2 * Copyright 2004-present Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
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13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 #include <folly/Likely.h>
19 #include <folly/Range.h>
20 #include <folly/experimental/logging/LogStream.h>
21 #include <folly/experimental/logging/Logger.h>
22 #include <folly/experimental/logging/LoggerDB.h>
26 * This file contains the XLOG() and XLOGF() macros.
28 * These macros make it easy to use the logging library without having to
29 * manually pick log category names. All XLOG() and XLOGF() statements in a
30 * given file automatically use a LogCategory based on the current file name.
32 * For instance, in src/foo/bar.cpp, the default log category name will be
35 * If desired, the log category name used by XLOG() in a .cpp file may be
36 * overridden using XLOG_SET_CATEGORY_NAME() macro.
40 * Log a message to this file's default log category.
42 * By default the log category name is automatically picked based on the
43 * current filename. In src/foo/bar.cpp the log category name "src.foo.bar"
44 * will be used. In "lib/stuff/foo.h" the log category name will be
47 * Note that the filename is based on the __FILE__ macro defined by the
48 * compiler. This is typically dependent on the filename argument that you
49 * give to the compiler. For example, if you compile src/foo/bar.cpp by
50 * invoking the compiler inside src/foo and only give it "bar.cpp" as an
51 * argument, the category name will simply be "bar". In general XLOG() works
52 * best if you always invoke the compiler from the root directory of your
55 #define XLOG(level, ...) \
57 ::folly::LogLevel::level, \
58 ::folly::LogStreamProcessor::APPEND, \
62 * Log a message to this file's default log category, using a format string.
64 #define XLOGF(level, fmt, arg1, ...) \
66 ::folly::LogLevel::level, \
67 ::folly::LogStreamProcessor::FORMAT, \
73 * Helper macro used to implement XLOG() and XLOGF()
75 * Beware that the level argument is evalutated twice.
77 * This macro is somewhat tricky:
79 * - In order to support streaming argument support (with the << operator),
80 * the macro must expand to a single ternary ? expression. This is the only
81 * way we can avoid evaluating the log arguments if the log check fails,
82 * and still have the macro behave as expected when used as the body of an if
85 * - We need to store some static-scope local state in order to track the
86 * LogCategory to use. This is a bit tricky to do and still meet the
87 * requirements of being a single expression, but fortunately static
88 * variables inside a lambda work for this purpose.
90 * Inside header files, each XLOG() statement defines to static variables:
91 * - the LogLevel for this category
92 * - a pointer to the LogCategory
94 * If the __INCLUDE_LEVEL__ macro is available (both gcc and clang support
95 * this), then we we can detect when we are inside a .cpp file versus a
96 * header file. If we are inside a .cpp file, we can avoid declaring these
97 * variables once per XLOG() statement, and instead we only declare one copy
98 * of these variables for the entire file.
100 * - We want to make sure this macro is safe to use even from inside static
101 * initialization code that runs before main. We also want to make the log
102 * admittance check as cheap as possible, so that disabled debug logs have
103 * minimal overhead, and can be left in place even in performance senstive
106 * In order to do this, we rely on zero-initialization of variables with
107 * static storage duration. The LogLevel variable will always be
108 * 0-initialized before any code runs. Therefore the very first time an
109 * XLOG() statement is hit the initial log level check will always pass
110 * (since all level values are greater or equal to 0), and we then do a
111 * second check to see if the log level and category variables need to be
112 * initialized. On all subsequent calls, disabled log statements can be
113 * skipped with just a single check of the LogLevel.
115 #define XLOG_IMPL(level, type, ...) \
116 (!XLOG_IS_ON_IMPL(level)) \
117 ? ::folly::logDisabledHelper( \
118 std::integral_constant<bool, ::folly::isLogLevelFatal(level)>{}) \
119 : ::folly::LogStreamVoidify<::folly::isLogLevelFatal(level)>{} & \
120 ::folly::LogStreamProcessor( \
122 static ::folly::XlogCategoryInfo<XLOG_IS_IN_HEADER_FILE> \
124 return _xlogCategory_.getInfo( \
125 &xlog_detail::xlogFileScopeInfo); \
128 xlog_detail::getXlogCategoryName(__FILE__, 0), \
129 xlog_detail::isXlogCategoryOverridden(0), \
137 * Check if and XLOG() statement with the given log level would be enabled.
139 * The level parameter must be an unqualified LogLevel enum value.
141 #define XLOG_IS_ON(level) XLOG_IS_ON_IMPL(::folly::LogLevel::level)
144 * Helper macro to implement of XLOG_IS_ON()
146 * This macro is used in the XLOG() implementation, and therefore must be as
147 * cheap as possible. It stores the category's LogLevel as a local static
148 * variable. The very first time this macro is evaluated it will look up the
149 * correct LogCategory and initialize the LogLevel. Subsequent calls then
150 * are only a single conditional log level check.
