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Section: Perl Programmers Reference Guide (3pm)
Updated: 2018-03-23


warnings - Perl pragma to control optional warnings 


    use warnings;    no warnings;    use warnings "all";    no warnings "all";    use warnings::register;    if (warnings::enabled()) {        warnings::warn("some warning");    }    if (warnings::enabled("void")) {        warnings::warn("void", "some warning");    }    if (warnings::enabled($object)) {        warnings::warn($object, "some warning");    }    warnings::warnif("some warning");    warnings::warnif("void", "some warning");    warnings::warnif($object, "some warning");


The "warnings" pragma gives control over which warnings are enabled inwhich parts of a Perl program. It's a more flexible alternative forboth the command line flag -w and the equivalent Perl variable,$^W.

This pragma works just like the "strict" pragma.This means that the scope of the warning pragma is limited to theenclosing block. It also means that the pragma setting will notleak across files (via "use", "require" or "do"). This allowsauthors to independently define the degree of warning checks that willbe applied to their module.

By default, optional warnings are disabled, so any legacy code thatdoesn't attempt to control the warnings will work unchanged.

All warnings are enabled in a block by either of these:

    use warnings;    use warnings 'all';

Similarly all warnings are disabled in a block by either of these:

    no warnings;    no warnings 'all';

For example, consider the code below:

    use warnings;    my @a;    {        no warnings;        my $b = @a[0];    }    my $c = @a[0];

The code in the enclosing block has warnings enabled, but the innerblock has them disabled. In this case that means the assignment to thescalar $c will trip the "Scalar value @a[0] better written as $a[0]"warning, but the assignment to the scalar $b will not. 

Default Warnings and Optional Warnings

Before the introduction of lexical warnings, Perl had two classes ofwarnings: mandatory and optional.

As its name suggests, if your code tripped a mandatory warning, youwould get a warning whether you wanted it or not.For example, the code below would always produce an "isn't numeric"warning about the ``2:''.

    my $a = "2:" + 3;

With the introduction of lexical warnings, mandatory warnings now becomedefault warnings. The difference is that although the previouslymandatory warnings are still enabled by default, they can then besubsequently enabled or disabled with the lexical warning pragma. Forexample, in the code below, an "isn't numeric" warning will onlybe reported for the $a variable.

    my $a = "2:" + 3;    no warnings;    my $b = "2:" + 3;

Note that neither the -w flag or the $^W can be used todisable/enable default warnings. They are still mandatory in this case. 

What's wrong with -w and $^W

Although very useful, the big problem with using -w on the commandline to enable warnings is that it is all or nothing. Take the typicalscenario when you are writing a Perl program. Parts of the code youwill write yourself, but it's very likely that you will make use ofpre-written Perl modules. If you use the -w flag in this case, youend up enabling warnings in pieces of code that you haven't written.

Similarly, using $^W to either disable or enable blocks of code isfundamentally flawed. For a start, say you want to disable warnings ina block of code. You might expect this to be enough to do the trick:

     {         local ($^W) = 0;         my $a =+ 2;         my $b; chop $b;     }

When this code is run with the -w flag, a warning will be producedfor the $a line: "Reversed += operator".

The problem is that Perl has both compile-time and run-time warnings. Todisable compile-time warnings you need to rewrite the code like this:

     {         BEGIN { $^W = 0 }         my $a =+ 2;         my $b; chop $b;     }

The other big problem with $^W is the way you can inadvertentlychange the warning setting in unexpected places in your code. For example,when the code below is run (without the -w flag), the second callto "doit" will trip a "Use of uninitialized value" warning, whereasthe first will not.

    sub doit    {        my $b; chop $b;    }    doit();    {        local ($^W) = 1;        doit()    }

This is a side-effect of $^W being dynamically scoped.

Lexical warnings get around these limitations by allowing finer controlover where warnings can or can't be tripped. 

Controlling Warnings from the Command Line

There are three Command Line flags that can be used to control whenwarnings are (or aren't) produced:
This is the existing flag. If the lexical warnings pragma is notused in any of you code, or any of the modules that you use, this flagwill enable warnings everywhere. See ``Backward Compatibility'' fordetails of how this flag interacts with lexical warnings.
If the -W flag is used on the command line, it will enable all warningsthroughout the program regardless of whether warnings were disabledlocally using "no warnings" or "$^W =0".This includes all files that getincluded via "use", "require" or "do".Think of it as the Perl equivalent of the ``lint'' command.
Does the exact opposite to the -W flag, i.e. it disables all warnings.

