glog(Google Logging Library)

对MPI支持一样好用  记得要传递FLAGS_v

http://code.google.com/p/google-glog/downloads/detail?name=glog-0.3.3.tar.gz&can=2&q=

How To Use Google Logging Library (glog)

(as ofFri Jan 25 2013)

Introduction

Google glog is a library that implements application-levellogging. This library provides logging APIs based on C++-stylestreams and various helper macros.You can log a message by simply streaming things to LOG(<aparticularseverity level>), e.g.

   #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";   }

Google glog defines a series of macros that simplify many common loggingtasks. You can log messages by severity level, control loggingbehavior from the command line, log based on conditionals, abort theprogram when expected conditions are not met, introduce your ownverbose logging levels, and more. This document describes thefunctionality supported by glog. Please note that this documentdoesn't describe all features in this library, but the most usefulones. If you want to find less common features, please checkheader files under src/glog directory.

Severity Level

You can specify one of the following severity levels (inincreasing order of severity):INFO,WARNING,ERROR, andFATAL.Logging aFATAL message terminates the program (after themessage is logged).Note that messages of a given severity are logged not only in thelogfile for that severity, but also in all logfiles of lower severity.E.g., a message of severityFATAL will be logged to thelogfiles of severityFATAL,ERROR,WARNING, and INFO.

The DFATAL severity logs a FATAL error indebug mode (i.e., there is noNDEBUG macro defined), butavoids halting the program in production by automatically reducing theseverity toERROR.

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 levelERROR or FATAL to standard error (stderr)in addition to log files.

Setting Flags

Several flags influence glog's output behavior.If the Googlegflags library is installed on your machine, theconfigure script (see the INSTALL file in the package fordetail of this script) will automatically detect and use it,allowing you to pass flags on the command line. For example, if youwant to turn the flag --logtostderr on, you can startyour application with the following command line:

   ./your_application --logtostderr=1

If the Google gflags library isn't installed, you set flags viaenvironment variables, prefixing the flag name with "GLOG_", e.g.

   GLOG_logtostderr=1 ./your_application

The following flags are most commonly used:

logtostderr (bool, default=false)

Log messages to stderr instead of logfiles.
Note: you can set binary flags to true by specifying1, true, or yes (caseinsensitive).Also, you can set binary flags tofalse by specifying0,false, orno (again, caseinsensitive).

stderrthreshold (int, default=2, whichis ERROR)

Copy log messages at or above this level to stderr inaddition to logfiles. The numbers of severity levelsINFO,WARNING,ERROR, andFATAL are 0, 1, 2, and 3, respectively.

minloglevel (int, default=0, whichis INFO)

Log messages at or above this level. Again, the numbers ofseverity levels INFO, WARNING,ERROR, and FATAL are 0, 1, 2, and 3,respectively.

log_dir (string, default="")

If specified, logfiles are written into this directory insteadof the default logging directory.

v (int, default=0)

Show all VLOG(m) messages for m less orequal the value of this flag. Overridable by --vmodule.Seethe section about verbose logging for moredetail.

vmodule (string, default="")

Per-module verbose level. The argument has to contain acomma-separated list of <module name>=<log level>.<module name>is a glob pattern (e.g.,gfs* for all modules whose namestarts with "gfs"), matched against the filename base(that is, name ignoring .cc/.h./-inl.h).<log level> overrides any value given by --v.See alsothe section about verbose logging.

There are some other flags defined in logging.cc. Please grep thesource code for "DEFINE_" to see a complete list of all flags.

You can also modify flag values in your program by modifying globalvariables FLAGS_* . Most settings start workingimmediately after you update FLAGS_* . The exceptions arethe flags related to destination files. For example, you might want tosetFLAGS_log_dir beforecallinggoogle::InitGoogleLogging . Here is an example:

   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";

Conditional / Occasional Logging

Sometimes, you may only want to log a message under certainconditions. You can use the following macros to perform conditionallogging:

   LOG_IF(INFO, num_cookies > 10) << "Got lots of cookies";

The "Got lots of cookies" message is logged only when the variablenum_cookies exceeds 10.If a line of code is executed many times, it may be useful to only loga message at certain intervals. This kind of logging is most usefulfor informational messages.

   LOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie";

The above line outputs a log messages on the 1st, 11th,21st, ... times it is executed. Note that the specialgoogle::COUNTER value is used to identify which repetition ishappening.

You can combine conditional and occasional logging with thefollowing macro.

