Adjusting Output¶

Several flags influence glog's output behavior.

Using Command-line Parameters and Environment Variables¶

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.

Activate --logtostderr in an application from the command line

A binary you_application that uses glog can be started using

./your_application --logtostderr=1

to log to stderr instead of writing the output to a log file.

Tip

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.

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

Activate logtostderr without gflags

GLOG_logtostderr=1 ./your_application

The following flags are most commonly used:

logtostderr (bool, default=false): Log messages to stderr instead of logfiles.
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.

Modifying Flags Programmatically¶

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 instance, you might want to set FLAGS_log_dir before calling google::InitGoogleLogging.

Setting log_dir at runtime

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";