Backward Compatibility

If you are used to working with a version of Perl prior to theintroduction of lexically scoped warnings, or have code that uses bothlexical warnings and $^W, this section will describe how they interact.

How Lexical Warnings interact with -w/$^W:

If none of the three command line flags (-w, -W or -X) thatcontrol warnings is used and neither $^W nor the "warnings" pragmaare used, then default warnings will be enabled and optional warningsdisabled.This means that legacy code that doesn't attempt to control the warningswill work unchanged.
The -w flag just sets the global $^W variable as in 5.005. Thismeans that any legacy code that currently relies on manipulating $^Wto control warning behavior will still work as is.
Apart from now being a boolean, the $^W variable operates in exactlythe same horrible uncontrolled global way, except that it cannotdisable/enable default warnings.
If a piece of code is under the control of the "warnings" pragma,both the $^W variable and the -w flag will be ignored for thescope of the lexical warning.
The only way to override a lexical warnings setting is with the -Wor -X command line flags.

The combined effect of 3 & 4 is that it will allow code which usesthe "warnings" pragma to control the warning behavior of $^W-typecode (using a "local $^W=0") if it really wants to, but not vice-versa. 

Category Hierarchy

A hierarchy of ``categories'' have been defined to allow groups of warningsto be enabled/disabled in isolation.

The current hierarchy is:

    all -+         |         +- closure         |         +- deprecated         |         +- exiting         |         +- experimental --+         |                 |         |                 +- experimental::bitwise         |                 |         |                 +- experimental::const_attr         |                 |         |                 +- experimental::declared_refs         |                 |         |                 +- experimental::lexical_subs         |                 |         |                 +- experimental::postderef         |                 |         |                 +- experimental::re_strict         |                 |         |                 +- experimental::refaliasing         |                 |         |                 +- experimental::regex_sets         |                 |         |                 +- experimental::signatures         |                 |         |                 +- experimental::smartmatch         |                 |         |                 +- experimental::win32_perlio         |         +- glob         |         +- imprecision         |         +- io ------------+         |                 |         |                 +- closed         |                 |         |                 +- exec         |                 |         |                 +- layer         |                 |         |                 +- newline         |                 |         |                 +- pipe         |                 |         |                 +- syscalls         |                 |         |                 +- unopened         |         +- locale         |         +- misc         |         +- missing         |         +- numeric         |         +- once         |         +- overflow         |         +- pack         |         +- portable         |         +- recursion         |         +- redefine         |         +- redundant         |         +- regexp         |         +- severe --------+         |                 |         |                 +- debugging         |                 |         |                 +- inplace         |                 |         |                 +- internal         |                 |         |                 +- malloc         |         +- signal         |         +- substr         |         +- syntax --------+         |                 |         |                 +- ambiguous         |                 |         |                 +- bareword         |                 |         |                 +- digit         |                 |         |                 +- illegalproto         |                 |         |                 +- parenthesis         |                 |         |                 +- precedence         |                 |         |                 +- printf         |                 |         |                 +- prototype         |                 |         |                 +- qw         |                 |         |                 +- reserved         |                 |         |                 +- semicolon         |         +- taint         |         +- threads         |         +- uninitialized         |         +- unpack         |         +- untie         |         +- utf8 ----------+         |                 |         |                 +- non_unicode         |                 |         |                 +- nonchar         |                 |         |                 +- surrogate         |         +- void

Just like the ``strict'' pragma any of these categories can be combined

    use warnings qw(void redefine);    no warnings qw(io syntax untie);

Also like the ``strict'' pragma, if there is more than one instance of the"warnings" pragma in a given scope the cumulative effect is additive.

    use warnings qw(void); # only "void" warnings enabled    ...    use warnings qw(io);   # only "void" & "io" warnings enabled    ...    no warnings qw(void);  # only "io" warnings enabled

To determine which category a specific warning has been assigned to seeperldiag.

Note: Before Perl 5.8.0, the lexical warnings category ``deprecated'' was asub-category of the ``syntax'' category. It is now a top-level categoryin its own right.

Note: Before 5.21.0, the ``missing'' lexical warnings category wasinternally defined to be the same as the ``uninitialized'' category. Itis now a top-level category in its own right. 

Fatal Warnings

The presence of the word ``FATAL'' in the category list will escalatewarnings in those categories into fatal errors in that lexical scope.

