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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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</style>
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</head>
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<body>
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How To Use Google Logging Library (glog)
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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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Google glog 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 severity level>), e.g.
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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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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 src/glog directory.
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You can specify one of the following severity levels (in
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increasing order of severity): INFO, WARNING,
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ERROR, and FATAL.
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Logging a FATAL 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 FATAL will be logged to the
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logfiles of severity FATAL, ERROR,
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WARNING, and INFO.
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The DFATAL severity logs a FATAL error in
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debug mode (i.e., there is no NDEBUG macro defined), but
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avoids halting the program in production by automatically reducing the
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severity to ERROR.
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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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ERROR or FATAL to standard error (stderr)
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in addition to log files.
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Several flags influence glog's output behavior.
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If the Google
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gflags library is installed on your machine, the
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configure 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 --logtostderr on, you can start
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your application with the following command line:
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./your_application --logtostderr=1
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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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GLOG_logtostderr=1 ./your_application
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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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The following flags are most commonly used:
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logtostderr (bool, default=false)
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Log messages to stderr instead of logfiles.
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Note: you can set binary flags to true by specifying
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1, true, or yes (case
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insensitive).
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Also, you can set binary flags to false by specifying
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0, false, or no (again, case
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insensitive).
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stderrthreshold (int, default=2, which
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is ERROR)
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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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INFO, WARNING, ERROR, and
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FATAL are 0, 1, 2, and 3, respectively.
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minloglevel (int, default=0, which
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is INFO)
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Log messages at or above this level. Again, the numbers of
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severity levels INFO, WARNING,
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ERROR, and FATAL are 0, 1, 2, and 3,
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respectively.
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log_dir (string, default="")
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If specified, logfiles are written into this directory instead
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of the default logging directory.
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v (int, default=0)
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Show all VLOG(m) messages for m less or
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equal the value of this flag. Overridable by --vmodule.
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See the section about verbose logging for more
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detail.
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vmodule (string, default="")
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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., gfs* 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 the section about verbose logging.
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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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You can also modify flag values in your program by modifying global
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variables FLAGS_* . Most settings start working
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immediately after you update FLAGS_* . The exceptions are
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the flags related to destination files. For example, you might want to
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set FLAGS_log_dir before
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calling google::InitGoogleLogging . Here is an example:
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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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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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LOG_IF(INFO, num_cookies > 10) << "Got lots of cookies";
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The "Got lots of cookies" message is logged only when the variable
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num_cookies 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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LOG_EVERY_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie";
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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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google::COUNTER value is used to identify which repetition is
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happening.
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You can combine conditional and occasional logging with the
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following macro.
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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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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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LOG_FIRST_N(INFO, 20) << "Got the " << google::COUNTER << "th cookie";
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Outputs log messages for the first 20 times it is executed. Again,
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the google::COUNTER identifier indicates which repetition is
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happening.
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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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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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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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CHECK macro provides the ability to abort the application
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when a condition is not met, similar to the assert macro
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defined in the standard C library.
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CHECK aborts the application if a condition is not
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true. Unlike assert, it is *not* controlled by
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NDEBUG, so the check will be executed regardless of
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compilation mode. Therefore, fp->Write(x) in the
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following example is always executed:
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CHECK(fp->Write(x) == 4) << "Write failed!";
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There are various helper macros for
|
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equality/inequality checks - CHECK_EQ,
|
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CHECK_NE, CHECK_LE, CHECK_LT,
|
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CHECK_GE, and CHECK_GT.
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They compare two values, and log a
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FATAL message including the two values when the result is
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not as expected. The values must have operator<<(ostream,
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...) defined.
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You may append to the error message like so:
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CHECK_NE(1, 2) << ": The world must be ending!";
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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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CHECK_EQ(string("abc")[1], 'b');
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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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CHECK_EQ(some_ptr, static_cast<SomeType*>(NULL));
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Better yet, use the CHECK_NOTNULL macro:
|
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|
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CHECK_NOTNULL(some_ptr);
|
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some_ptr->DoSomething();
|
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|
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Since this macro returns the given pointer, this is very useful in
|
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constructor initializer lists.
|
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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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|
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Note that you cannot use this macro as a C++ stream due to this
|
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feature. Please use CHECK_EQ described above to log a
|
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custom message before aborting the application.
|
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9def5d |
|
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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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CHECK_STREQ, CHECK_STRNE,
|
|
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CHECK_STRCASEEQ, and CHECK_STRCASENE. The
|
|
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CASE versions are case-insensitive. You can safely pass NULL
|
|
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pointers for this macro. They treat NULL and any
|
|
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non-NULL string as not equal. Two NULLs are
|
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equal.
|
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|
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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., CHECK_STREQ(Foo().c_str(), Bar().c_str()) where
|
|
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Foo and Bar return C++'s
|
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std::string).
|
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|
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The CHECK_DOUBLE_EQ macro checks the equality of two
|
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floating point values, accepting a small error margin.
