Occasional Vlog
President David Snell wanted to show our covenant partners around our headquarters in Americus, Ga., in his new vlog (video blog) 'A Word from the Wieland House,'. In practice, there is a fine line between a vlog and any other video channel. Many YouTube channels could be considered to be vlogs, despite not featuring “day in the life” videos. If a channel contains the same kinds of content that a blog would – but in a video format – you could consider it to be a vlog. All six members still upload to their own respective channels and just use the Misfits channel to post the occasional vlog of the group in real life. The group also has a merchandise store known as Scuffed Worldwide. Welcome to my stream of consciousness. Have you ever wanted to hear my opinions? Well oh boy, hold onto your seats. Among this collection you will find things I find humorous, deep dives into my life experiences, introspections about things I notice and maybe the occasional vlog. I'm glad you're here. Adding such a clip to the beginning of your vlog makes it look a lot more professional and sleek. 8) Keep your video steady, not shaky. Shaky video is something that stops people from watching your video longer. The occasional shake when walking down a path is fine, but a video that that constantly jerky is bound to put people off.
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(<aparticular severity level>), e.g.
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, and FATAL.Logging a FATAL 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 severity FATAL will be logged to thelogfiles of severity FATAL, ERROR,WARNING, and INFO.
The DFATAL severity logs a FATAL error indebug mode (i.e., there is no NDEBUG macro defined), butavoids halting the program in production by automatically reducing theseverity to ERROR.
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:If the Google gflags library isn't installed, you set flags viaenvironment variables, prefixing the flag name with 'GLOG_', e.g.
The following flags are most commonly used:
logtostderr(bool, default=false)- Log messages to stderr instead of logfiles.
Note: you can set binary flags totrueby specifying1,true, oryes(caseinsensitive).Also, you can set binary flags tofalseby specifying0,false, orno(again, caseinsensitive). stderrthreshold(int, default=2, whichisERROR)- Copy log messages at or above this level to stderr inaddition to logfiles. The numbers of severity levels
INFO,WARNING,ERROR, andFATALare 0, 1, 2, and 3, respectively. minloglevel(int, default=0, whichisINFO)- Log messages at or above this level. Again, the numbers ofseverity levels
INFO,WARNING,ERROR, andFATALare 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 formless orequal the value of this flag. Overridable by --vmodule.See the 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 also the 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 toset FLAGS_log_dir beforecalling google::InitGoogleLogging . Here is an example:
Conditional / Occasional Logging
Sometimes, you may only want to log a message under certainconditions. You can use the following macros to perform conditionallogging: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.
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.
Instead of outputting a message every nth time, you can also limitthe output to the first n occurrences:
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.
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 the assert 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:
There are various helper macros forequality/inequality checks - CHECK_EQ,CHECK_NE, CHECK_LE, CHECK_LT,CHECK_GE, and CHECK_GT.They compare two values, and log aFATAL message including the two values when the result isnot as expected. The values must have operator<<(ostream,...) defined.
You may append to the error message like so:
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:
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.
Better yet, use the CHECK_NOTNULL macro:
Since this macro returns the given pointer, this is very useful inconstructor initializer lists.
Note that you cannot use this macro as a C++ stream due to thisfeature. Please use CHECK_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, and CHECK_STRCASENE. TheCASE versions are case-insensitive. You can safely pass NULLpointers for this macro. They treat NULL and anynon-NULL string as not equal. Two NULLs 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 and Bar 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:
With VLOG, the lower the verbose level, the morelikely messages are to be logged. For example, if--v1, VLOG(1) will log, butVLOG(2) will not log. This is opposite of the severitylevel, where INFO is 0, and ERROR is 2.--minloglevel of 1 will log WARNING andabove. Though you can specify any integers for both VLOGmacro and --v flag, the common values for them are smallpositive integers. For example, if you write VLOG(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. The VLOG 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:
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 about command line flags.
There's also VLOG_IS_ON(n) 'verbose level' conditionmacro. This macro returns true when the --v is equal orgreater than n. To be used as
Verbose level condition macros VLOG_IF,VLOG_EVERY_N and VLOG_IF_EVERY_N behaveanalogous to LOG_IF, LOG_EVERY_N,LOF_IF_EVERY, but accept a numeric verbosity level asopposed to a severity level.
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.
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.
User-defined Failure Function
FATAL severity level messages or unsatisfiedCHECK condition terminate your program. You can changethe behavior of the termination byInstallFailureFunction.
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 their LOG* andCHECK equivalents with the addition that they append adescription of the current state of errno to their output lines.E.g.
This check fails with the following error message.
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.
Occasional Vlog Ideas
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:
The compiler will remove the log messages whose severities are lessthan the specified integer value. SinceVLOG logs at the severity level INFO(numeric value 0),setting GOOGLE_STRIP_LOG to 1 or greater removesall log messages associated with VLOGs as well asINFO log statements.
Notes for Windows users
Google glog defines a severity level ERROR, which isalso defined in windows.h . You can make glog not defineINFO, WARNING, ERROR,and FATAL by definingGLOG_NO_ABBREVIATED_SEVERITIES beforeincluding glog/logging.h . Even with this macro, you canstill use the iostream like logging facilities:
However, you cannotuse INFO, WARNING, ERROR,and FATAL anymore for functions definedin glog/logging.h .
If you don't need ERROR definedby windows.h, there are a couple of more workaroundswhich sometimes don't work:
Occasional Vlog 4
- #define
WIN32_LEAN_AND_MEANorNOGDIbefore you #includewindows.h. - #undef
ERRORafter you #includewindows.h.
See this issue for more detail.
Shinichiro Hamaji