[PATCH] Minor error in serial_configure

[Jef Driesen]

On 2014-05-14 10:49, Jef Driesen wrote:
> On 2014-05-13 13:37, Jef Driesen wrote:
>> On 2014-05-13 11:39, Venkatesh Shukla wrote:
>>> I have attached a patch to a minute mistake that I noticed in the 
>>> code.
>>> 
>>> -	if (memcmp (&tty, &active, sizeof (struct termios) != 0)) {
>>> +	if (memcmp (&tty, &active, sizeof (struct termios)) !=0) {
>> 
>> Nice catch! I'll apply the patch.
>> 
>> Due to this bug, the memcmp only checked the first byte. I wonder
>> whether fixing this will have any negative side effect, with this
>> discussion in mind:
>> 
>> http://libdivecomputer.org/pipermail/devel/2014-March/000268.html
>> 
>> Anyway, we'll find out soon enough.
> 
> Great, I already discovered a major problem. After applying this
> patch, the memcmp always fails on my system. Not good.
> 
> The problem seems to be that the termios struct contains some padding
> bytes on my system. Because the content of padding bytes is
> unspecified, they may contain some random data, which causes the
> memcmp to fail. Comparing the individual termios members, which would
> be the portable way to compare structs, isn't an option here, because
> the contents of the termios struct is platform dependent.
> 
> Explicitly initializing the termios structure with memset, will also
> set the padding bytes to zero. This seems to work fine on my system.
> But the C standard doesn't make any guarantee regarding padding bytes.
> Thus in theory the compiler is free to change those padding values at
> will. I doubt this will be a problem in practice, but just in case,
> please test the attached patch.
> 
> The alternative solution is removing the memcmp check completely.

Has anyone tried the patch? On my system it still fails for all backends 
that require odd or even parity (e.g. vyper and iconhd). I assume this 
is only a pty limitation (see Linus comments in the link above), but 
since I don't have one of those devices at my disposal, I can't try on 
real hardware.

So most likely, either this memcmp will have to be removed completely, 
or at least conditionally compiled on the ENABLE_PTY macro.

Jef