🚀 go-pugleaf

cowen@clark.edu (James M. Cowen) wrote:

[ ... ] 
>In short, I'm confused, can someone clarify how one would use fflush()
>in a portable fashion ... no that's not what I want to ask.  Don't use
>fflush() on a read/update stream,  that seems obvious given ANSI C, but 
>it "works" on POSIX.1 systems, but POSIX.1 indicates it accepts the
>ANSI C standard, but ... I hope you get the drift of my question.

>In passing, I'm teaching a class using W. Richard Stevens book, "Advanced 
>Programming in a Unix Environment", and was unable to advance a satisfactory
>explanation of fflush() with respect to ANSI C and POSIX.1.  Indeed, my
>response, "interesting" was not only unsatisfactory to my students, it
>was inappropriate. 

Everything here sounds perfectly reasonable to me.  POSIX.1 includes the
ISO standard for C, therefore everything that's defined in the C
standard is also defined in POSIX.  However, POSIX also defines quite a
bit more, that is NOT defined in the C standard.  In many cases the new
definitions are embodied in functions that are not defined in the C
standard at all, and typically using headers that are not defined in the
C standard either.  

However, some of the behavior specified by POSIX involves defining
behavior of functions that are in the C standard, but which are more
restrictive of the functions (and therefore less restrictive on calling
code) than the C standard.  For instance, the C standard mandates the
presence of `errno' and specifies its purpose, but has only a minimal
definition of how its value may be affected by standard functions.
POSIX is more restrictive in specifying considerably more about what
functions will set errno to particular values and under what
circumstances they will do so.

Likewise, with fflush, the C standard simply doesn't define the results
unless it's applied to a stream opened for writing, or opened for update
and for which the last operation was a write.  POSIX apparently
restricts the function further, by saying that it must provide a defined
result when applied to a stream opened for reading, or from which
reading has been done.