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

[Carlos O'Donell <carlos at redhat dot com>]

On 10/18/2017 07:18 AM, 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.
> 
> 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 );
> }
 
Only (b).

(a) The kernel knows, but doesn't tell you.

(b) Only via architecture-specific checks which involve inspecting the
    call frame to determine if there is a signal frame on the stack.

(c) glibc *could* know, but it probably needs kernel help for other
    purposes.

This issue was discussed orthogonality here:
https://sourceware.org/ml/libc-alpha/2014-02/msg00798.html

Where glibc would create a wrapper for all signal handlers to save and
restore errno to work around bugs in signal handlers that modify errno.

There is no solution yet.

-- 
Cheers,
Carlos.