Re: glibc strerrorname_np

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 04/11/2021 17:23, Jonny Grant wrote:
> Hi Carlos
> I was pleased to see you added strerrorname_np()
> 
> May I ask, I couldn't find the file your implementation is in - could you point it out to me in the glibc repository please?

It is on string/strerrorname_np.c, which calls __get_errname() defined at
stdio-common/errlist.c.

> 
> I noticed on the man page it may return NULL, which is a shame, as then it means we always need to check that before using in every printf etc :-
> 
> printf("err %s\n", strerrorname_np(myerr)?strerrorname_np(myerr), "Unknown err");

I didn't considered printf() when I added strerrorname_np(). Maybe an empty string ("")
would be better than NULL.

>  
> I'd done my own version a while ago as strerrno_s(), and assumed I could never get it accepted anywhere like glibc.
> Probably I should have tried to submit it to glibc!
> https://github.com/jonnygrant/safec/blob/master/strerrno.c
> 
> Would something like my implementation ever be accepted?
> errno_t strerrno_s(char * const buf, const rsize_t buflen, const errno_t errnum)

So this is basically:

  int strerrno_s (char *buf, size_t buflen, int errnum)
  {
    const char *r = strerrorname_np (errnum);
    __snprintf (buf, buflen, "%s", r == NULL ? "" : r);
    return errnum;
  }

I don't see much gain on adding another wrapper to format errno, the idea of
strerrorname_np() was to provide a async-signal-safe way to map errno to
string (by avoiding translation).

> 
> 
> Last quick question, do you know why strerror_r() is considered safer than strerror()? I guess someone could trash the memory returned by it?
> 
> char * errstr = strerror(EINVAL);
> errstr[0] = '\0';  // trashed the process copy of the string. (or even SEGV it, if it was in some static section of the ELF?)

It is undefined-behavior when you modify the return value (both C and POSIX
are explicit about it). On glibc, both strerror() and strerror_l() returns
the same thread local buffer (so you might modify in a thread-safe manner).