|
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CHECK_NEAR accepts a third floating point argument, which
|
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specifies the acceptable error margin.
|
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|
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|
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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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9def5d |
development. For such verbose logging, glog provides the
|
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VLOG macro, which allows you to define your own numeric
|
|
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logging levels. The --v command line option controls
|
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which verbose messages are logged:
|
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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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With VLOG, the lower the verbose level, the more
|
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likely messages are to be logged. For example, if
|
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--v==1, VLOG(1) will log, but
|
|
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VLOG(2) will not log. This is opposite of the severity
|
|
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level, where INFO is 0, and ERROR is 2.
|
|
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--minloglevel of 1 will log WARNING and
|
|
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above. Though you can specify any integers for both VLOG
|
|
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macro and --v flag, the common values for them are small
|
|
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positive integers. For example, if you write VLOG(0),
|
|
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you should specify --v=-1 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 VLOG macros always log at the
|
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INFO log level (when they log at all).
|
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Verbose logging can be controlled from the command line on a
|
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per-module basis:
|
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--vmodule=mapreduce=2,file=1,gfs*=3 --v=0
|
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|
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|
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will:
|
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|
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a. Print VLOG(2) and lower messages from mapreduce.{h,cc}
|
|
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b. Print VLOG(1) and lower messages from file.{h,cc}
|
|
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c. Print VLOG(3) and lower messages from files prefixed with "gfs"
|
|
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d. Print VLOG(0) and lower messages from elsewhere
|
|
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|
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|
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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
|
|
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href="#flags">command line flags.
|
|
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|
|
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There's also VLOG_IS_ON(n) "verbose level" condition
|
|
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macro. This macro returns true when the --v is equal or
|
|
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greater than n. To be used as
|
|
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|
|
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|
|
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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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|
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|
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Verbose level condition macros VLOG_IF,
|
|
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VLOG_EVERY_N and VLOG_IF_EVERY_N behave
|
|
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analogous to LOG_IF, LOG_EVERY_N,
|
|
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LOF_IF_EVERY, but accept a numeric verbosity level as
|
|
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opposed to a severity level.
|
|
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|
|
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|
|
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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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9def5d |
"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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9def5d |
"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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9def5d |
" 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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|
|
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|
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|
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|
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|
|
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The library provides a convenient signal handler that will dump useful
|
|
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9def5d |
information when the program crashes on certain signals such as SIGSEGV.
|
|
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9def5d |
The signal handler can be installed by
|
|
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9def5d |
google::InstallFailureSignalHandler(). The following is an example of output
|
|
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9def5d |
from the signal handler.
|
|
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|
|
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|
|
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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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9def5d |
PC: @ 0x412eb1 TestWaitingLogSink::send()
|
|
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@ 0x7f892fb417d0 (unknown)
|
|
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9def5d |
@ 0x412eb1 TestWaitingLogSink::send()
|
|
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9def5d |
@ 0x7f89304f7f06 google::LogMessage::SendToLog()
|
|
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9def5d |
@ 0x7f89304f35af google::LogMessage::Flush()
|
|
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9def5d |
@ 0x7f89304f3739 google::LogMessage::~LogMessage()
|
|
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9def5d |
@ 0x408cf4 TestLogSinkWaitTillSent()
|
|
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9def5d |
@ 0x4115de main
|
|
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9def5d |
@ 0x7f892f7ef1c4 (unknown)
|
|
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@ 0x4046f9 (unknown)
|
|
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|
|
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|
|
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|
|
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By default, the signal handler writes the failure dump to the standard
|
|
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9def5d |
error. You can customize the destination by InstallFailureWriter().
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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The conditional logging macros provided by glog (e.g.,
|
|
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9def5d |
CHECK, LOG_IF, VLOG, ...) are
|
|
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9def5d |
carefully implemented and don't execute the right hand side
|
|
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9def5d |
expressions when the conditions are false. So, the following check
|
|
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9def5d |
may not sacrifice the performance of your application.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
CHECK(obj.ok) << obj.CreatePrettyFormattedStringButVerySlow();
|
|
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9def5d |
|
|
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|
|
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9def5d |
|
|
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9def5d |
|
|
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FATAL severity level messages or unsatisfied
|
|
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9def5d |
CHECK condition terminate your program. You can change
|
|
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9def5d |
the behavior of the termination by
|
|
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9def5d |
InstallFailureFunction.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
void YourFailureFunction() {
|
|
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9def5d |
// Reports something...
|
|
Packit Service |
9def5d |
exit(1);
|
|
Packit Service |
9def5d |
}
|
|
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9def5d |
|
|
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9def5d |
int main(int argc, char* argv[]) {
|
|
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9def5d |
google::InstallFailureFunction(&YourFailureFunction);
|
|
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9def5d |
}
|
|
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9def5d |
|
|
Packit Service |
9def5d |
|
|
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9def5d |
By default, glog tries to dump stacktrace and makes the program
|
|
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9def5d |
exit with status 1. The stacktrace is produced only when you run the
|
|
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9def5d |
program on an architecture for which glog supports stack tracing (as
|
|
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9def5d |
of September 2008, glog supports stack tracing for x86 and x86_64).
