631 lines
22 KiB
HTML
631 lines
22 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<html>
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<head>
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<title>How To Use Google Logging Library (glog)</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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font-family: "Times Roman", times, serif;
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</style>
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</head>
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<body>
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<h1>How To Use Google Logging Library (glog)</h1>
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<small>(as of
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<script type=text/javascript>
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var lm = new Date(document.lastModified);
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document.write(lm.toDateString());
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</script>)
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</small>
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<br>
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<h2> <A NAME=intro>Introduction</A> </h2>
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<p><b>Google glog</b> is a library that implements application-level
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logging. This library provides logging APIs based on C++-style
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streams and various helper macros.
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You can log a message by simply streaming things to LOG(<a
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particular <a href="#severity">severity level</a>>), e.g.
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<pre>
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#include <glog/logging.h>
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int main(int argc, char* argv[]) {
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// Initialize Google's logging library.
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google::InitGoogleLogging(argv[0]);
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// ...
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LOG(INFO) << "Found " << num_cookies << " cookies";
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}
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</pre>
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<p>Google glog defines a series of macros that simplify many common logging
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tasks. You can log messages by severity level, control logging
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behavior from the command line, log based on conditionals, abort the
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program when expected conditions are not met, introduce your own
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verbose logging levels, and more. This document describes the
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functionality supported by glog. Please note that this document
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doesn't describe all features in this library, but the most useful
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ones. If you want to find less common features, please check
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header files under <code>src/glog</code> directory.
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<h2> <A NAME=severity>Severity Level</A> </h2>
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<p>
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You can specify one of the following severity levels (in
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increasing order of severity): <code>INFO</code>, <code>WARNING</code>,
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<code>ERROR</code>, and <code>FATAL</code>.
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Logging a <code>FATAL</code> message terminates the program (after the
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message is logged).
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Note that messages of a given severity are logged not only in the
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logfile for that severity, but also in all logfiles of lower severity.
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E.g., a message of severity <code>FATAL</code> will be logged to the
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logfiles of severity <code>FATAL</code>, <code>ERROR</code>,
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<code>WARNING</code>, and <code>INFO</code>.
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<p>
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The <code>DFATAL</code> severity logs a <code>FATAL</code> error in
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debug mode (i.e., there is no <code>NDEBUG</code> macro defined), but
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avoids halting the program in production by automatically reducing the
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severity to <code>ERROR</code>.
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<p>Unless otherwise specified, glog writes to the filename
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"/tmp/<program name>.<hostname>.<user name>.log.<severity level>.<date>.<time>.<pid>"
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(e.g., "/tmp/hello_world.example.com.hamaji.log.INFO.20080709-222411.10474").
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By default, glog copies the log messages of severity level
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<code>ERROR</code> or <code>FATAL</code> to standard error (stderr)
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in addition to log files.
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<h2><A NAME=flags>Setting Flags</A></h2>
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<p>Several flags influence glog's output behavior.
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If the <a href="https://github.com/gflags/gflags">Google
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gflags library</a> is installed on your machine, the
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<code>configure</code> script (see the INSTALL file in the package for
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detail of this script) will automatically detect and use it,
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allowing you to pass flags on the command line. For example, if you
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want to turn the flag <code>--logtostderr</code> on, you can start
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your application with the following command line:
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<pre>
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./your_application --logtostderr=1
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</pre>
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If the Google gflags library isn't installed, you set flags via
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environment variables, prefixing the flag name with "GLOG_", e.g.
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<pre>
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GLOG_logtostderr=1 ./your_application
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</pre>
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<!-- TODO(hamaji): Fill the version number
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<p>By glog version 0.x.x, you can use GLOG_* environment variables
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even if you have gflags. If both an environment variable and a flag
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are specified, the value specified by a flag wins. E.g., if GLOG_v=0
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and --v=1, the verbosity will be 1, not 0.
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-->
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<p>The following flags are most commonly used:
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<dl>
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<dt><code>logtostderr</code> (<code>bool</code>, default=<code>false</code>)
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<dd>Log messages to stderr instead of logfiles.<br>
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Note: you can set binary flags to <code>true</code> by specifying
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<code>1</code>, <code>true</code>, or <code>yes</code> (case
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insensitive).
