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From: stroyan@hpfcdc.HP.COM (Mike Stroyan)
Newsgroups: comp.sys.hp
Subject: Re: Strange behaviour on HP9000/825
Message-ID: <5570246@hpfcdc.HP.COM>
Date: 26 Jul 89 20:00:48 GMT
References: <227@innovus.UUCP>
Organization: HP Ft. Collins, Co.
Lines: 20

> File opens, errno=0, fp=1073742056
> File read, errno=25, fp=1073742056, buffer=#include <stdio.h>
> File read2, errno=0, fp=1073742056, buffer=#include <errno.h>
> 
> Anybody care to comment about why this is happening?

The fopen is checking isatty() to see if it should buffer the stream
output.  The isatty call is using a failed ioctl() call to see that the
file is not a tty.  The failed ioctl() call is setting errno to ENOTTY.

Your program is checking the value of errno without first checking
ferror() or the return value from fgets().  You can't depend on the
value of errno being to be 0 when an error has not occured.  You must
first determine that an error occured, then use errno to find out more
about the error.  The errno value is not always defined to reflect the
state of libraries such as stdio.  While some library calls will refer
the user to errno for more information, it is really intended to return
information about kernel calls rather than arbitrary library calls.

Mike Stroyan, stroyan@hpfcla.hp.com