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Description
Descriptionperldoc -f open:
As a special case the three-argument form with a read/write mode and the third argument being
undef:open(my$tmp,"+>",undef)ordie ...opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which makes
+>the only useful and sensible mode to use.) You will need toseekto do the reading.
This feature was added in 5.8.0 and is intended to only trigger when a literalundef is passed as the filename. It does not happen if a variable whose contents areundef is used:
$ perl -we 'open(my $fh, "+>", my $bogus) or die $!'Use of uninitialized value $bogus in open at -e line 1.No such file or directory at -e line 1.$ perl -we 'my @bogus = undef; open(my $fh, "+>", $bogus[0]) or die $!'Use of uninitialized value $bogus[0] in open at -e line 1.No such file or directory at -e line 1.$ perl -we 'my %bogus = (a => undef); open(my $fh, "+>", $bogus{a}) or die $!'Use of uninitialized value $bogus{"a"} in open at -e line 1.No such file or directory at -e line 1.Steps to Reproduce
However, if you use an array or hash element that doesn't exist at all (as opposed to existing but containingundef), the special behavior kicks in andopen silently succeeds, creating a temp file:
$ perl -we 'my @bogus; open(my $fh, "+>", $bogus[0]) or die $!'$ perl -we 'my %bogus; open(my $fh, "+>", $bogus{a}) or die $!'$ perl -we 'open(my $fh, "+>", $ARGV[0]) or die $!'$Even this behavior is not entirely consistent, however. If you use a runtime call to&CORE::open, it starts failing again (but with duplicate warning messages for some reason):
$ perl -we 'my @bogus; &CORE::open(my $fh, "+>", $bogus[0]) or die $!'Use of uninitialized value in open at -e line 1.Use of uninitialized value in open at -e line 1.No such file or directory at -e line 1.$ perl -we 'my %bogus; &CORE::open(my $fh, "+>", $bogus{a}) or die $!'Use of uninitialized value in open at -e line 1.Use of uninitialized value in open at -e line 1.No such file or directory at -e line 1.Expected behavior
A call toopen that does not pass a literalundef, but an element of an array or hash, should complain about the undefined value and fail. Basically like the&CORE::open version, but there I'd expect to see only one copy of the warning.
Perl configuration
Summary of my perl5 (revision 5 version 40 subversion 0) configuration: Platform: osname=linux osvers=6.5.0-10040-tuxedo archname=x86_64-linux-thread-multi-ld uname='linux luum 6.5.0-10040-tuxedo #44 smp preempt_dynamic wed may 8 17:36:39 utc 2024 x86_64 x86_64 x86_64 gnulinux ' config_args='-de -Dprefix=/home/mauke/perl5/perlbrew/perls/perl-5.40.0 -Dcc=cgcc -Dman1dir=none -Dman3dir=none -Dusethreads -Duselongdouble -Aoptimize=-flto -Aeval:scriptdir=/home/mauke/perl5/perlbrew/perls/perl-5.40.0/bin' hint=recommended useposix=true d_sigaction=define useithreads=define usemultiplicity=define use64bitint=define use64bitall=define uselongdouble=define usemymalloc=n default_inc_excludes_dot=define Compiler: cc='cgcc' ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64' optimize='-O2 -flto' cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include' ccversion='' gccversion='11.4.0' gccosandvers='' intsize=4 longsize=8 ptrsize=8 doublesize=8 byteorder=12345678 doublekind=3 d_longlong=define longlongsize=8 d_longdbl=define longdblsize=16 longdblkind=3 ivtype='long' ivsize=8 nvtype='long double' nvsize=16 Off_t='off_t' lseeksize=8 alignbytes=16 prototype=define Linker and Libraries: ld='cgcc' ldflags =' -fstack-protector-strong -L/usr/local/lib' libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /usr/lib64 libs=-lpthread -ldb -ldl -lm -lcrypt -lutil -lc perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc libc=/lib/x86_64-linux-gnu/libc.so.6 so=so useshrplib=false libperl=libperl.a gnulibc_version='2.35' Dynamic Linking: dlsrc=dl_dlopen.xs dlext=so d_dlsymun=undef ccdlflags='-Wl,-E' cccdlflags='-fPIC' lddlflags='-shared -O2 -flto -L/usr/local/lib -fstack-protector-strong'Characteristics of this binary (from libperl): Compile-time options: HAS_LONG_DOUBLE HAS_STRTOLD HAS_TIMES MULTIPLICITY PERLIO_LAYERS PERL_COPY_ON_WRITE PERL_DONT_CREATE_GVSV PERL_HASH_FUNC_SIPHASH13 PERL_HASH_USE_SBOX32 PERL_MALLOC_WRAP PERL_OP_PARENT PERL_PRESERVE_IVUV PERL_USE_SAFE_PUTENV USE_64_BIT_ALL USE_64_BIT_INT USE_ITHREADS USE_LARGE_FILES USE_LOCALE USE_LOCALE_COLLATE USE_LOCALE_CTYPE USE_LOCALE_NUMERIC USE_LOCALE_TIME USE_LONG_DOUBLE USE_PERLIO USE_PERL_ATOF USE_REENTRANT_API USE_THREAD_SAFE_LOCALE Built under linux Compiled at Jun 9 2024 23:04:17 %ENV: PERLBREW_BASHRC_VERSION="0.74" PERLBREW_HOME="/home/mauke/.perlbrew" PERLBREW_MANPATH="/home/mauke/perl5/perlbrew/perls/perl-5.40.0/man" PERLBREW_PATH="/home/mauke/perl5/perlbrew/bin:/home/mauke/perl5/perlbrew/perls/perl-5.40.0/bin" PERLBREW_PERL="perl-5.40.0" PERLBREW_ROOT="/home/mauke/perl5/perlbrew" PERLBREW_VERSION="0.94" PERLDOC="-oman" PERL_UNICODE="SAL" @INC: /home/mauke/perl5/perlbrew/perls/perl-5.40.0/lib/site_perl/5.40.0/x86_64-linux-thread-multi-ld /home/mauke/perl5/perlbrew/perls/perl-5.40.0/lib/site_perl/5.40.0 /home/mauke/perl5/perlbrew/perls/perl-5.40.0/lib/5.40.0/x86_64-linux-thread-multi-ld /home/mauke/perl5/perlbrew/perls/perl-5.40.0/lib/5.40.0