readline setting $! on success contrary to perlfunc

[p5pRT]

Migrated from rt.perl.org#69208 (status was 'resolved')

Searchable as RT69208$

[p5pRT]

From user42@zip.com.au

Created by user42@zip.com.au

The readline() function sometimes sets $! to EBADF when it succeeds,
which means the error checking method recommended in perlfunc readline()
doesn't work.

For example the program foo.pl below prints

  first​: Bad file descriptor

where I expected that it would print an empty string, or perhaps
"Success" from $! having been left unchanged from undef.

foo.pl gets it at the start of a file, I struck it when a readline
crossed a 1024 byte boundary later in a file.

gdb claims errno is changed by PerlIO_fast_gets() under buffer filling
of PerlIOBuf_fill. I suppose if fast_gets fails and the fill must fall
back on PerlIO_read then errno should be restored, to let that
readline() error approach work, and perserve the principle that system
calls or pseudo-system-calls don't change errno if they succeed.

Perl Info

Flags:
    category=core
    severity=medium

Site configuration information for perl 5.10.1:

Configured by Debian Project at Sun Sep  6 13:25:58 UTC 2009.

Summary of my perl5 (revision 5 version 10 subversion 1) configuration:
   
  Platform:
    osname=linux, osvers=2.6.18.3xenu, archname=i486-linux-gnu-thread-multi
    uname='linux demosthenes.ayous.org 2.6.18.3xenu #2 smp fri nov 24 22:26:50 utc 2006 i686 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dcccdlflags=-fPIC -Darchname=i486-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.10 -Darchlib=/usr/lib/perl/5.10 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.10.1 -Dsitearch=/usr/local/lib/perl/5.10.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.10.1 -Dd_dosuid -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=undef, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -g',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.3.4', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib /usr/lib64
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=/lib/libc-2.9.so, so=so, useshrplib=true, libperl=libperl.so.5.10.1
    gnulibc_version='2.9'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -g -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

[p5pRT]

From user42@zip.com.au

foo.pl

[p5pRT]

From @ikegami

On Thu, Sep 17, 2009 at 7​:53 PM, Kevin Ryde <perlbug-followup@​perl.org>wrote​:

# New Ticket Created by Kevin Ryde
# Please include the string​: [perl #69208]
# in the subject line of all future correspondence about this issue.
# <URL​: http​://rt.perl.org/rt3/Ticket/Display.html?id=69208 >

This is a bug report for perl from user42@​zip.com.au,
generated with the help of perlbug 1.39 running under perl 5.10.1.

-----------------------------------------------------------------
[Please describe your issue here]

The readline() function sometimes sets $! to EBADF when it succeeds,
which means the error checking method recommended in perlfunc readline()
doesn't work.

It's on my todo list to create a documentation patch for this. Basically,
the solution is​:

while (!eof($fh)) {
  defined( $_ = <$fh> )
  or die("readline​: $!\n");
  ...
}

More details here​:
http​://markmail.org/message/bu2vnbgabfakzicc

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From user42@zip.com.au

"Eric Brine via RT" <perlbug-followup@​perl.org> writes​:

while (!eof($fh)) {

Is eof the best there? For myself I ended up checking the result string
does end with $/ (using a chomp).

If readline has never done anything particular with $! then I suppose it
may be a bit late to bother trying to make it so.

[p5pRT]

From @briandfoy

I've fixed the documentation portion of this in 0f03d33.

[p5pRT]

From @dcollinsn

On Fri Sep 25 13​:19​:21 2009, comdog wrote​:

I've fixed the documentation portion of this in
0f03d33.

There are a number of older bugs like this, where someone is checking $! after a call that didn't fail, and is surprised when there's something silly in there. The docs now say that $! is only meaningful immediately after a failure, and in the case of readline, that means it is only meaningful after readline has returned undef.

Closing resolved.

--
Respectfully,
Dan Collins

[p5pRT]

@dcollinsn - Status changed from 'open' to 'resolved'