Google Logging (glog) is a C++14 library that implements application-level logging. The 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 severity 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";
}

The library can be installed using various package managers or compiled from source. For a detailed overview of glog features and their usage, please refer to the user guide.

The above example requires further Bazel or CMake setup for use in own projects.

Assuming that glog was previously built using CMake or installed using a package manager, you can use the CMake command find_package to build against glog in your CMake project as follows:

cmake_minimum_required (VERSION 3.16)
project (myproj VERSION 1.0)

find_package (glog 0.7.0 REQUIRED)

add_executable (myapp main.cpp)
target_link_libraries (myapp glog::glog)

Compile definitions and options will be added automatically to your target as needed.

Alternatively, glog can be incorporated into using the CMake command add_subdirectory to include glog directly from a subdirectory of your project by replacing the find_package call from the previous snippet by add_subdirectory. The glog::glog target is in this case an ALIAS library target for the glog library target.

To use glog within a project which uses the Bazel build tool, add the following lines to your WORKSPACE file:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "com_github_gflags_gflags",
    sha256 = "34af2f15cf7367513b352bdcd2493ab14ce43692d2dcd9dfc499492966c64dcf",
    strip_prefix = "gflags-2.2.2",
    urls = ["https://github.com/gflags/gflags/archive/v2.2.2.tar.gz"],
)

http_archive(
    name = "com_github_google_glog",
    sha256 = "122fb6b712808ef43fbf80f75c52a21c9760683dae470154f02bddfc61135022",
    strip_prefix = "glog-0.6.0",
    urls = ["https://github.com/google/glog/archive/v0.6.0.zip"],
)

You can then add @com_github_google_glog//:glog to the deps section of a cc_binary or cc_library rule, and #include <glog/logging.h> to include it in your source code. Here’s a simple example:

cc_binary(
    name = "main",
    srcs = ["main.cc"],
    deps = ["@com_github_google_glog//:glog"],
)

glog can be compiled using CMake on a wide range of platforms. The typical workflow for building glog on a Unix-like system with GNU Make as build tool is as follows:

1. Clone the repository and change into source directory.

git clone https://github.com/google/glog.git
cd glog

2. Run CMake to configure the build tree.

cmake -S . -B build -G "Unix Makefiles"

CMake provides different generators, and by default will pick the most relevant one to your environment. If you need a specific version of Visual Studio, use cmake . -G <generator-name>, and see cmake --help for the available generators. Also see -T <toolset-name>, which can be used to request the native x64 toolchain with -T host=x64.

3. Afterwards, generated files can be used to compile the project.

cmake --build build

4. Test the build software (optional).

cmake --build build --target test

5. Install the built files (optional).

cmake --build build --target install

Once successfully built, glog can be integrated into own projects.

You can download and install glog using the conan package manager:

pip install conan
conan install -r conancenter glog/<glog-version>@

The glog recipe in conan center is kept up to date by conan center index community contributors. If the version is out of date, please create an issue or pull request on the conan-center-index repository.

You can download and install glog using the vcpkg dependency manager:

git clone https://github.com/Microsoft/vcpkg.git
cd vcpkg
./bootstrap-vcpkg.sh
./vcpkg integrate install
./vcpkg install glog

The glog port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please create an issue or pull request on the vcpkg repository.

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, customize the prefix attached to log messages, and more.

Following sections describe the functionality supported by glog. Please note this description may not be complete but limited to the most useful ones. If you want to find less common features, please check header files under src/glog directory.

You can specify one of the following severity levels (in increasing order of severity):

1. INFO,
2. WARNING,
3. ERROR, and
4. FATAL.

Logging a FATAL message terminates the program (after the message is logged).

Messages of a given severity are logged not only to corresponding severity logfile but also to other logfiles of lower severity. For instance, a message of severity FATAL will be logged to logfiles of severity FATAL, ERROR, WARNING, and INFO.

The DFATAL severity logs a FATAL error in debug mode (i.e., there is no NDEBUG macro defined), but avoids halting the program in production by automatically reducing the severity to ERROR.

Unless otherwise specified, glog uses the format

<tmp>/<program name>.<hostname>.<user name>.log.<severity level>.<date>-<time>.<pid>

for log filenames written to a directory designated as <tmp> and determined according to the following rules.

