perldiag.pod

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=head1 NAME

perldiag - various Perl diagnostics

=head1 DESCRIPTION

These messages are classified as follows (listed in increasing order of
desperation):

    (W) A warning (optional).
    (D) A deprecation (optional).
    (S) A severe warning (default).
    (F) A fatal error (trappable).
    (P) An internal error you should never see (trappable).
    (X) A very fatal error (nontrappable).
    (A) An alien error message (not generated by Perl).

The majority of messages from the first three classifications above
(W, D & S) can be controlled using the C<warnings> pragma. 

If a message can be controlled by the C<warnings> pragma, its warning
category is included with the classification letter in the description
below.

Optional warnings are enabled by using the C<warnings> pragma or the B<-w>
and B<-W> switches. Warnings may be captured by setting C<$SIG{__WARN__}>
to a reference to a routine that will be called on each warning instead
of printing it.  See L<perlvar>.

Default warnings are always enabled unless they are explicitly disabled
with the C<warnings> pragma or the B<-X> switch.

Trappable errors may be trapped using the eval operator.  See
L<perlfunc/eval>.  In almost all cases, warnings may be selectively
disabled or promoted to fatal errors using the C<warnings> pragma.
See L<warnings>.

The messages are in alphabetical order, without regard to upper or
lower-case.  Some of these messages are generic.  Spots that vary are
denoted with a %s or other printf-style escape.  These escapes are
ignored by the alphabetical order, as are all characters other than
letters.  To look up your message, just ignore anything that is not a
letter.

=over 4

=item accept() on closed socket %s

(W closed) You tried to do an accept on a closed socket.  Did you forget
to check the return value of your socket() call?  See
L<perlfunc/accept>.

=item Allocation too large: %lx

(X) You can't allocate more than 64K on an MS-DOS machine.

=item '!' allowed only after types %s

(F) The '!' is allowed in pack() and unpack() only after certain types.
See L<perlfunc/pack>.

=item Ambiguous call resolved as CORE::%s(), qualify as such or use &

(W ambiguous) A subroutine you have declared has the same name as a Perl
keyword, and you have used the name without qualification for calling
one or the other.  Perl decided to call the builtin because the
subroutine is not imported.

To force interpretation as a subroutine call, either put an ampersand
before the subroutine name, or qualify the name with its package.
Alternatively, you can import the subroutine (or pretend that it's
imported with the C<use subs> pragma).

To silently interpret it as the Perl operator, use the C<CORE::> prefix
on the operator (e.g. C<CORE::log($x)>) or by declaring the subroutine
to be an object method (see L<perlsub/"Subroutine Attributes"> or
L<attributes>).

=item Ambiguous use of %s resolved as %s

(W ambiguous)(S) You said something that may not be interpreted the way
you thought.  Normally it's pretty easy to disambiguate it by supplying
a missing quote, operator, parenthesis pair or declaration.

=item '|' and '<' may not both be specified on command line

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and found that STDIN was a pipe, and that you also tried to
redirect STDIN using '<'.  Only one STDIN stream to a customer, please.

=item '|' and '>' may not both be specified on command line

(F) An error peculiar to VMS.  Perl does its own command line
redirection, and thinks you tried to redirect stdout both to a file and
into a pipe to another command.  You need to choose one or the other,
though nothing's stopping you from piping into a program or Perl script
which 'splits' output into two streams, such as

    open(OUT,">$ARGV[0]") or die "Can't write to $ARGV[0]: $!";
    while (<STDIN>) {
        print;
        print OUT;
    }
    close OUT;

=item Applying %s to %s will act on scalar(%s)

(W misc) The pattern match (//), substitution (s///), and
transliteration (tr///) operators work on scalar values.  If you apply
one of them to an array or a hash, it will convert the array or hash to
a scalar value -- the length of an array, or the population info of a
hash -- and then work on that scalar value.  This is probably not what
you meant to do.  See L<perlfunc/grep> and L<perlfunc/map> for
alternatives.

