errno in system calls.

[Giacomo Tesio]

Hi, I'm not a newlib developer, just one walking through the same path, so
take this with a grain of salt.

I'm using newlib as a POSIX facade to my non-POSIX system (a Plan 9
derivative).

Here you can see how I implemented the reentrant syscalls are using errno,
for example

https://github.com/JehanneOS/newlib/blob/master/newlib/libc/sys/jehanne/syscalls.c#L58-L63


Here an example of a syscall that does not provide the reentrant version in
newlib

https://github.com/JehanneOS/newlib/blob/master/newlib/libc/sys/jehanne/syscalls.c#L211-L216


(the POSIX_* functions are provided by a library, libposix, that contains
the code that I hope to reuse)



Giacomo

2017-08-29 16:56 GMT+02:00, David Venhoek <david@venhoek.nl>:
> Dear newlib developers,
>
> I am currently working on porting newlib towards a hobby operating
> system, and I am having the feeling that I don't have a full grasp on
> how the interactions between errno and the reentrancy system works.
>
> I currently implements the system calls required by newlib as
> non-reentrant functions, using their direct (non-underscored names,
> e.g. open). For this I undef errno, and refer to the extern int errno
> as specified in the documentation.
>
> As I understand it, this works correctly when interacting with these
> functions through the reentrant covers, as the rest of the newlib
> does, as these functions take care of picking up the errno. However, I
> don't fully understand how this is supposed to work when calling these
> functions directly from user programs. How do these programs pick up
> the value of errno? Or is this use case not possible when implementing
> them these ways, and if so, what is the recommended approach to
> implementing these system calls so as to achieve this?
>
> Thank you in advance for your time.
>
> Kind regards,
> David Venhoek
>