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Also, you can set binary flags to <code>false</code> by specifying
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<code>0</code>, <code>false</code>, or <code>no</code> (again, case
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insensitive).
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<dt><code>stderrthreshold</code> (<code>int</code>, default=2, which
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is <code>ERROR</code>)
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<dd>Copy log messages at or above this level to stderr in
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addition to logfiles. The numbers of severity levels
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<code>INFO</code>, <code>WARNING</code>, <code>ERROR</code>, and
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<code>FATAL</code> are 0, 1, 2, and 3, respectively.
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<dt><code>minloglevel</code> (<code>int</code>, default=0, which
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is <code>INFO</code>)
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<dd>Log messages at or above this level. Again, the numbers of
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severity levels <code>INFO</code>, <code>WARNING</code>,
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<code>ERROR</code>, and <code>FATAL</code> are 0, 1, 2, and 3,
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respectively.
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<dt><code>log_dir</code> (<code>string</code>, default="")
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<dd>If specified, logfiles are written into this directory instead
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of the default logging directory.
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<dt><code>v</code> (<code>int</code>, default=0)
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<dd>Show all <code>VLOG(m)</code> messages for <code>m</code> less or
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equal the value of this flag. Overridable by --vmodule.
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See <a href="#verbose">the section about verbose logging</a> for more
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detail.
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<dt><code>vmodule</code> (<code>string</code>, default="")
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<dd>Per-module verbose level. The argument has to contain a
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comma-separated list of <module name>=<log level>.
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<module name>
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is a glob pattern (e.g., <code>gfs*</code> for all modules whose name
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starts with "gfs"), matched against the filename base
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(that is, name ignoring .cc/.h./-inl.h).
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<log level> overrides any value given by --v.
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See also <a href="#verbose">the section about verbose logging</a>.
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</dl>
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<p>There are some other flags defined in logging.cc. Please grep the
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source code for "DEFINE_" to see a complete list of all flags.
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<p>You can also modify flag values in your program by modifying global
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variables <code>FLAGS_*</code> . Most settings start working
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immediately after you update <code>FLAGS_*</code> . The exceptions are
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the flags related to destination files. For example, you might want to
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set <code>FLAGS_log_dir</code> before
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calling <code>google::InitGoogleLogging</code> . Here is an example:
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<pre>
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LOG(INFO) << "file";
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// Most flags work immediately after updating values.
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FLAGS_logtostderr = 1;
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LOG(INFO) << "stderr";
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FLAGS_logtostderr = 0;
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// This won't change the log destination. If you want to set this
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// value, you should do this before google::InitGoogleLogging .
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FLAGS_log_dir = "/some/log/directory";
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LOG(INFO) << "the same file";
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</pre>
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<h2><A NAME=conditional>Conditional / Occasional Logging</A></h2>
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<p>Sometimes, you may only want to log a message under certain
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conditions. You can use the following macros to perform conditional
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logging:
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<pre>
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LOG_IF(INFO, num_cookies > 10) << "Got lots of cookies";
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</pre>
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The "Got lots of cookies" message is logged only when the variable
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<code>num_cookies</code> exceeds 10.
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If a line of code is executed many times, it may be useful to only log
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a message at certain intervals. This kind of logging is most useful
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for informational messages.
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<pre>
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LOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie";
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</pre>
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<p>The above line outputs a log messages on the 1st, 11th,
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21st, ... times it is executed. Note that the special
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<code>google::COUNTER</code> value is used to identify which repetition is
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happening.
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<p>You can combine conditional and occasional logging with the
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following macro.
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<pre>
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LOG_IF_EVERY_N(INFO, (size > 1024), 10) << "Got the " << google::COUNTER
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<< "th big cookie";
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</pre>
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<p>Instead of outputting a message every nth time, you can also limit
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the output to the first n occurrences:
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<pre>
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LOG_FIRST_N(INFO, 20) << "Got the " << google::COUNTER << "th cookie";
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</pre>
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<p>Outputs log messages for the first 20 times it is executed. Again,
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the <code>google::COUNTER</code> identifier indicates which repetition is
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happening.
