Re: Confusing "Success" error message

[Andries E. Brouwer]

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.

Andries