Re: Confusing "Success" error message

[Eli Zaretskii]

> Date: Fri, 8 Nov 2019 16:47:30 +0100
> From: "Andries E. Brouwer" <address@hidden>
> Cc: "Andries E. Brouwer" <address@hidden>, address@hidden,
>         address@hidden, address@hidden
> 
> On Fri, Nov 08, 2019 at 04:34:10PM +0200, Eli Zaretskii wrote:
> 
> > > Libc functions are free to call other functions internally,
> > > and such internal calls may fail where the outer level call
> > > does not fail. So even if a libc function does not return
> > > an error, errno can have changed.
> > 
> > That would be a bug in libc, I think.  Its functions should save and
> > restore errno if other functions they call error out without causing
> > the calling function to fail.
> 
> % man 3 errno
> ...
>        A common mistake is to do
> 
>            if (somecall() == -1) {
>                printf("somecall() failed\n");
>                if (errno == ...) { ... }
>            }
> 
>        where errno no longer needs to have the value it had upon  return  from
>        somecall()  (i.e.,  it may have been changed by the printf(3)).  If the
>        value of errno should be preserved across a library call,  it  must  be
>        saved:
> 
>            if (somecall() == -1) {
>                int errsv = errno;
>                printf("somecall() failed\n");
>                if (errsv == ...) { ... }
>            }
> 
> That was the Linux man page. Here is the POSIX man page:
> 
> ...
>        The  value  in  errno  is significant only when the return value of the
>        call indicated an error (i.e., -1 from most system calls;  -1  or  NULL
>        from  most  library  functions); a function that succeeds is allowed to
>        change errno.

Thanks, but AFAIU this says the same as I did: if a function succeeds,
it should not modify errno.

In the above example from a man page, the "may have been changed by
printf" part alludes to the possibility that printf fails in some way,
e.g. because the format is in error or stdout is closed or somesuch.