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.IX Title "MDC 3"
.TH MDC 3 2026-08-14 "perl v5.42.2" "User Contributed Perl Documentation"
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.SH NAME
Log::Log4perl::MDC \- Mapped Diagnostic Context
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Log::Log4perl allows loggers to maintain global thread\-specific data, 
called the Nested Diagnostic Context (NDC) and 
Mapped Diagnostic Context (MDC).
.PP
The MDC is a simple thread\-specific hash table, in which the application
can stuff values under certain keys and retrieve them later
via the \f(CW"%X{key}"\fR placeholder in 
\&\f(CW\*(C`Log::Log4perl::Layout::PatternLayout\*(C'\fRs.
.ie n .IP "Log::Log4perl::MDC\->put($key, $value);" 4
.el .IP "Log::Log4perl::MDC\->put($key, \f(CW$value\fR);" 4
.IX Item "Log::Log4perl::MDC->put($key, $value);"
Store a value \f(CW$value\fR under key \f(CW$key\fR in the map.
.ie n .IP "my $value = Log::Log4perl::MDC\->get($key);" 4
.el .IP "my \f(CW$value\fR = Log::Log4perl::MDC\->get($key);" 4
.IX Item "my $value = Log::Log4perl::MDC->get($key);"
Retrieve the content of the map under the specified key.
Typically done by \f(CW%X{key}\fR in
\&\f(CW\*(C`Log::Log4perl::Layout::PatternLayout\*(C'\fR.
If no value exists to the given key, \f(CW\*(C`undef\*(C'\fR is returned.
.ie n .IP "my $text = Log::Log4perl::MDC\->\fBremove()\fR;" 4
.el .IP "my \f(CW$text\fR = Log::Log4perl::MDC\->\fBremove()\fR;" 4
.IX Item "my $text = Log::Log4perl::MDC->remove();"
Delete all entries from the map.
.IP Log::Log4perl::MDC\->\fBget_context()\fR; 4
.IX Item "Log::Log4perl::MDC->get_context();"
Returns a reference to the hash table.
.PP
Please note that all of the methods above are class methods, there\*(Aqs no
instances of this class. Since the thread model in perl 5.8.0 is
"no shared data unless explicitly requested" the data structures
used are just global (and therefore thread\-specific).
.SH LICENSE
.IX Header "LICENSE"
Copyright 2002\-2026 by Mike Schilli <m@perlmeister.com>
and Kevin Goess <cpan@goess.org>.
.PP
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.