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| author | Vincent Ambo <tazjin@google.com> | 2020-05-27T00·26+0100 |
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| committer | Vincent Ambo <tazjin@google.com> | 2020-05-27T00·26+0100 |
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diff --git a/doc/glog.html b/doc/glog.html new file mode 100644 index 000000000000..699d2ceb80b1 --- /dev/null +++ b/doc/glog.html @@ -0,0 +1,631 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> + +<html> +<head> +<title>How To Use Google Logging Library (glog)</title> + +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="http://www.google.com/favicon.ico" type="image/x-icon" + rel="shortcut icon"> +<link href="designstyle.css" type="text/css" rel="stylesheet"> +<style type="text/css"> +<!-- + ol.bluelist li { + color: #3366ff; + font-family: sans-serif; + } + ol.bluelist li p { + color: #000; + font-family: "Times Roman", times, serif; + } + ul.blacklist li { + color: #000; + font-family: "Times Roman", times, serif; + } +//--> +</style> +</head> + +<body> + +<h1>How To Use Google Logging Library (glog)</h1> +<small>(as of +<script type=text/javascript> + var lm = new Date(document.lastModified); + document.write(lm.toDateString()); +</script>) +</small> +<br> + +<h2> <A NAME=intro>Introduction</A> </h2> + +<p><b>Google glog</b> is a library that implements application-level +logging. This library provides logging APIs based on C++-style +streams and various helper macros. +You can log a message by simply streaming things to LOG(<a +particular <a href="#severity">severity level</a>>), e.g. + +<pre> + #include <glog/logging.h> + + int main(int argc, char* argv[]) { + // Initialize Google's logging library. + google::InitGoogleLogging(argv[0]); + + // ... + LOG(INFO) << "Found " << num_cookies << " cookies"; + } +</pre> + +<p>Google glog defines a series of macros that simplify many common logging +tasks. You can log messages by severity level, control logging +behavior from the command line, log based on conditionals, abort the +program when expected conditions are not met, introduce your own +verbose logging levels, and more. This document describes the +functionality supported by glog. Please note that this document +doesn't describe all features in this library, but the most useful +ones. If you want to find less common features, please check +header files under <code>src/glog</code> directory. + +<h2> <A NAME=severity>Severity Level</A> </h2> + +<p> +You can specify one of the following severity levels (in +increasing order of severity): <code>INFO</code>, <code>WARNING</code>, +<code>ERROR</code>, and <code>FATAL</code>. +Logging a <code>FATAL</code> message terminates the program (after the +message is logged). +Note that messages of a given severity are logged not only in the +logfile for that severity, but also in all logfiles of lower severity. +E.g., a message of severity <code>FATAL</code> will be logged to the +logfiles of severity <code>FATAL</code>, <code>ERROR</code>, +<code>WARNING</code>, and <code>INFO</code>. + +<p> +The <code>DFATAL</code> severity logs a <code>FATAL</code> error in +debug mode (i.e., there is no <code>NDEBUG</code> macro defined), but +avoids halting the program in production by automatically reducing the +severity to <code>ERROR</code>. + +<p>Unless otherwise specified, glog writes to the filename +"/tmp/<program name>.<hostname>.<user name>.log.<severity level>.<date>.<time>.<pid>" +(e.g., "/tmp/hello_world.example.com.hamaji.log.INFO.20080709-222411.10474"). +By default, glog copies the log messages of severity level +<code>ERROR</code> or <code>FATAL</code> to standard error (stderr) +in addition to log files. + +<h2><A NAME=flags>Setting Flags</A></h2> + +<p>Several flags influence glog's output behavior. +If the <a href="https://github.com/gflags/gflags">Google +gflags library</a> is installed on your machine, the +<code>configure</code> script (see the INSTALL file in the package for +detail of this script) will automatically detect and use it, +allowing you to pass flags on the command line. For example, if you +want to turn the flag <code>--logtostderr</code> on, you can start +your application with the following command line: + +<pre> + ./your_application --logtostderr=1 +</pre> + +If the Google gflags library isn't installed, you set flags via +environment variables, prefixing the flag name with "GLOG_", e.g. + +<pre> + GLOG_logtostderr=1 ./your_application +</pre> + +<!