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<h2><A NAME=debug>Debug Mode Support</A></h2>
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<p>Special "debug mode" logging macros only have an effect in debug
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mode and are compiled away to nothing for non-debug mode
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compiles. Use these macros to avoid slowing down your production
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application due to excessive logging.
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<pre>
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DLOG(INFO) << "Found cookies";
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DLOG_IF(INFO, num_cookies > 10) << "Got lots of cookies";
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DLOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie";
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</pre>
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<h2><A NAME=check>CHECK Macros</A></h2>
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<p>It is a good practice to check expected conditions in your program
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frequently to detect errors as early as possible. The
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<code>CHECK</code> macro provides the ability to abort the application
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when a condition is not met, similar to the <code>assert</code> macro
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defined in the standard C library.
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<p><code>CHECK</code> aborts the application if a condition is not
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true. Unlike <code>assert</code>, it is *not* controlled by
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<code>NDEBUG</code>, so the check will be executed regardless of
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compilation mode. Therefore, <code>fp->Write(x)</code> in the
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following example is always executed:
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<pre>
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CHECK(fp->Write(x) == 4) << "Write failed!";
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</pre>
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<p>There are various helper macros for
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equality/inequality checks - <code>CHECK_EQ</code>,
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<code>CHECK_NE</code>, <code>CHECK_LE</code>, <code>CHECK_LT</code>,
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<code>CHECK_GE</code>, and <code>CHECK_GT</code>.
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They compare two values, and log a
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<code>FATAL</code> message including the two values when the result is
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not as expected. The values must have <code>operator<<(ostream,
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...)</code> defined.
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<p>You may append to the error message like so:
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<pre>
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CHECK_NE(1, 2) << ": The world must be ending!";
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</pre>
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<p>We are very careful to ensure that each argument is evaluated exactly
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once, and that anything which is legal to pass as a function argument is
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legal here. In particular, the arguments may be temporary expressions
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which will end up being destroyed at the end of the apparent statement,
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for example:
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<pre>
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CHECK_EQ(string("abc")[1], 'b');
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</pre>
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<p>The compiler reports an error if one of the arguments is a
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pointer and the other is NULL. To work around this, simply static_cast
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NULL to the type of the desired pointer.
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<pre>
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CHECK_EQ(some_ptr, static_cast<SomeType*>(NULL));
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</pre>
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<p>Better yet, use the CHECK_NOTNULL macro:
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<pre>
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CHECK_NOTNULL(some_ptr);
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some_ptr->DoSomething();
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</pre>
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<p>Since this macro returns the given pointer, this is very useful in
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constructor initializer lists.
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<pre>
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struct S {
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S(Something* ptr) : ptr_(CHECK_NOTNULL(ptr)) {}
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Something* ptr_;
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};
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</pre>
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<p>Note that you cannot use this macro as a C++ stream due to this
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feature. Please use <code>CHECK_EQ</code> described above to log a
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custom message before aborting the application.
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<p>If you are comparing C strings (char *), a handy set of macros
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performs case sensitive as well as case insensitive comparisons -
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<code>CHECK_STREQ</code>, <code>CHECK_STRNE</code>,
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<code>CHECK_STRCASEEQ</code>, and <code>CHECK_STRCASENE</code>. The
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CASE versions are case-insensitive. You can safely pass <code>NULL</code>
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pointers for this macro. They treat <code>NULL</code> and any
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non-<code>NULL</code> string as not equal. Two <code>NULL</code>s are
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equal.
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<p>Note that both arguments may be temporary strings which are
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destructed at the end of the current "full expression"
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(e.g., <code>CHECK_STREQ(Foo().c_str(), Bar().c_str())</code> where
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<code>Foo</code> and <code>Bar</code> return C++'s
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<code>std::string</code>).
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<p>The <code>CHECK_DOUBLE_EQ</code> macro checks the equality of two
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floating point values, accepting a small error margin.
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<code>CHECK_NEAR</code> accepts a third floating point argument, which
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specifies the acceptable error margin.
