Google Logging (glog) is a C 98 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";
}

For a detailed overview of glog features and their usage, please refer to the user guide.

glog supports multiple build systems for compiling the project from source: Bazel, CMake, and vcpkg.

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 = "21bc744fb7f2fa701ee8db339ded7dce4f975d0d55837a97be7d46e8382dea5a",
    strip_prefix = "glog-0.5.0",
    urls = ["https://github.com/google/glog/archive/v0.5.0.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 also supports CMake that can be used to build the project on a wide range of platforms. If you don’t have CMake installed already, you can download it for from CMake’s official website.

CMake works by generating native makefiles or build projects that can be used in the compiler environment of your choice. You can either build glog with CMake as a standalone project or it can be incorporated into an existing CMake build for another project.

When building glog as a standalone project, on Unix-like systems with GNU Make as build tool, the typical workflow is:

1. Get the source code and change to it. e.g., cloning with git:

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

If you have glog installed in your system, you can use the CMake command find_package to build against glog in your CMake Project as follows:

cmake_minimum_required (VERSION 3.0.2)
project (myproj VERSION 1.0)

find_package (glog 0.6.0 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.

You can also use the CMake command add_subdirectory to include glog directly from a subdirectory of your project by replacing the find_package call from the previous example by add_subdirectory. The glog::glog target is in this case an ALIAS library target for the glog library target.

Again, compile definitions and options will be added automatically to your target as needed.

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): INFO, WARNING, ERROR, and FATAL. Logging a FATAL message terminates the program (after the message is logged). Note that messages of a given severity are logged not only in the logfile for that severity, but also in all logfiles of lower severity. E.g., a message of severity FATAL will be logged to the 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 writes to the filename /tmp/\<program name\>.\<hostname\>.\<user name\>.log.\<severity level\>.\<date\>.\<time\>.\<pid\> (e.g., /tmp/hello_world.example.com.hamaji.log.INFO.20080709-222411.10474). By default, glog copies the log messages of severity level ERROR or FATAL to standard error (stderr) in addition to log files.

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 turn the flag --logtostderr on, 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. Note: you can set binary flags to true by specifying 1, true, or yes (case insensitive). Also, you can set binary flags to false by specifying 0, false, or no (again, 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. See the section about 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 the section about verbose logging.

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

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. Note that the special google::COUNTER value is used to identify which repetition is happening.

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. Again, the google::COUNTER identifier indicates which repetition is happening.

Other times, it is desired to only log a message periodically based on a time. So for example, 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";

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 NULL. To work around this, simply static_cast NULL 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 in constructor 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 this feature. Please use CHECK_EQ described above to log a custom message before aborting the application.

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

Note that both arguments may be temporary strings which are destructed at the end of the current "full expression" (e.g., CHECK_STREQ(Foo().c_str(), Bar().c_str()) where Foo and Bar return C ’s 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. This is opposite of the severity level, where INFO is 0, and ERROR is 2. --minloglevel of 1 will 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

will:

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 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" condition macro. This macro returns true when the --v is equal or greater than n. 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 and VLOG_IF_EVERY_N behave analogous to LOG_IF, LOG_EVERY_N, LOF_IF_EVERY, 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 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;

glog supports changing the format of the prefix attached to log messages by receiving a user-provided callback to be used to generate such strings. That feature must be enabled at compile time by the WITH_CUSTOM_PREFIX flag.

For each log entry, the callback will be invoked with a LogMessageInfo struct 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:

/* This function writes a prefix that matches glog's default format.
 * (The third parameter can be used to receive user-supplied data, and is
 * NULL by default.)
 */
void CustomPrefix(std::ostream &s, const LogMessageInfo &l, void*) {
   s << l.severity[0]
   << setw(4) << 1900   l.time.year()
   << setw(2) << 1   l.time.month()
   << setw(2) << l.time.day()
   << ' '
   << setw(2) << l.time.hour() << ':'
   << setw(2) << l.time.min()  << ':'
   << setw(2) << l.time.sec() << "."
   << setw(6) << l.time.usec()
   << ' '
   << setfill(' ') << setw(5)
   << l.thread_id << setfill('0')
   << ' '
   << l.filename << ':' << l.line_number << "]";
}

To enable the use of CustomPrefix(), simply give glog a pointer to it during initialization: InitGoogleLogging(argv[0], &CustomPrefix);.

Optionally, InitGoogleLogging() takes a third argument of type void* to pass on to the callback function.

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(1);
}

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

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

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.in 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, 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_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:

google::EnableLogCleaner(3); // 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 you #include windows.h.
• #undef ERROR after you #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: 1. glibc built-in stack-unwinder and 2. frame pointer based stack-unwinder.

1. 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.
2. 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.