Windows

glog uses the GetTempPathA API function to retrieve the directory for temporary files with a fallback to

1. C:\TMP\
2. C:\TEMP\

(in the order given.)

non-Windows

The directory is determined by referencing the environment variables

1. TMPDIR
2. TMP

if set with a fallback to /tmp/.

The default path to a log file on Linux, for instance, could be

/tmp/hello_world.example.com.hamaji.log.INFO.20080709-222411.10474

By default, glog echos ERROR and FATAL messages to standard error in addition to log files.

Log lines have this form:

Lyyyymmdd hh:mm:ss.uuuuuu threadid file:line] msg...

where the fields are defined as follows:

Example:

I1103 11:57:31.739339 24395 google.cc:2341] Command line: ./some_prog
I1103 11:57:31.739403 24395 google.cc:2342] Process id 24395

Although microseconds are useful for comparing events on a single machine, clocks on different machines may not be well synchronized. Hence, use with caution when comparing the low bits of timestamps from different machines.

The predefined log line prefix can be replaced using a user-provided callback that formats the corresponding output.

For each log entry, the callback will be invoked with a reference to a google::LogMessage instance containing the severity, filename, line number, thread ID, and time of the event. It will also be given a reference to the output stream, whose contents will be prepended to the actual message in the final log line.

For example, the following function outputs a prefix that matches glog's default format. The third parameter data can be used to access user-supplied data which unless specified defaults to nullptr.

void MyPrefixFormatter(std::ostream& s, const google::LogMessage& m, void* /*data*/) {
   s << google::GetLogSeverityName(m.severity())[0]
   << setw(4) << 1900 + m.time().year()
   << setw(2) << 1 + m.time().month()
   << setw(2) << m.time().day()
   << ' '
   << setw(2) << m.time().hour() << ':'
   << setw(2) << m.time().min()  << ':'
   << setw(2) << m.time().sec() << "."
   << setw(6) << m.time().usec()
   << ' '
   << setfill(' ') << setw(5)
   << m.thread_id() << setfill('0')
   << ' '
   << m.basename() << ':' << m.line() << "]";
}

To enable the use of a prefix formatter, use the

google::InstallPrefixFormatter(&MyPrefixFormatter);

function to pass a pointer to the corresponding MyPrefixFormatter callback during initialization. InstallPrefixFormatter takes a second optional argument of type void* that allows supplying user data to the callback.

Several flags influence glog’s output behavior. If the Google gflags library is installed on your machine, the build system will automatically detect and use it, allowing you to pass flags on the command line. For example, if you want to activate --logtostderr, you can start your application with the following command line:

./your_application --logtostderr=1

If the Google gflags library isn’t installed, you set flags via environment 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.

You can set boolean flags to true by specifying 1, true, or yes. To set boolean flags to false, specify 0, false, or no. In either case the spelling is case-insensitive.

stderrthreshold (int, default=2, which is ERROR)

Copy log messages at or above this level to stderr in addition to logfiles. The numbers of severity levels INFO, WARNING, ERROR, and FATAL are 0, 1, 2, and 3, respectively.

minloglevel (int, default=0, which is INFO)

Log messages at or above this level. Again, the numbers of severity 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 instead of the default logging directory.

v (int, default=0)

Show all VLOG(m) messages for m less or equal the value of this flag. Overridable by --vmodule. Refer to verbose logging for more detail.

vmodule (string, default="")

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., gfs* 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 verbose logging for more details.

Additional flags are defined in flags.cc. Please see the source for their complete list.

You can also modify flag values in your program by modifying global variables FLAGS_* . Most settings start working immediately after you update FLAGS_* . The exceptions are the flags related to destination files. For example, you might want to set FLAGS_log_dir before calling google::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";

Sometimes, you may only want to log a message under certain conditions. You can use the following macros to perform conditional logging:

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

The "Got lots of cookies" message is logged only when the variable num_cookies 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.

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.

The placeholder google::COUNTER identifies the recurring repetition.

You can combine conditional and occasional logging with the following 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 limit the 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. The google::COUNTER identifier indicates which repetition is happening.

Other times, it is desired to only log a message periodically based on a time. For instance, to log a message every 10ms:

LOG_EVERY_T(INFO, 0.01) << "Got a cookie";

Or every 2.35s:

LOG_EVERY_T(INFO, 2.35) << "Got a cookie";

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.

