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Configure Logging

Several flags affect the output behavior of log.

Configure Log Output Using Flags​

The following flags are the most commonly used:

logtostderr (bool, default=false)

: Log messages to stderr instead of logfiles.

stderr_threshold​

  • Type: int, default = 2 (i.e., ERROR). Copy log messages at or above this level to stderr in addition to log files. The numeric values for log levels INFO, WARNING, ERROR, and FATAL are 0, 1, 2, and 3 respectively.
  • Supports dynamic update.

min_log_level​

  • Type: int, default = 0 (i.e., INFO).
  • Log messages at or above this level. Again, the numeric values for log levels INFO, WARNING, ERROR, and FATAL are 0, 1, 2, and 3 respectively.
  • Supports dynamic update.

log_with_prefix​

  • Type: bool, default = true.
  • Add a log prefix to the beginning of each log line.
  • Supports dynamic update.

verbosity​

  • Type: int, default = 0 (i.e., no verbose logs output).
  • Display all VKLOG(m) messages where m <= this value. May be overridden by vlog_module.
  • Supports dynamic update.

vlog_module​

  • Type: string, default = "" (i.e., no verbose logs output).
  • Per-module log verbosity level. The parameter is a comma-separated list of <module name>=<log level>. <module name> is a glob pattern that matches against the filename base (i.e., the name with .cc/.h/.-inl.h stripped). Patterns without slashes match only the filename part; otherwise, they match the entire file path relative to the workspace root (still with .cc/.h/.-inl.h stripped). ? and * in the glob pattern match any single character or sequence of characters (including slashes) respectively. <log level> overrides any value given by --verbosity.

backtrace_log_at​

  • Type: string, default = "" (i.e., no stack trace logs output).
  • Print a stack trace when logging is triggered at file:linenum.
  • Supports dynamic update.

log_type​

Parameter used during log initialization.

  • Type: int, default = 0.
  • Log type corresponding to LogSinkType:
    • 0: Console log
    • 1: Daily log file
    • 2: Hourly log file
    • 3: Rotating log file
  • Does not support dynamic update.

Modify Flags Programmatically​

You can also modify flag values within the program by updating global variables FLAGS_*. Most settings take effect immediately after updating FLAGS_*, except for flags related to target files.

!!! Example: "Set Log Level at Runtime"

KLOG(INFO) << "file";
// Most flags work immediately after updating values.
turbo::set_flag(&FLAGS_min_log_level,static_cast<int>(turbo::LogSeverity::kWarning));
KLOG(INFO) << "should not display";
turbo::set_flag(&FLAGS_min_log_level,static_cast<int>(turbo::LogSeverity::kInfo));
KLOG(INFO) << "should display";