=item Args must match #! line

(F) The setuid emulator requires that the arguments Perl was invoked
with match the arguments specified on the #! line.  Since some systems
impose a one-argument limit on the #! line, try combining switches;
for example, turn C<-w -U> into C<-wU>.

=item Arg too short for msgsnd

(F) msgsnd() requires a string at least as long as sizeof(long).

=item %s argument is not a HASH or ARRAY element

(F) The argument to exists() must be a hash or array element, such as:

    $foo{$bar}
    $ref->{"susie"}[12]

=item %s argument is not a HASH or ARRAY element or slice

(F) The argument to delete() must be either a hash or array element,
such as:

    $foo{$bar}
    $ref->{"susie"}[12]

or a hash or array slice, such as:

    @foo[$bar, $baz, $xyzzy]
    @{$ref->[12]}{"susie", "queue"}

=item %s argument is not a subroutine name

(F) The argument to exists() for C<exists &sub> must be a subroutine
name, and not a subroutine call.  C<exists &sub()> will generate this
error.

=item Argument "%s" isn't numeric%s

(W numeric) The indicated string was fed as an argument to an operator
that expected a numeric value instead.  If you're fortunate the message
will identify which operator was so unfortunate.

=item Array @%s missing the @ in argument %d of %s()

(D deprecated) Really old Perl let you omit the @ on array names in some
spots.  This is now heavily deprecated.

=item assertion botched: %s

(P) The malloc package that comes with Perl had an internal failure.

=item Assertion failed: file "%s"

(P) A general assertion failed.  The file in question must be examined.

=item Assignment to both a list and a scalar

(F) If you assign to a conditional operator, the 2nd and 3rd arguments
must either both be scalars or both be lists.  Otherwise Perl won't
know which context to supply to the right side.

=item Negative offset to vec in lvalue context

(F) When vec is called in an lvalue context, the second argument must be
greater than or equal to zero.

=item Attempt to free non-arena SV: 0x%lx

(P internal) All SV objects are supposed to be allocated from arenas
that will be garbage collected on exit.  An SV was discovered to be
outside any of those arenas.

=item Attempt to free nonexistent shared string

(P internal) Perl maintains a reference counted internal table of
strings to optimize the storage and access of hash keys and other
strings.  This indicates someone tried to decrement the reference count
of a string that can no longer be found in the table.

=item Attempt to free temp prematurely

(W debugging) Mortalized values are supposed to be freed by the
free_tmps() routine.  This indicates that something else is freeing the
SV before the free_tmps() routine gets a chance, which means that the
free_tmps() routine will be freeing an unreferenced scalar when it does
try to free it.

=item Attempt to free unreferenced glob pointers

(P internal) The reference counts got screwed up on symbol aliases.

=item Attempt to free unreferenced scalar

(W internal) Perl went to decrement the reference count of a scalar to
see if it would go to 0, and discovered that it had already gone to 0
earlier, and should have been freed, and in fact, probably was freed.
This could indicate that SvREFCNT_dec() was called too many times, or
that SvREFCNT_inc() was called too few times, or that the SV was
mortalized when it shouldn't have been, or that memory has been
corrupted.

=item Attempt to join self

(F) You tried to join a thread from within itself, which is an
impossible task.  You may be joining the wrong thread, or you may need
to move the join() to some other thread.

=item Attempt to pack pointer to temporary value

(W pack) You tried to pass a temporary value (like the result of a
function, or a computed expression) to the "p" pack() template.  This
means the result contains a pointer to a location that could become
invalid anytime, even before the end of the current statement.  Use
literals or global values as arguments to the "p" pack() template to
avoid this warning.

=item Attempt to use reference as lvalue in substr

(W substr) You supplied a reference as the first argument to substr()
used as an lvalue, which is pretty strange.  Perhaps you forgot to
dereference it first.  See L<perlfunc/substr>.