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<h2><A NAME=verbose>Verbose Logging</A></h2>
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<p>When you are chasing difficult bugs, thorough log messages are very
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useful. However, you may want to ignore too verbose messages in usual
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development. For such verbose logging, glog provides the
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<code>VLOG</code> macro, which allows you to define your own numeric
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logging levels. The <code>--v</code> command line option controls
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which verbose messages are logged:
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<pre>
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VLOG(1) << "I'm printed when you run the program with --v=1 or higher";
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VLOG(2) << "I'm printed when you run the program with --v=2 or higher";
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</pre>
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<p>With <code>VLOG</code>, the lower the verbose level, the more
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likely messages are to be logged. For example, if
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<code>--v==1</code>, <code>VLOG(1)</code> will log, but
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<code>VLOG(2)</code> will not log. This is opposite of the severity
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level, where <code>INFO</code> is 0, and <code>ERROR</code> is 2.
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<code>--minloglevel</code> of 1 will log <code>WARNING</code> and
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above. Though you can specify any integers for both <code>VLOG</code>
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macro and <code>--v</code> flag, the common values for them are small
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positive integers. For example, if you write <code>VLOG(0)</code>,
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you should specify <code>--v=-1</code> or lower to silence it. This
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is less useful since we may not want verbose logs by default in most
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cases. The <code>VLOG</code> macros always log at the
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<code>INFO</code> log level (when they log at all).
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<p>Verbose logging can be controlled from the command line on a
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per-module basis:
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<pre>
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--vmodule=mapreduce=2,file=1,gfs*=3 --v=0
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</pre>
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<p>will:
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<ul>
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<li>a. Print VLOG(2) and lower messages from mapreduce.{h,cc}
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<li>b. Print VLOG(1) and lower messages from file.{h,cc}
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<li>c. Print VLOG(3) and lower messages from files prefixed with "gfs"
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<li>d. Print VLOG(0) and lower messages from elsewhere
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</ul>
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<p>The wildcarding functionality shown by (c) supports both '*'
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(matches 0 or more characters) and '?' (matches any single character)
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wildcards. Please also check the section about <a
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href="#flags">command line flags</a>.
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<p>There's also <code>VLOG_IS_ON(n)</code> "verbose level" condition
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macro. This macro returns true when the <code>--v</code> is equal or
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greater than <code>n</code>. To be used as
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<pre>
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if (VLOG_IS_ON(2)) {
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// do some logging preparation and logging
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// that can't be accomplished with just VLOG(2) << ...;
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}
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</pre>
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<p>Verbose level condition macros <code>VLOG_IF</code>,
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<code>VLOG_EVERY_N</code> and <code>VLOG_IF_EVERY_N</code> behave
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analogous to <code>LOG_IF</code>, <code>LOG_EVERY_N</code>,
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<code>LOF_IF_EVERY</code>, but accept a numeric verbosity level as
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opposed to a severity level.
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<pre>
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VLOG_IF(1, (size > 1024))
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<< "I'm printed when size is more than 1024 and when you run the "
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"program with --v=1 or more";
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VLOG_EVERY_N(1, 10)
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<< "I'm printed every 10th occurrence, and when you run the program "
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"with --v=1 or more. Present occurence is " << google::COUNTER;
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VLOG_IF_EVERY_N(1, (size > 1024), 10)
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<< "I'm printed on every 10th occurence of case when size is more "
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" than 1024, when you run the program with --v=1 or more. ";
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"Present occurence is " << google::COUNTER;
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</pre>
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<h2> <A name="signal">Failure Signal Handler</A> </h2>
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<p>
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The library provides a convenient signal handler that will dump useful
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information when the program crashes on certain signals such as SIGSEGV.
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The signal handler can be installed by
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google::InstallFailureSignalHandler(). The following is an example of output
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from the signal handler.