-- TODO(hamaji): Fill the version number +<p>By glog version 0.x.x, you can use GLOG_* environment variables +even if you have gflags. If both an environment variable and a flag +are specified, the value specified by a flag wins. E.g., if GLOG_v=0 +and --v=1, the verbosity will be 1, not 0. +--> + +<p>The following flags are most commonly used: + +<dl> +<dt><code>logtostderr</code> (<code>bool</code>, default=<code>false</code>) +<dd>Log messages to stderr instead of logfiles.<br> +Note: you can set binary flags to <code>true</code> by specifying +<code>1</code>, <code>true</code>, or <code>yes</code> (case +insensitive). +Also, you can set binary flags to <code>false</code> by specifying +<code>0</code>, <code>false</code>, or <code>no</code> (again, case +insensitive). +<dt><code>stderrthreshold</code> (<code>int</code>, default=2, which +is <code>ERROR</code>) +<dd>Copy log messages at or above this level to stderr in +addition to logfiles. The numbers of severity levels +<code>INFO</code>, <code>WARNING</code>, <code>ERROR</code>, and +<code>FATAL</code> are 0, 1, 2, and 3, respectively. +<dt><code>minloglevel</code> (<code>int</code>, default=0, which +is <code>INFO</code>) +<dd>Log messages at or above this level. Again, the numbers of +severity levels <code>INFO</code>, <code>WARNING</code>, +<code>ERROR</code>, and <code>FATAL</code> are 0, 1, 2, and 3, +respectively. +<dt><code>log_dir</code> (<code>string</code>, default="") +<dd>If specified, logfiles are written into this directory instead +of the default logging directory. +<dt><code>v</code> (<code>int</code>, default=0) +<dd>Show all <code>VLOG(m)</code> messages for <code>m</code> less or +equal the value of this flag. Overridable by --vmodule. +See <a href="#verbose">the section about verbose logging</a> for more +detail. +<dt><code>vmodule</code> (<code>string</code>, default="") +<dd>Per-module verbose level. The argument has to contain a +comma-separated list of <module name>=<log level>. +<module name> +is a glob pattern (e.g., <code>gfs*</code> for all modules whose name +starts with "gfs"), matched against the filename base +(that is, name ignoring .cc/.h./-inl.h). +<log level> overrides any value given by --v. +See also <a href="#verbose">the section about verbose logging</a>. +</dl> + +<p>There are some other flags defined in logging.cc. Please grep the +source code for "DEFINE_" to see a complete list of all flags. + +<p>You can also modify flag values in your program by modifying global +variables <code>FLAGS_*</code> . Most settings start working +immediately after you update <code>FLAGS_*</code> . The exceptions are +the flags related to destination files. For example, you might want to +set <code>FLAGS_log_dir</code> before +calling <code>google::InitGoogleLogging</code> . Here is an example: + +<pre> + LOG(INFO) << "file"; + // Most flags work immediately after updating values. + FLAGS_logtostderr = 1; + LOG(INFO) << "stderr"; + FLAGS_logtostderr = 0; + // This won't change the log destination. If you want to set this + // value, you should do this before google::InitGoogleLogging . + FLAGS_log_dir = "/some/log/directory"; + LOG(INFO) << "the same file"; +</pre> + +<h2><A NAME=conditional>Conditional / Occasional Logging</A></h2> + +<p>Sometimes, you may only want to log a message under certain +conditions. You can use the following macros to perform conditional +logging: + +<pre> + LOG_IF(INFO, num_cookies > 10) << "Got lots of cookies"; +</pre> + +The "Got lots of cookies" message is logged only when the variable +<code>num_cookies</code> exceeds 10. + +If a line of code is executed many times, it may be useful to only log +a message at certain intervals. This kind of logging is most useful +for informational messages. + +<pre> + LOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie"; +</pre> + +<p>The above line outputs a log messages on the 1st, 11th, +21st, ... times it is executed. Note that the special +<code>google::COUNTER</code> value is used to identify which repetition is +happening. + +<p>You can combine conditional and occasional logging with the +following