   LOG_IF_EVERY_N(INFO, (size > 1024), 10) << "Got the " << google::COUNTER                                           << "th big cookie";

Instead of outputting a message every nth time, you can also limitthe output to the first n occurrences:

   LOG_FIRST_N(INFO, 20) << "Got the " << google::COUNTER << "th cookie";

Outputs log messages for the first 20 times it is executed. Again,the google::COUNTER identifier indicates which repetition ishappening.

Debug Mode Support

Special "debug mode" logging macros only have an effect in debugmode and are compiled away to nothing for non-debug modecompiles. Use these macros to avoid slowing down your productionapplication due to excessive logging.

   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";

CHECK Macros

It is a good practice to check expected conditions in your programfrequently to detect errors as early as possible. TheCHECK macro provides the ability to abort the applicationwhen a condition is not met, similar to theassert macrodefined in the standard C library.

CHECK aborts the application if a condition is nottrue. Unlike assert, it is *not* controlled byNDEBUG, so the check will be executed regardless ofcompilation mode. Therefore,fp->Write(x) in thefollowing example is always executed:

   CHECK(fp->Write(x) == 4) << "Write failed!";

There are various helper macros forequality/inequality checks - CHECK_EQ,CHECK_NE,CHECK_LE,CHECK_LT,CHECK_GE, andCHECK_GT.They compare two values, and log aFATAL message including the two values when the result isnot as expected. The values must haveoperator<<(ostream,...) defined.

You may append to the error message like so:

   CHECK_NE(1, 2) << ": The world must be ending!";

We are very careful to ensure that each argument is evaluated exactlyonce, and that anything which is legal to pass as a function argument islegal here. In particular, the arguments may be temporary expressionswhich will end up being destroyed at the end of the apparent statement,for example:

   CHECK_EQ(string("abc")[1], 'b');

The compiler reports an error if one of the arguments is apointer and the other is NULL. To work around this, simply static_castNULL to the type of the desired pointer.

   CHECK_EQ(some_ptr, static_cast<SomeType*>(NULL));

Better yet, use the CHECK_NOTNULL macro:

   CHECK_NOTNULL(some_ptr);   some_ptr->DoSomething();

Since this macro returns the given pointer, this is very useful inconstructor initializer lists.

   struct S {     S(Something* ptr) : ptr_(CHECK_NOTNULL(ptr)) {}     Something* ptr_;   };

Note that you cannot use this macro as a C++ stream due to thisfeature. Please useCHECK_EQ described above to log acustom message before aborting the application.

If you are comparing C strings (char *), a handy set of macrosperforms case sensitive as well as case insensitive comparisons -CHECK_STREQ,CHECK_STRNE,CHECK_STRCASEEQ, andCHECK_STRCASENE. TheCASE versions are case-insensitive. You can safely passNULLpointers for this macro. They treatNULL and anynon-NULL string as not equal. TwoNULLs areequal.

Note that both arguments may be temporary strings which aredestructed at the end of the current "full expression"(e.g.,CHECK_STREQ(Foo().c_str(), Bar().c_str()) whereFoo andBar return C++'sstd::string).

The CHECK_DOUBLE_EQ macro checks the equality of twofloating point values, accepting a small error margin.CHECK_NEAR accepts a third floating point argument, whichspecifies the acceptable error margin.

Verbose Logging

When you are chasing difficult bugs, thorough log messages are veryuseful. However, you may want to ignore too verbose messages in usualdevelopment. For such verbose logging, glog provides theVLOG macro, which allows you to define your own numericlogging levels. The --v command line option controlswhich verbose messages are logged:

   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";

With VLOG, the lower the verbose level, the morelikely messages are to be logged. For example, if--v==1,VLOG(1) will log, butVLOG(2) will not log. This is opposite of the severitylevel, whereINFO is 0, and ERROR is 2.--minloglevel of 1 will logWARNING andabove. Though you can specify any integers for bothVLOGmacro and--v flag, the common values for them are smallpositive integers. For example, if you writeVLOG(0),you should specify--v=-1 or lower to silence it. Thisis less useful since we may not want verbose logs by default in mostcases. TheVLOG macros always log at theINFO log level (when they log at all).

Verbose logging can be controlled from the command line on aper-module basis:

   --vmodule=mapreduce=2,file=1,gfs*=3 --v=0

will:

• a. Print VLOG(2) and lower messages from mapreduce.{h,cc}
• b. Print VLOG(1) and lower messages from file.{h,cc}
• c. Print VLOG(3) and lower messages from files prefixed with "gfs"
• d. Print VLOG(0) and lower messages from elsewhere

The wildcarding functionality shown by (c) supports both '*'(matches 0 or more characters) and '?' (matches any single character)wildcards. Please also check the section aboutcommand line flags.