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<pre>
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*** Aborted at 1225095260 (unix time) try "date -d @1225095260" if you are using GNU date ***
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*** SIGSEGV (@0x0) received by PID 17711 (TID 0x7f893090a6f0) from PID 0; stack trace: ***
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PC: @ 0x412eb1 TestWaitingLogSink::send()
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@ 0x7f892fb417d0 (unknown)
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@ 0x412eb1 TestWaitingLogSink::send()
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@ 0x7f89304f7f06 google::LogMessage::SendToLog()
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@ 0x7f89304f35af google::LogMessage::Flush()
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@ 0x7f89304f3739 google::LogMessage::~LogMessage()
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@ 0x408cf4 TestLogSinkWaitTillSent()
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@ 0x4115de main
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@ 0x7f892f7ef1c4 (unknown)
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@ 0x4046f9 (unknown)
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</pre>
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<p>
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By default, the signal handler writes the failure dump to the standard
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error. You can customize the destination by InstallFailureWriter().
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<h2> <A name="misc">Miscellaneous Notes</A> </h2>
|
|
|
|
<h3><A NAME=message>Performance of Messages</A></h3>
|
|
|
|
<p>The conditional logging macros provided by glog (e.g.,
|
|
<code>CHECK</code>, <code>LOG_IF</code>, <code>VLOG</code>, ...) are
|
|
carefully implemented and don't execute the right hand side
|
|
expressions when the conditions are false. So, the following check
|
|
may not sacrifice the performance of your application.
|
|
|
|
<pre>
|
|
CHECK(obj.ok) << obj.CreatePrettyFormattedStringButVerySlow();
|
|
</pre>
|
|
|
|
<h3><A NAME=failure>User-defined Failure Function</A></h3>
|
|
|
|
<p><code>FATAL</code> severity level messages or unsatisfied
|
|
<code>CHECK</code> condition terminate your program. You can change
|
|
the behavior of the termination by
|
|
<code>InstallFailureFunction</code>.
|
|
|
|
<pre>
|
|
void YourFailureFunction() {
|
|
// Reports something...
|
|
exit(1);
|
|
}
|
|
|
|
int main(int argc, char* argv[]) {
|
|
google::InstallFailureFunction(&YourFailureFunction);
|
|
}
|
|
</pre>
|
|
|
|
<p>By default, glog tries to dump stacktrace and makes the program
|
|
exit with status 1. The stacktrace is produced only when you run the
|
|
program on an architecture for which glog supports stack tracing (as
|
|
of September 2008, glog supports stack tracing for x86 and x86_64).
|
|
|
|
<h3><A NAME=raw>Raw Logging</A></h3>
|
|
|
|
<p>The header file <code><glog/raw_logging.h></code> can be
|
|
used for thread-safe logging, which does not allocate any memory or
|
|
acquire any locks. Therefore, the macros defined in this
|
|
header file can be used by low-level memory allocation and
|
|
synchronization code.
|
|
Please check <code>src/glog/raw_logging.h.in</code> for detail.
|
|
</p>
|
|
|
|
<h3><A NAME=plog>Google Style perror()</A></h3>
|
|
|
|
<p><code>PLOG()</code> and <code>PLOG_IF()</code> and
|
|
<code>PCHECK()</code> behave exactly like their <code>LOG*</code> and
|
|
<code>CHECK</code> equivalents with the addition that they append a
|
|
description of the current state of errno to their output lines.
|
|
E.g.
|
|
|
|
<pre>
|
|
PCHECK(write(1, NULL, 2) >= 0) << "Write NULL failed";
|
|
</pre>
|
|
|
|
<p>This check fails with the following error message.
|
|
|
|
<pre>
|
|
F0825 185142 test.cc:22] Check failed: write(1, NULL, 2) >= 0 Write NULL failed: Bad address [14]
|
|
</pre>
|
|
|
|
<h3><A NAME=syslog>Syslog</A></h3>
|
|
|
|
<p><code>SYSLOG</code>, <code>SYSLOG_IF</code>, and
|
|
<code>SYSLOG_EVERY_N</code> macros are available.
|
|
These log to syslog in addition to the normal logs. Be aware that
|
|
logging to syslog can drastically impact performance, especially if
|
|
syslog is configured for remote logging! Make sure you understand the
|
|
implications of outputting to syslog before you use these macros. In
|
|
general, it's wise to use these macros sparingly.