macro. + +<pre> + LOG_IF_EVERY_N(INFO, (size > 1024), 10) << "Got the " << google::COUNTER + << "th big cookie"; +</pre> + +<p>Instead of outputting a message every nth time, you can also limit +the output to the first n occurrences: + +<pre> + LOG_FIRST_N(INFO, 20) << "Got the " << google::COUNTER << "th cookie"; +</pre> + +<p>Outputs log messages for the first 20 times it is executed. Again, +the <code>google::COUNTER</code> identifier indicates which repetition is +happening. + +<h2><A NAME=debug>Debug Mode Support</A></h2> + +<p>Special "debug mode" logging macros only have an effect in debug +mode and are compiled away to nothing for non-debug mode +compiles. Use these macros to avoid slowing down your production +application due to excessive logging. + +<pre> + DLOG(INFO) << "Found cookies"; + + DLOG_IF(INFO, num_cookies > 10) << "Got lots of cookies"; + + DLOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie"; +</pre> + +<h2><A NAME=check>CHECK Macros</A></h2> + +<p>It is a good practice to check expected conditions in your program +frequently to detect errors as early as possible. The +<code>CHECK</code> macro provides the ability to abort the application +when a condition is not met, similar to the <code>assert</code> macro +defined in the standard C library. + +<p><code>CHECK</code> aborts the application if a condition is not +true. Unlike <code>assert</code>, it is *not* controlled by +<code>NDEBUG</code>, so the check will be executed regardless of +compilation mode. Therefore, <code>fp->Write(x)</code> in the +following example is always executed: + +<pre> + CHECK(fp->Write(x) == 4) << "Write failed!"; +</pre> + +<p>There are various helper macros for +equality/inequality checks - <code>CHECK_EQ</code>, +<code>CHECK_NE</code>, <code>CHECK_LE</code>, <code>CHECK_LT</code>, +<code>CHECK_GE</code>, and <code>CHECK_GT</code>. +They compare two values, and log a +<code>FATAL</code> message including the two values when the result is +not as expected. The values must have <code>operator<<(ostream, +...)</code> defined. + +<p>You may append to the error message like so: + +<pre> + CHECK_NE(1, 2) << ": The world must be ending!"; +</pre> + +<p>We are very careful to ensure that each argument is evaluated exactly +once, and that anything which is legal to pass as a function argument is +legal here. In particular, the arguments may be temporary expressions +which will end up being destroyed at the end of the apparent statement, +for example: + +<pre> + CHECK_EQ(string("abc")[1], 'b'); +</pre> + +<p>The compiler reports an error if one of the arguments is a +pointer and the other is NULL. To work around this, simply static_cast +NULL to the type of the desired pointer. + +<pre> + CHECK_EQ(some_ptr, static_cast<SomeType*>(NULL)); +</pre> + +<p>Better yet, use the CHECK_NOTNULL macro: + +<pre> + CHECK_NOTNULL(some_ptr); + some_ptr->DoSomething(); +</pre> + +<p>Since this macro returns the given pointer, this is very useful in +constructor initializer lists. + +<pre> + struct S { + S(Something* ptr) : ptr_(CHECK_NOTNULL(ptr)) {} + Something* ptr_; + }; +</pre> + +<p>Note that you cannot use this macro as a C++ stream due to this +feature. Please use <code>CHECK_EQ</code> described above to log a +custom message before aborting the application. + +<p>If you are comparing C strings (char *), a handy set of macros +performs case sensitive as well as case insensitive comparisons - +<code>CHECK_STREQ</code>, <code>CHECK_STRNE</code>, +<code>CHECK_STRCASEEQ</code>, and <code>CHECK_STRCASENE</code>. The +CASE versions are case-insensitive. You can safely pass <code>NULL</code> +pointers for this macro. They treat <code>NULL</code> and any +non-<code>NULL</code> string as not equal. Two <code>NULL</code>s are +equal. + +<p>Note that both arguments may be temporary strings which are +destructed at the end of the current "full expression" +(e.g., <code>CHECK_STREQ(Foo().c_str(), Bar().c_str())</code> where +<code>Foo</code> and <code>Bar</code> return C++'s +<code>std::string</code>). + +<p>The <code>CHECK_DOUBLE_EQ</code> macro checks the equality of two +floating point values, accepting a small error margin. +<code>CHECK_NEAR</code> accepts a third floating point argument, which +specifies the acceptable error margin. + +<h2><A NAME=verbose>Verbose