NOTE: FATAL warnings should be used with care, particularly"FATAL => 'all'".

Libraries using warnings::warn for custom warning categoriesgenerally don't expect warnings::warn to be fatal and can wind upin an unexpected state as a result. For XS modules issuing categorizedwarnings, such unanticipated exceptions could also expose memory leak bugs.

Moreover, the Perl interpreter itself has had serious bugs involvingfatalized warnings. For a summary of resolved and unresolved problems asof January 2015, please seethis perl5-porters post <>.

While some developers find fatalizing some warnings to be a usefuldefensive programming technique, using "FATAL => 'all'" to fatalizeall possible warning categories --- including custom ones --- is particularlyrisky. Therefore, the use of "FATAL => 'all'" isdiscouraged.

The strictures module on CPAN offers one example ofa warnings subset that the module's authors believe is relatively safe tofatalize.

NOTE: users of FATAL warnings, especially those using"FATAL => 'all'", should be fully aware that they are risking futureportability of their programs by doing so. Perl makes absolutely nocommitments to not introduce new warnings or warnings categories in thefuture; indeed, we explicitly reserve the right to do so. Code that maynot warn now may warn in a future release of Perl if the Perl5 developmentteam deems it in the best interests of the community to do so. Should codeusing FATAL warnings break due to the introduction of a new warning we willNOT consider it an incompatible change. Users of FATAL warnings shouldtake special caution during upgrades to check to see if their code triggersany new warnings and should pay particular attention to the fine print ofthe documentation of the features they use to ensure they do not exploitfeatures that are documented as risky, deprecated, or unspecified, or wherethe documentation says ``so don't do that'', or anything with the same senseand spirit. Use of such features in combination with FATAL warnings isENTIRELY AT THE USER'S RISK.

The following documentation describes how to use FATAL warnings but theperl5 porters strongly recommend that you understand the risks before doingso, especially for library code intended for use by others, as there is noway for downstream users to change the choice of fatal categories.

In the code below, the use of "time", "length"and "join" can all produce a "Useless use of xxx in void context"warning.

    use warnings;    time;    {        use warnings FATAL => qw(void);        length "abc";    }    join "", 1,2,3;    print "done\n";

When run it produces this output

    Useless use of time in void context at fatal line 3.    Useless use of length in void context at fatal line 7.

The scope where "length" is used has escalated the "void" warningscategory into a fatal error, so the program terminates immediately when itencounters the warning.

To explicitly turn off a ``FATAL'' warning you just disable the warningit is associated with. So, for example, to disable the ``void'' warningin the example above, either of these will do the trick:

    no warnings qw(void);    no warnings FATAL => qw(void);

If you want to downgrade a warning that has been escalated into a fatalerror back to a normal warning, you can use the ``NONFATAL'' keyword. Forexample, the code below will promote all warnings into fatal errors,except for those in the ``syntax'' category.

    use warnings FATAL => 'all', NONFATAL => 'syntax';

As of Perl 5.20, instead of "use warnings FATAL => 'all';" you canuse:

   use v5.20;       # Perl 5.20 or greater is required for the following   use warnings 'FATAL';  # short form of "use warnings FATAL => 'all';"

If you want your program to be compatible with versions of Perl before5.20, you must use "use warnings FATAL => 'all';" instead. (Inprevious versions of Perl, the behavior of the statements"use warnings 'FATAL';", "use warnings 'NONFATAL';" and"no warnings 'FATAL';" was unspecified; they did not behave as ifthey included the "=> 'all'" portion. As of 5.20, they do.) 

Reporting Warnings from a Module

The "warnings" pragma provides a number of functions that are useful formodule authors. These are used when you want to report a module-specificwarning to a calling module has enabled warnings via the "warnings"pragma.

Consider the module "MyMod::Abc" below.

    package MyMod::Abc;    use warnings::register;    sub open {        my $path = shift;        if ($path !~ m#^/#) {            warnings::warn("changing relative path to /var/abc")                if warnings::enabled();            $path = "/var/abc/$path";        }    }    1;

The call to "warnings::register" will create a new warnings categorycalled ``MyMod::Abc'', i.e. the new category name matches the currentpackage name. The "open" function in the module will display a warningmessage if it gets given a relative path as a parameter. This warningswill only be displayed if the code that uses "MyMod::Abc" has actuallyenabled them with the "warnings" pragma like below.

    use MyMod::Abc;    use warnings 'MyMod::Abc';    ...    abc::open("../fred.txt");