There's also VLOG_IS_ON(n) "verbose level" conditionmacro. This macro returns true when the--v is equal orgreater thann. To be used as

   if (VLOG_IS_ON(2)) {     // do some logging preparation and logging     // that can't be accomplished with just VLOG(2) << ...;   }

Verbose level condition macros VLOG_IF,VLOG_EVERY_N andVLOG_IF_EVERY_N behaveanalogous toLOG_IF,LOG_EVERY_N,LOF_IF_EVERY, but accept a numeric verbosity level asopposed to a severity level.

   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;

Failure Signal Handler

The library provides a convenient signal handler that will dump usefulinformation when the program crashes on certain signals such as SIGSEGV.The signal handler can be installed bygoogle::InstallFailureSignalHandler(). The following is an example of outputfrom the signal handler.

*** 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)

By default, the signal handler writes the failure dump to the standarderror. You can customize the destination by InstallFailureWriter().

Miscellaneous Notes

Performance of Messages

The conditional logging macros provided by glog (e.g.,CHECK, LOG_IF, VLOG, ...) arecarefully implemented and don't execute the right hand sideexpressions when the conditions are false. So, the following checkmay not sacrifice the performance of your application.

   CHECK(obj.ok) << obj.CreatePrettyFormattedStringButVerySlow();

User-defined Failure Function

FATAL severity level messages or unsatisfiedCHECK condition terminate your program. You can changethe behavior of the termination byInstallFailureFunction.

   void YourFailureFunction() {     // Reports something...     exit(1);   }   int main(int argc, char* argv[]) {     google::InstallFailureFunction(&YourFailureFunction);   }

By default, glog tries to dump stacktrace and makes the programexit with status 1. The stacktrace is produced only when you run theprogram on an architecture for which glog supports stack tracing (asof September 2008, glog supports stack tracing for x86 and x86_64).

Raw Logging

The header file <glog/raw_logging.h> can beused for thread-safe logging, which does not allocate any memory oracquire any locks. Therefore, the macros defined in thisheader file can be used by low-level memory allocation andsynchronization code.Please check src/glog/raw_logging.h.in for detail.

Google Style perror()

PLOG() and PLOG_IF() andPCHECK() behave exactly like theirLOG* andCHECK equivalents with the addition that they append adescription of the current state of errno to their output lines.E.g.

   PCHECK(write(1, NULL, 2) >= 0) << "Write NULL failed";

This check fails with the following error message.

   F0825 185142 test.cc:22] Check failed: write(1, NULL, 2) >= 0 Write NULL failed: Bad address [14]

Syslog

SYSLOG, SYSLOG_IF, andSYSLOG_EVERY_N macros are available.These log to syslog in addition to the normal logs. Be aware thatlogging to syslog can drastically impact performance, especially ifsyslog is configured for remote logging! Make sure you understand theimplications of outputting to syslog before you use these macros. Ingeneral, it's wise to use these macros sparingly.

Strip Logging Messages

Strings used in log messages can increase the size of your binaryand present a privacy concern. You can therefore instruct glog toremove all strings which fall below a certain severity level by usingthe GOOGLE_STRIP_LOG macro:

If your application has code like this:

   #define GOOGLE_STRIP_LOG 1    // this must go before the #include!   #include <glog/logging.h>

The compiler will remove the log messages whose severities are lessthan the specified integer value. SinceVLOG logs at the severity levelINFO(numeric value0),settingGOOGLE_STRIP_LOG to 1 or greater removesall log messages associated withVLOGs as well asINFO log statements.

Notes for Windows users

Google glog defines a severity level ERROR, which isalso defined inwindows.h . You can make glog not defineINFO,WARNING,ERROR,andFATAL by definingGLOG_NO_ABBREVIATED_SEVERITIES beforeincludingglog/logging.h . Even with this macro, you canstill use the iostream like logging facilities:

  #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";

However, you cannotuse INFO, WARNING, ERROR,andFATAL anymore for functions definedinglog/logging.h .

  #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);

If you don't need ERROR definedby windows.h, there are a couple of more workaroundswhich sometimes don't work:

• #define WIN32_LEAN_AND_MEAN or NOGDI before you #includewindows.h .
• #undef ERROR after you #include windows.h .

See this issue for more detail.

---

Shinichiro Hamaji
Gregor Hohpe
Fri Jan 25 2013