Re: determine whether code is running in a signal handler context

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 18/10/17 15:18, Yubin Ruan wrote:
> Hi,
> 
> I am writing to see if this is any util functions in libc that can
> help to determine it is currently running in a signal.
> 
> I wrote some library and provide a function which will be used in many
> client code. However this function is not async-signal safe (it calls
> malloc(3)) so when it is called, I want to detect whether it is
> currently running in a signal handler. If it is, I can avoid calling
> those not-async-signal-safe functions which might cause deadlock.
> 

note that in posix as-safety is symmetric between the
interrupted code and interrupt handler: if any of the
interrupt and interrupt handler is as-safe then the
behaviour is well defined.

so calling non-as-safe code in an asynchronous signal
handler is perfectly fine if the interrupted code is
as-safe.

there are synchronous signals too, i.e. raise(sig),
and then the signal handler runs in well-defined state
(one can use signal masks to make sure a signal handler
only runs in such state)

so using "in_signal_handler_context()" is not a valid
way to verify the as-safety interface contract.

> that is, I want a `in_signal_handler_context()' utility that can be
> used as this:
> 
> int mylibfunc( void ) {
>     if( in_signal_handler_context() ) { return(-1) }
>     // rest of function goes here
>     return( 0 );
> }
> 
> 
> Yubin
>