It is also possible to test whether the pre-defined warnings categories areset in the calling module with the "warnings::enabled" function. Considerthis snippet of code:

    package MyMod::Abc;    sub open {        if (warnings::enabled("deprecated")) {            warnings::warn("deprecated",                           "open is deprecated, use new instead");        }        new(@_);    }    sub new    ...    1;

The function "open" has been deprecated, so code has been included todisplay a warning message whenever the calling module has (at least) the``deprecated'' warnings category enabled. Something like this, say.

    use warnings 'deprecated';    use MyMod::Abc;    ...    MyMod::Abc::open($filename);

Either the "warnings::warn" or "warnings::warnif" function should beused to actually display the warnings message. This is because they canmake use of the feature that allows warnings to be escalated into fatalerrors. So in this case

    use MyMod::Abc;    use warnings FATAL => 'MyMod::Abc';    ...    MyMod::Abc::open('../fred.txt');

the "warnings::warnif" function will detect this and die afterdisplaying the warning message.

The three warnings functions, "warnings::warn", "warnings::warnif"and "warnings::enabled" can optionally take an object reference in placeof a category name. In this case the functions will use the class nameof the object as the warnings category.

Consider this example:

    package Original;    no warnings;    use warnings::register;    sub new    {        my $class = shift;        bless [], $class;    }    sub check    {        my $self = shift;        my $value = shift;        if ($value % 2 && warnings::enabled($self))          { warnings::warn($self, "Odd numbers are unsafe") }    }    sub doit    {        my $self = shift;        my $value = shift;        $self->check($value);        # ...    }    1;    package Derived;    use warnings::register;    use Original;    our @ISA = qw( Original );    sub new    {        my $class = shift;        bless [], $class;    }    1;

The code below makes use of both modules, but it only enables warnings from"Derived".

    use Original;    use Derived;    use warnings 'Derived';    my $a = Original->new();    $a->doit(1);    my $b = Derived->new();    $a->doit(1);

When this code is run only the "Derived" object, $b, will generatea warning.

    Odd numbers are unsafe at line 7

Notice also that the warning is reported at the line where the object is firstused.

When registering new categories of warning, you can supply more names towarnings::register like this:

    package MyModule;    use warnings::register qw(format precision);    ...    warnings::warnif('MyModule::format', '...');


use warnings::register
Creates a new warnings category with the same name as the package wherethe call to the pragma is used.
Use the warnings category with the same name as the current package.

Return TRUE if that warnings category is enabled in the calling module.Otherwise returns FALSE.

Return TRUE if the warnings category, $category, is enabled in thecalling module.Otherwise returns FALSE.
Use the name of the class for the object reference, $object, as thewarnings category.

Return TRUE if that warnings category is enabled in the first scopewhere the object is used.Otherwise returns FALSE.

Return TRUE if the warnings category with the same name as the currentpackage has been set to FATAL in the calling module.Otherwise returns FALSE.
Return TRUE if the warnings category $category has been set to FATAL inthe calling module.Otherwise returns FALSE.
Use the name of the class for the object reference, $object, as thewarnings category.

Return TRUE if that warnings category has been set to FATAL in the firstscope where the object is used.Otherwise returns FALSE.

Print $message to STDERR.

Use the warnings category with the same name as the current package.

If that warnings category has been set to ``FATAL'' in the calling modulethen die. Otherwise return.

warnings::warn($category, $message)
Print $message to STDERR.

If the warnings category, $category, has been set to ``FATAL'' in thecalling module then die. Otherwise return.

warnings::warn($object, $message)
Print $message to STDERR.

Use the name of the class for the object reference, $object, as thewarnings category.

If that warnings category has been set to ``FATAL'' in the scope where $objectis first used then die. Otherwise return.

Equivalent to:

    if (warnings::enabled())      { warnings::warn($message) }
warnings::warnif($category, $message)
Equivalent to:

    if (warnings::enabled($category))      { warnings::warn($category, $message) }
warnings::warnif($object, $message)
Equivalent to:

    if (warnings::enabled($object))      { warnings::warn($object, $message) }
This registers warning categories for the given names and is primarily foruse by the warnings::register pragma.

See also ``Pragmatic Modules'' in perlmodlib and perldiag.



Default Warnings and Optional Warnings
What's wrong with -w and $^W
Controlling Warnings from the Command Line
Backward Compatibility
Category Hierarchy
Fatal Warnings
Reporting Warnings from a Module

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