[libcamera-devel] [PATCH 1/3] Early return if no cameras are found on the system.

[Laurent Pinchart]

Hi Kieran,

On Thu, Nov 19, 2020 at 12:40:08PM +0000, Kieran Bingham wrote:
> Hi Umang,
> 
> $SUBJECT is missing a "simple-cam:" prefix to match the others.
> 
> Was this patch already posted somewhere?
> 
> I don't seem to be able to find it, but I do half-recollect it ;-)
> I just can't figure out where from.
> 
> On 19/11/2020 12:34, Umang Jain wrote:
> > Failing to do so, the codepath will segfault while trying to acquire
> > a non-existent camera.
> > 
> > Signed-off-by: Umang Jain <email at uajain.com>
> > Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> > ---
> >  simple-cam.cpp | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/simple-cam.cpp b/simple-cam.cpp
> > index 727bb6d..af9c8b1 100644
> > --- a/simple-cam.cpp
> > +++ b/simple-cam.cpp
> > @@ -307,6 +307,13 @@ int main()
> >  	for (std::unique_ptr<Request> &request : requests)
> >  		camera->queueRequest(request.get());
> >  
> > +	if(!cm->cameras().size()) {
> > +		std::cout << "No cameras were identified on the system."
> > +			  << std::endl;
> > +		cm->stop();
> > +		return -1;
> 
> I think we should we return -ENODEV instead of -1 here. We already use
> errno values (for example -ENOMEM is used).

Note that the return value of the main() function is a bit special. C
and C++ define EXIT_SUCCESS and EXIT_FAILURE to denote success and
failure respectively (not a surprise :-)). The value of those macros is
unspecified. See https://en.cppreference.com/w/c/program/EXIT_status.

(Interestingly, the standard defines that both EXIT_SUCCESS and the
value zero indicate successful program execution status, but don't
require EXIT_SUCCESS to be equal to zero.)

Other values are allowed, but only the 8 least significant bits are
conveyed to the parent, so the range of usable values is effectively
0-255. Furthermore, shells may reserve some values. For instance, bash
sets $? to 126 if a command is found but not executable, 127 if the
command is not found, and 128+N if the command terminates with a fatal
signal N.

As I don't expect we'll need to convey failure reasons through the exit
status for simple cam, how about returning either 0 and 1, or
EXIT_SUCCESS and EXIT_FAILURE ?

> > +	}
> > +
> >  	/*
> >  	 * --------------------------------------------------------------------
> >  	 * Run an EventLoop

-- 
Regards,

Laurent Pinchart