /* matches open brace in LOCK */

Firstly, your editor will catch this; when you try to reindent the code,
it turns to shit. Secondly, there are a few things you can do to reduce
this threat and improve it in other ways, though I wouldn't go to too
many lengths. One simple idea:

#define LOCK(mutex)    do { mutex_lock(&(mutex))

#define UNLOCK(mutex)  mutex_unlock(&(mutex)); } while (0)

Now you can't close a LOCK(mutex) with just a curly brace, because
do { } is a syntax error without a while (...);.

If you want access to the mutex_lock return value, you have
to do it otherwise, like put the lock call outside of the braces:

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal

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scopped lock in plain C (similar to std::lock_guard in C++) Kirill Frolov <fk0@fk0.name> - 2021-12-13 00:46 +0000
  Re: scopped lock in plain C (similar to std::lock_guard in C++) Kaz Kylheku <480-992-1380@kylheku.com> - 2021-12-17 07:36 +0000

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