[Gambas-user] Get last errno from external function call

[Matheus Castello]

For example some calls from libgpiod return structs, if the return is a 
null pointer then you have an error. For this cases libgpiod set the 
error code to the `errno` and you need to read it as soon the function 
returns the null pointer.

Reference: 
https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/tree/include/gpiod.h#n28


On 10/5/2022 12:25 AM, Jussi Lahtinen wrote:
> You mean an error occurring inside the library? I don't think there is 
> any other way, than the library itself reporting the error code.
> I don't see why using __errno_location inside the library would fail. 
> Are you calling it from the Gambas code?

Yeah, I add an `External` declaration to it to try to get the errno, but 
as I said "surely the runtime (I mean the Gambas3 runtime) must be 
making other calls that are overwriting the errno" and I'm always 
getting `errno = 0` that means success.

.NET for example have an annotation to store the errno from an external 
function the `SetLastError`: 
https://github.com/dotnet/iot/blob/main/src/System.Device.Gpio/Interop/Unix/libgpiod/Interop.libgpiod.cs#L49 

I was looking for something similar in Gambas3.

BR,
Matheus Castello


> If this is about a missing error message, then are you reading it from 
> stderr?
> You need to give more details and/or some code to see.
> 
> Jussi
> 
> 
> 
> 
> On Wed, Oct 5, 2022 at 5:54 AM Matheus Castello <matheus at castello.eng.br 
> <mailto:matheus at castello.eng.br>> wrote:
> 
>     Hi guys,
> 
>     I'm new to Gambas3, I'm trying to use it for embedded devices and I'm
>     doing some external function calls. There is some way in Gambas3 to
>     get,
>     or Gambas runtime stores, the last errno from a external function call?
> 
>     I tried to get it directly from libc `__errno_location` but surely the
>     runtime must be making other calls that are overwriting the errno.
> 
>     Thanks in advance,
>     Matheus Castello
> 
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