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";
DLOG_FIRST_N(INFO, 10) << "Got the " << google::COUNTER << "th cookie";
DLOG_EVERY_T(INFO, 0.01) << "Got a cookie";

It is a good practice to check expected conditions in your program frequently to detect errors as early as possible. The CHECK macro provides the ability to abort the application when a condition is not met, similar to the assert macro defined in the standard C library.

CHECK aborts the application if a condition is not true. Unlike assert, it is *not* controlled by NDEBUG, so the check will be executed regardless of compilation mode. Therefore, fp->Write(x) in the following example is always executed:

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

There are various helper macros for equality/inequality checks -CHECK_EQ, CHECK_NE, CHECK_LE, CHECK_LT, CHECK_GE, and CHECK_GT. They compare two values, and log a FATAL message including the two values when the result is not as expected. The values must have operator<<(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 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:

CHECK_EQ(string("abc")[1], ’b’);

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

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

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 in constructor initializer lists.

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

Due to the argument forwarding, CHECK_NOTNULL cannot be used to simultaneously stream an additional custom message. To provide a custom message, one can use the macro CHECK_EQ prior to the failing check.

If you are comparing C strings (char *), a handy set of macros performs both case sensitive and insensitive comparisons - CHECK_STREQ, CHECK_STRNE, CHECK_STRCASEEQ, and CHECK_STRCASENE. The CHECK_*CASE* macro variants are case-insensitive. You can safely pass nullptr pointers to this macro. They treat nullptr and any non-nullptr string as not equal. Two nullptrs are equal.

Both arguments may be temporary objects which are destructed at the end of the current "full expression", such as

CHECK_STREQ(Foo().c_str(), Bar().c_str());

where Foo and Bar return std::string.

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

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 VLOG macro, which allows you to define your own numeric logging levels. The --v command line option controls which 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 more likely messages are to be logged. For example, if --v==1, VLOG(1) will log, but VLOG(2) will not log.

The VLOG behavior is opposite of the severity level logging, where INFO, ERROR, etc. are defined in increasing order and thus --minloglevel of 1 will only log WARNING and above.

Though you can specify any integers for both VLOG macro and --v flag, the common values for them are small positive integers. For example, if you write VLOG(0), you should specify --v=-1 or lower to silence it. This is less useful since we may not want verbose logs by default in most cases. The VLOG macros always log at the INFO log level (when they log at all).

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

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

Specifying these options will specficially:

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

The wildcarding functionality 3. supports both * (matches 0 or more characters) and ? (matches any single character) wildcards. Please also refer to command line flags for more information.

There’s also VLOG_IS_ON(n) "verbose level" condition macro. This macro returns true when the --v is equal to or greater than n. The macro can be used as follows:

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 and VLOG_IF_EVERY_N behave analogous to LOG_IF, LOG_EVERY_N, LOG_IF_EVERY_N, but accept a numeric verbosity level as opposed 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 occurrence is " << google::COUNTER;
VLOG_IF_EVERY_N(1, (size > 1024), 10)
   << "I’m printed on every 10th occurrence of case when size is more "
      " than 1024, when you run the program with --v=1 or more. ";
      "Present occurrence is " << google::COUNTER;

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.

*** 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 standard error. You can customize the destination by InstallFailureWriter().

The conditional logging macros provided by glog (e.g., CHECK, LOG_IF, VLOG, etc.) 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.

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

FATAL severity level messages or unsatisfied CHECK condition terminate your program. You can change the behavior of the termination by InstallFailureFunction.

void YourFailureFunction() {
  // Reports something...
  exit(EXIT_FAILURE);
}

int main(int argc, char* argv[]) {
  google::InstallFailureFunction(&YourFailureFunction);
}

By default, glog tries to dump the stacktrace and calls std::abort. The stacktrace is generated only when running the application on a system supported by glog. Currently, glog supports x86, x86_64, PowerPC architectures, libunwind, and the Debug Help Library (dbghelp) on Windows for extracting the stack trace.

The header file <glog/raw_logging.h> 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 src/glog/raw_logging.h for detail.

PLOG() and PLOG_IF() and PCHECK() behave exactly like their LOG* and CHECK equivalents with the addition that they append a description of the current state of errno to their output lines. E.g.

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

This check fails with the following error message.

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

SYSLOG, SYSLOG_IF, and SYSLOG_EVERY_N 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.