|
|
|
|
<h3><A NAME=strip>Strip Logging Messages</A></h3>
|
|
|
|
<p>Strings used in log messages can increase the size of your binary
|
|
and present a privacy concern. You can therefore instruct glog to
|
|
remove all strings which fall below a certain severity level by using
|
|
the GOOGLE_STRIP_LOG macro:
|
|
|
|
<p>If your application has code like this:
|
|
|
|
<pre>
|
|
#define GOOGLE_STRIP_LOG 1 // this must go before the #include!
|
|
#include <glog/logging.h>
|
|
</pre>
|
|
|
|
<p>The compiler will remove the log messages whose severities are less
|
|
than the specified integer value. Since
|
|
<code>VLOG</code> logs at the severity level <code>INFO</code>
|
|
(numeric value <code>0</code>),
|
|
setting <code>GOOGLE_STRIP_LOG</code> to 1 or greater removes
|
|
all log messages associated with <code>VLOG</code>s as well as
|
|
<code>INFO</code> log statements.
|
|
|
|
<h3><A NAME=strip>Automatically Remove Old Logs</A></h3>
|
|
|
|
<p>To enable the log cleaner:
|
|
|
|
<pre>
|
|
google::EnableLogCleaner(3); // keep your logs for 3 days
|
|
</pre>
|
|
|
|
And then Google glog will check if there are overdue logs whenever
|
|
a flush is performed. In this example, any log file from your project whose
|
|
last modified time is greater than 3 days will be unlink()ed.
|
|
|
|
<p>This feature can be disabled at any time (if it has been enabled)
|
|
|
|
<pre>
|
|
google::DisableLogCleaner();
|
|
</pre>
|
|
|
|
<h3><A NAME=windows>Notes for Windows users</A></h3>
|
|
|
|
<p>Google glog defines a severity level <code>ERROR</code>, which is
|
|
also defined in <code>windows.h</code> . You can make glog not define
|
|
<code>INFO</code>, <code>WARNING</code>, <code>ERROR</code>,
|
|
and <code>FATAL</code> by defining
|
|
<code>GLOG_NO_ABBREVIATED_SEVERITIES</code> before
|
|
including <code>glog/logging.h</code> . Even with this macro, you can
|
|
still use the iostream like logging facilities:
|
|
|
|
<pre>
|
|
#define GLOG_NO_ABBREVIATED_SEVERITIES
|
|
#include <windows.h>
|
|
#include <glog/logging.h>
|
|
|
|
// ...
|
|
|
|
LOG(ERROR) << "This should work";
|
|
LOG_IF(ERROR, x > y) << "This should be also OK";
|
|
</pre>
|
|
|
|
<p>
|
|
However, you cannot
|
|
use <code>INFO</code>, <code>WARNING</code>, <code>ERROR</code>,
|
|
and <code>FATAL</code> anymore for functions defined
|
|
in <code>glog/logging.h</code> .
|
|
|
|
<pre>
|
|
#define GLOG_NO_ABBREVIATED_SEVERITIES
|
|
#include <windows.h>
|
|
#include <glog/logging.h>
|
|
|
|
// ...
|
|
|
|
// This won't work.
|
|
// google::FlushLogFiles(google::ERROR);
|
|
|
|
// Use this instead.
|
|
google::FlushLogFiles(google::GLOG_ERROR);
|
|
</pre>
|
|
|
|
<p>
|
|
If you don't need <code>ERROR</code> defined
|
|
by <code>windows.h</code>, there are a couple of more workarounds
|
|
which sometimes don't work:
|
|
|
|
<ul>
|
|
<li>#define <code>WIN32_LEAN_AND_MEAN</code> or <code>NOGDI</code>
|
|
<strong>before</strong> you #include <code>windows.h</code> .
|
|
<li>#undef <code>ERROR</code> <strong>after</strong> you #include
|
|
<code>windows.h</code> .
|
|
</ul>
|
|
|
|
<p>See <a href="http://code.google.com/p/google-glog/issues/detail?id=33">
|
|
this issue</a> for more detail.
|
|
|
|
<hr>
|
|
<address>
|
|
Shinichiro Hamaji<br>
|
|
Gregor Hohpe<br>
|
|
<script type=text/javascript>
|
|
var lm = new Date(document.lastModified);
|
|
document.write(lm.toDateString());
|
|
</script>
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|