Re: ANSI C and POSIX (was Re: C/C++ knocks the crap out of Ada)

[dewar@cs.nyu.edu (Robert Dewar)]

Mike notes/asks

>I'm a latecomer to this dispute, but this is not smart:
>
>    char buf[100];
>    read(fd,buf,1000);
>
>By the way, how did all this get started?

Good question, I don't particularly see any dispute here. Obviously
noone ever wrote code so obviously peculiar as your above example!

The exact code, which appeared in the GNAT runtime was a loop to
read a file that had in the loop

     read(fd,p,16*1024);

to read the file in chunks of 16K. The last call exactly filled the
remaining buffer which had been sized to exactly fit the size of the
file.

This code worked fine for a long time on nearly all systems, and in fact
if you read the spec should work on most systems. But it failed on Linux
(and was immediately corrected, no big deal!) and the reason is that
Linux has subtly different semantics. The issue was whether any of the
so-called specs for read (POSIX or SPEC1170) were clear enough to
determine which of the two observed implementation models was "right".

Turns out that no spec we could find (no one seems to have SPEC1170)
precisely defined the specs.

The wandering off into whether the code involved was or was not a good
idea (obviously it was not!) is irrelevant. I was just using this as
an example of a portability problem caused by weak specifications.
Peter takes the odd view that it is not a portability problem because
the code was not well defined, and therefore not portable. This seems
a bit of a useless observation--Peter has determined that only
portable code is portable, not terribly useful :-).

Kazimir at first claimed that the spec clearly favored the Linux
interpretation, then could not find any evidence and appealed to
unwritten rules :-)


But just to be clear, there is no dispute over whether the code as
written was a good idea, it was simply an interesting example of
a portability problem resulting from weak specs. The disussion
showed to an extent that surprised even me that the specs for
such routines are really horribly weak, at least in POSIX.