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:

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 less than the specified integer value. Since VLOG logs at the severity level INFO (numeric value 0), setting GOOGLE_STRIP_LOG to 1 or greater removes all log messages associated with VLOGs as well as INFO log statements.

To enable the log cleaner:

using namespace std::chrono_literals;
google::EnableLogCleaner(24h * 3); // keep your logs for 3 days

In C++20 (and later) this can be shortened to:

using namespace std::chrono_literals;
google::EnableLogCleaner(3d); // keep your logs for 3 days

And then 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.

This feature can be disabled at any time (if it has been enabled)

google::DisableLogCleaner();

glog defines a severity level ERROR, which is also defined in windows.h . You can make glog not define INFO, WARNING, ERROR, and FATAL by defining GLOG_NO_ABBREVIATED_SEVERITIES before including glog/logging.h . Even with this macro, you can still 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 cannot use INFO, WARNING, ERROR, and FATAL anymore for functions defined in glog/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 defined by windows.h, there are a couple of more workarounds which sometimes don’t work:

• #define WIN32_LEAN_AND_MEAN or NOGDI before #include <windows.h>.
• #undef ERROR after #include <windows.h>.

See this issue for more detail.

The glibc built-in stack-unwinder on 64-bit systems has some problems with glog. (In particular, if you are using InstallFailureSignalHandler(), the signal may be raised in the middle of malloc, holding some malloc-related locks when they invoke the stack unwinder. The built-in stack unwinder may call malloc recursively, which may require the thread to acquire a lock it already holds: deadlock.)

For that reason, if you use a 64-bit system and you need InstallFailureSignalHandler(), we strongly recommend you install libunwind before trying to configure or install google glog. libunwind can be found here.

Even if you already have libunwind installed, you will probably still need to install from the snapshot to get the latest version.

Caution: if you install libunwind from the URL above, be aware that you may have trouble if you try to statically link your binary with glog: that is, if you link with gcc -static -lgcc_eh .... This is because both libunwind and libgcc implement the same C++ exception handling APIs, but they implement them differently on some platforms. This is not likely to be a problem on ia64, but may be on x86-64.

Also, if you link binaries statically, make sure that you add -Wl,--eh-frame-hdr to your linker options. This is required so that libunwind can find the information generated by the compiler required for stack unwinding.

Using -static is rare, though, so unless you know this will affect you it probably won’t.

If you cannot or do not wish to install libunwind, you can still try to use two kinds of stack-unwinder:

glibc built-in stack-unwinder

As we already mentioned, glibc’s unwinder has a deadlock issue. However, if you don’t use InstallFailureSignalHandler() or you don’t worry about the rare possibilities of deadlocks, you can use this stack-unwinder. If you specify no options and libunwind isn’t detected on your system, the configure script chooses this unwinder by default.

frame pointer based stack-unwinder

The frame pointer based stack unwinder requires that your application, the glog library, and system libraries like libc, all be compiled with a frame pointer. This is not the default for x86-64.

We’d love to accept your patches and contributions to this project. There are a just a few small guidelines you need to follow.

Contributions to any Google project must be accompanied by a Contributor License Agreement. This is not a copyright assignment, it simply gives Google permission to use and redistribute your contributions as part of the project.

• If you are an individual writing original source code and you’re sure you own the intellectual property, then you’ll need to sign an individual CLA.
• If you work for a company that wants to allow you to contribute your work, then you’ll need to sign a corporate CLA.

You generally only need to submit a CLA once, so if you’ve already submitted one (even if it was for a different project), you probably don’t need to do it again.

Once your CLA is submitted (or if you already submitted one for another Google project), make a commit adding yourself to the AUTHORS and CONTRIBUTORS files. This commit can be part of your first pull request.

1. It’s generally best to start by opening a new issue describing the bug or feature you’re intending to fix. Even if you think it’s relatively minor, it’s helpful to know what people are working on. Mention in the initial issue that you are planning to work on that bug or feature so that it can be assigned to you.
2. Follow the normal process of forking the project, and setup a new branch to work in. It’s important that each group of changes be done in separate branches in order to ensure that a pull request only includes the commits related to that bug or feature.
3. Do your best to have well-formed commit messages for each change. This provides consistency throughout the project, and ensures that commit messages are able to be formatted properly by various git tools.
4. Finally, push the commits to your fork and submit a pull request.