=item Bad arg length for %s, is %d, should be %d

(F) You passed a buffer of the wrong size to one of msgctl(), semctl()
or shmctl().  In C parlance, the correct sizes are, respectively,
S<sizeof(struct msqid_ds *)>, S<sizeof(struct semid_ds *)>, and
S<sizeof(struct shmid_ds *)>.

=item Bad evalled substitution pattern

(F) You've used the /e switch to evaluate the replacement for a
substitution, but perl found a syntax error in the code to evaluate,
most likely an unexpected right brace '}'.

=item Bad filehandle: %s

(F) A symbol was passed to something wanting a filehandle, but the
symbol has no filehandle associated with it.  Perhaps you didn't do an
open(), or did it in another package.

=item Bad free() ignored

(S malloc) An internal routine called free() on something that had never
been malloc()ed in the first place. Mandatory, but can be disabled by
setting environment variable C<PERL_BADFREE> to 0.

This message can be seen quite often with DB_File on systems with "hard"
dynamic linking, like C<AIX> and C<OS/2>. It is a bug of C<Berkeley DB>
which is left unnoticed if C<DB> uses I<forgiving> system malloc().

=item Bad hash

(P) One of the internal hash routines was passed a null HV pointer.

=item Bad index while coercing array into hash

(F) The index looked up in the hash found as the 0'th element of a
pseudo-hash is not legal.  Index values must be at 1 or greater.
See L<perlref>.

=item Badly placed ()'s

(A) You've accidentally run your script through B<csh> instead
of Perl.  Check the #! line, or manually feed your script into
Perl yourself.

=item Bad name after %s::

(F) You started to name a symbol by using a package prefix, and then
didn't finish the symbol.  In particular, you can't interpolate outside
of quotes, so

    $var = 'myvar';
    $sym = mypack::$var;

is not the same as

    $var = 'myvar';
    $sym = "mypack::$var";

=item Bad realloc() ignored

(S malloc) An internal routine called realloc() on something that had
never been malloc()ed in the first place. Mandatory, but can be disabled
by setting environment variable C<PERL_BADFREE> to 1.

=item Bad symbol for array

(P) An internal request asked to add an array entry to something that
wasn't a symbol table entry.

=item Bad symbol for filehandle

(P) An internal request asked to add a filehandle entry to something
that wasn't a symbol table entry.

=item Bad symbol for hash

(P) An internal request asked to add a hash entry to something that
wasn't a symbol table entry.

=item Bareword found in conditional

(W bareword) The compiler found a bareword where it expected a
conditional, which often indicates that an || or && was parsed as part
of the last argument of the previous construct, for example:

    open FOO || die;

It may also indicate a misspelled constant that has been interpreted as
a bareword:

    use constant TYPO => 1;
    if (TYOP) { print "foo" }

The C<strict> pragma is useful in avoiding such errors.

=item Bareword "%s" not allowed while "strict subs" in use

(F) With "strict subs" in use, a bareword is only allowed as a
subroutine identifier, in curly brackets or to the left of the "=>"
symbol.  Perhaps you need to predeclare a subroutine?

=item Bareword "%s" refers to nonexistent package

(W bareword) You used a qualified bareword of the form C<Foo::>, but the
compiler saw no other uses of that namespace before that point.  Perhaps
you need to predeclare a package?

=item BEGIN failed--compilation aborted

(F) An untrapped exception was raised while executing a BEGIN
subroutine.  Compilation stops immediately and the interpreter is
exited.

=item BEGIN not safe after errors--compilation aborted

(F) Perl found a C<BEGIN {}> subroutine (or a C<use> directive, which
implies a C<BEGIN {}>) after one or more compilation errors had already
occurred.  Since the intended environment for the C<BEGIN {}> could not
be guaranteed (due to the errors), and since subsequent code likely
depends on its correct operation, Perl just gave up.

=item \1 better written as $1

(W syntax) Outside of patterns, backreferences live on as variables.
The use of backslashes is grandfathered on the right-hand side of a