[DTrace-devel] [PATCH] Add support for built-in variable errno

[Eugene Loh]

On 4/24/21 9:57 PM, Kris Van Hees wrote:

> On Fri, Apr 23, 2021 at 12:50:12PM -0400, eugene.loh at oracle.com wrote:
>> From: Eugene Loh <eugene.loh at oracle.com>
>>
>> The built-in variable errno should be:
>>
>>    *)  the errno for failed system calls for syscall:::return probes
>>
>>    *)  0 otherwise
>>
>> This behavior is slightly different from the errno in C library functions
>> (where errno is not reset by successful system calls) and from the
>> behavior described in the DTrace Guide (which allows errno to be non-0
>> for non-syscall probes).
> The behaviour of errno in the C library is not relevant here because this is
> the D variable 'errno' which has no relation with the C library variable of the
> same name.

It seems to me that the two definitely have similarities. Meanwhile, the 
text is trying to clarify that the two are different.  It seems to me 
that far, far more users are familiar with libc's errno than are 
familiar with the semantic differences between it and DTrace's.  I vote 
for keeping the sentence, but if you want then let me know and I'll 
strike it.

> As far as the DTrace guide goes... the description there is not
> as accurate as I would like because the reference implementation (DTrace on
> Oracle Solaris) is very OS specific and Linux does not have any mechanism to
> provide the exact same functionality.

Yeah.  Anyhow, that's the situation.  So, thanks for the clarification, 
but I assume there are no implications for this patch?

>> Introduce an mst->syserrno, and have the provider trampolines set it to 0.
> Let's name it mst->errno for consistency with the errno variable itself.
Yeah.

Unfortunately, the naming collides with errno.h, which defines

         # define errno (*__errno_location ())

and makes using the string "errno" tricky.  E.g.,

     $ cat play1.c
     typedef struct dt_mstate {
         int errno;
     } dt_mstate_t;
     $ gcc -c play1.c

is okay.  Contrast that to when we add "#include <errno.h>":

     $ cat play2.c
     #include <errno.h>
     typedef struct dt_mstate {
         int errno;
     } dt_mstate_t;
     $ gcc -c play2.c
     In file included from play2.c:1:
     play2.c:3:6: error: field ‘__errno_location’ declared as a function
       int errno;
           ^~~~~

And some files (like dt_cg.c, among others) want to include both errno.h 
and dt_dctx.h.  (So, once again, DTrace can like using the name "errno" 
but the 900-pound gorilla in the room has already claimed it.)