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
The header file <glog/raw_logging.h> can be
|
|
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9def5d |
used for thread-safe logging, which does not allocate any memory or
|
|
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9def5d |
acquire any locks. Therefore, the macros defined in this
|
|
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9def5d |
header file can be used by low-level memory allocation and
|
|
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9def5d |
synchronization code.
|
|
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9def5d |
Please check src/glog/raw_logging.h.in for detail.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
PLOG() and PLOG_IF() and
|
|
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9def5d |
PCHECK() behave exactly like their LOG* and
|
|
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9def5d |
CHECK equivalents with the addition that they append a
|
|
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9def5d |
description of the current state of errno to their output lines.
|
|
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9def5d |
E.g.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
PCHECK(write(1, NULL, 2) >= 0) << "Write NULL failed";
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
This check fails with the following error message.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
F0825 185142 test.cc:22] Check failed: write(1, NULL, 2) >= 0 Write NULL failed: Bad address [14]
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
SYSLOG, SYSLOG_IF, and
|
|
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9def5d |
SYSLOG_EVERY_N macros are available.
|
|
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9def5d |
These log to syslog in addition to the normal logs. Be aware that
|
|
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9def5d |
logging to syslog can drastically impact performance, especially if
|
|
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9def5d |
syslog is configured for remote logging! Make sure you understand the
|
|
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9def5d |
implications of outputting to syslog before you use these macros. In
|
|
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9def5d |
general, it's wise to use these macros sparingly.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
Strings used in log messages can increase the size of your binary
|
|
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9def5d |
and present a privacy concern. You can therefore instruct glog to
|
|
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9def5d |
remove all strings which fall below a certain severity level by using
|
|
Packit Service |
9def5d |
the GOOGLE_STRIP_LOG macro:
|
|
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9def5d |
|
|
Packit Service |
9def5d |
If your application has code like this:
|
|
Packit Service |
9def5d |
|
|
Packit Service |
9def5d |
|
|
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9def5d |
#define GOOGLE_STRIP_LOG 1 // this must go before the #include!
|
|
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9def5d |
#include <glog/logging.h>
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
The compiler will remove the log messages whose severities are less
|
|
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9def5d |
than the specified integer value. Since
|
|
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9def5d |
VLOG logs at the severity level INFO
|
|
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9def5d |
(numeric value 0),
|
|
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9def5d |
setting GOOGLE_STRIP_LOG to 1 or greater removes
|
|
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9def5d |
all log messages associated with VLOGs as well as
|
|
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9def5d |
INFO log statements.
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
Google glog defines a severity level ERROR, which is
|
|
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9def5d |
also defined in windows.h . You can make glog not define
|
|
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9def5d |
INFO, WARNING, ERROR,
|
|
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9def5d |
and FATAL by defining
|
|
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9def5d |
GLOG_NO_ABBREVIATED_SEVERITIES before
|
|
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9def5d |
including glog/logging.h . Even with this macro, you can
|
|
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9def5d |
still use the iostream like logging facilities:
|
|
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9def5d |
|
|
Packit Service |
9def5d |
|
|
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9def5d |
#define GLOG_NO_ABBREVIATED_SEVERITIES
|
|
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9def5d |
#include <windows.h>
|
|
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9def5d |
#include <glog/logging.h>
|
|
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9def5d |
|
|
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9def5d |
// ...
|
|
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9def5d |
|
|
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9def5d |
LOG(ERROR) << "This should work";
|
|
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9def5d |
LOG_IF(ERROR, x > y) << "This should be also OK";
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
However, you cannot
|
|
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9def5d |
use INFO, WARNING, ERROR,
|
|
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9def5d |
and FATAL anymore for functions defined
|
|
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9def5d |
in glog/logging.h .
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
#define GLOG_NO_ABBREVIATED_SEVERITIES
|
|
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9def5d |
#include <windows.h>
|
|
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9def5d |
#include <glog/logging.h>
|
|
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9def5d |
|
|
Packit Service |
9def5d |
// ...
|
|
Packit Service |
9def5d |
|
|
Packit Service |
9def5d |
// This won't work.
|
|
Packit Service |
9def5d |
// google::FlushLogFiles(google::ERROR);
|
|
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9def5d |
|
|
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9def5d |
// Use this instead.
|
|
Packit Service |
9def5d |
google::FlushLogFiles(google::GLOG_ERROR);
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
|
|
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9def5d |
If you don't need ERROR defined
|
|
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by windows.h, there are a couple of more workarounds
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which sometimes don't work:
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#define WIN32_LEAN_AND_MEAN or NOGDI
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before you #include windows.h .
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#undef ERROR after you #include
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windows.h .
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See
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this issue for more detail.
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<address>
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Shinichiro Hamaji
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Gregor Hohpe
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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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</address>
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</body>
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</html>
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