Logging</A></h2> + +<p>When you are chasing difficult bugs, thorough log messages are very +useful. However, you may want to ignore too verbose messages in usual +development. For such verbose logging, glog provides the +<code>VLOG</code> macro, which allows you to define your own numeric +logging levels. The <code>--v</code> command line option controls +which verbose messages are logged: + +<pre> + VLOG(1) << "I'm printed when you run the program with --v=1 or higher"; + VLOG(2) << "I'm printed when you run the program with --v=2 or higher"; +</pre> + +<p>With <code>VLOG</code>, the lower the verbose level, the more +likely messages are to be logged. For example, if +<code>--v==1</code>, <code>VLOG(1)</code> will log, but +<code>VLOG(2)</code> will not log. This is opposite of the severity +level, where <code>INFO</code> is 0, and <code>ERROR</code> is 2. +<code>--minloglevel</code> of 1 will log <code>WARNING</code> and +above. Though you can specify any integers for both <code>VLOG</code> +macro and <code>--v</code> flag, the common values for them are small +positive integers. For example, if you write <code>VLOG(0)</code>, +you should specify <code>--v=-1</code> or lower to silence it. This +is less useful since we may not want verbose logs by default in most +cases. The <code>VLOG</code> macros always log at the +<code>INFO</code> log level (when they log at all). + +<p>Verbose logging can be controlled from the command line on a +per-module basis: + +<pre> + --vmodule=mapreduce=2,file=1,gfs*=3 --v=0 +</pre> + +<p>will: + +<ul> + <li>a. Print VLOG(2) and lower messages from mapreduce.{h,cc} + <li>b. Print VLOG(1) and lower messages from file.{h,cc} + <li>c. Print VLOG(3) and lower messages from files prefixed with "gfs" + <li>d. Print VLOG(0) and lower messages from elsewhere +</ul> + +<p>The wildcarding functionality shown by (c) supports both '*' +(matches 0 or more characters) and '?' (matches any single character) +wildcards. Please also check the section about <a +href="#flags">command line flags</a>. + +<p>There's also <code>VLOG_IS_ON(n)</code> "verbose level" condition +macro. This macro returns true when the <code>--v</code> is equal or +greater than <code>n</code>. To be used as + +<pre> + if (VLOG_IS_ON(2)) { + // do some logging preparation and logging + // that can't be accomplished with just VLOG(2) << ...; + } +</pre> + +<p>Verbose level condition macros <code>VLOG_IF</code>, +<code>VLOG_EVERY_N</code> and <code>VLOG_IF_EVERY_N</code> behave +analogous to <code>LOG_IF</code>, <code>LOG_EVERY_N</code>, +<code>LOF_IF_EVERY</code>, but accept a numeric verbosity level as +opposed to a severity level. + +<pre> + VLOG_IF(1, (size > 1024)) + << "I'm printed when size is more than 1024 and when you run the " + "program with --v=1 or more"; + VLOG_EVERY_N(1, 10) + << "I'm printed every 10th occurrence, and when you run the program " + "with --v=1 or more. Present occurence is " << google::COUNTER; + VLOG_IF_EVERY_N(1, (size > 1024), 10) + << "I'm printed on every 10th occurence of case when size is more " + " than 1024, when you run the program with --v=1 or more. "; + "Present occurence is " << google::COUNTER; +</pre> + +<h2> <A name="signal">Failure Signal Handler</A> </h2> + +<p> +The library provides a convenient signal handler that will dump useful +information when the program crashes on certain signals such as SIGSEGV. +The signal handler can be installed by +google::InstallFailureSignalHandler(). The following is an example of output +from the signal handler. + +<pre> +*** Aborted at 1225095260 (unix time) try "date -d @1225095260" if you are using GNU date *** +*** SIGSEGV (@0x0) received by PID 17711 (TID 0x7f893090a6f0) from PID 0; stack trace: *** +PC: @ 0x412eb1 TestWaitingLogSink::send() + @ 0x7f892fb417d0 (unknown) + @ 0x412eb1 TestWaitingLogSink::send() + @ 0x7f89304f7f06 google::LogMessage::SendToLog() + @ 0x7f89304f35af google::LogMessage::Flush() + @ 0x7f89304f3739 google::LogMessage::~LogMessage() + @ 0x408cf4 TestLogSinkWaitTillSent() + @ 0x4115de main + @ 0x7f892f7ef1c4 (unknown) + @ 0x4046f9 (unknown) +</pre> + +<p> +By default, the signal handler writes the failure dump to the standard +error. You can customize the destination by InstallFailureWriter(). + +<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> |