152 * The LogCategory object keeps track of this local LogLevel variable and
153 * automatically keeps it up-to-date when the category's effective level is
156 * See XlogLevelInfo for the implementation details.
158 #define XLOG_IS_ON_IMPL(level) \
160 static ::folly::XlogLevelInfo<XLOG_IS_IN_HEADER_FILE> _xlogLevel_; \
161 return _xlogLevel_.check( \
163 xlog_detail::getXlogCategoryName(__FILE__, 0), \
164 xlog_detail::isXlogCategoryOverridden(0), \
165 &xlog_detail::xlogFileScopeInfo); \
169 * Get the name of the log category that will be used by XLOG() statements
172 #define XLOG_GET_CATEGORY_NAME() \
173 (xlog_detail::isXlogCategoryOverridden(0) \
174 ? xlog_detail::getXlogCategoryName(__FILE__, 0) \
175 : ::folly::getXlogCategoryNameForFile(__FILE__))
178 * Get a pointer to the LogCategory that will be used by XLOG() statements in
181 * This is just a small wrapper around a LoggerDB::getCategory() call.
182 * This must be implemented as a macro since it uses __FILE__, and that must
183 * expand to the correct filename based on where the macro is used.
185 #define XLOG_GET_CATEGORY() \
186 folly::LoggerDB::get()->getCategory(XLOG_GET_CATEGORY_NAME())
189 * XLOG_SET_CATEGORY_NAME() can be used to explicitly define the log category
190 * name used by all XLOG() and XLOGF() calls in this translation unit.
192 * This overrides the default behavior of picking a category name based on the
195 * This should be used at the top-level scope in a .cpp file, before any XLOG()
196 * or XLOGF() macros have been used in the file.
198 * XLOG_SET_CATEGORY_NAME() cannot be used inside header files.
200 #ifdef __INCLUDE_LEVEL__
201 #define XLOG_SET_CATEGORY_CHECK \
203 __INCLUDE_LEVEL__ == 0, \
204 "XLOG_SET_CATEGORY_NAME() should not be used in header files");
206 #define XLOG_SET_CATEGORY_CHECK
209 #define XLOG_SET_CATEGORY_NAME(category) \
211 namespace xlog_detail { \
212 XLOG_SET_CATEGORY_CHECK \
213 constexpr inline folly::StringPiece getXlogCategoryName( \
214 folly::StringPiece, \
218 constexpr inline bool isXlogCategoryOverridden(int) { \
225 * XLOG_IS_IN_HEADER_FILE evaluates to false if we can definitively tell if we
226 * are not in a header file. Otherwise, it evaluates to true.
228 #ifdef __INCLUDE_LEVEL__
229 #define XLOG_IS_IN_HEADER_FILE bool(__INCLUDE_LEVEL__ > 0)
231 // Without __INCLUDE_LEVEL__ we canot tell if we are in a header file or not,
232 // and must pessimstically assume we are always in a header file.
233 #define XLOG_IS_IN_HEADER_FILE true
238 class XlogFileScopeInfo {
240 #ifdef __INCLUDE_LEVEL__
241 std::atomic<::folly::LogLevel> level;
242 ::folly::LogCategory* category;
247 * A file-static XlogLevelInfo and XlogCategoryInfo object is declared for each
250 * We intentionally do not provide constructors for these structures, and rely
251 * on their members to be zero-initialized when the program starts. This
252 * ensures that everything will work as desired even if XLOG() statements are
253 * used during dynamic object initialization before main().
255 template <bool IsInHeaderFile>
256 class XlogLevelInfo {
259 LogLevel levelToCheck,
260 folly::StringPiece categoryName,
262 XlogFileScopeInfo*) {
263 // Do an initial relaxed check. If this fails we know the category level
264 // is initialized and the log admittance check failed.
265 // Use LIKELY() to optimize for the case of disabled debug statements:
266 // we disabled debug statements to be cheap. If the log message is
267 // enabled then this check will still be minimal perf overhead compared to
268 // the overall cost of logging it.
269 if (LIKELY(levelToCheck < level_.load(std::memory_order_relaxed))) {
273 // If we are still here, then either:
274 // - The log level check actually passed, or
275 // - level_ has not been initialized yet, and we have to initialize it and
276 // then re-perform the check.
278 // Do this work in a separate helper method. It is intentionally defined
279 // in the xlog.cpp file to avoid inlining, to reduce the amount of code
280 // emitted for each XLOG() statement.
281 auto currentLevel = loadLevelFull(categoryName, isOverridden);
282 return levelToCheck >= currentLevel;
286 LogLevel loadLevelFull(folly::StringPiece categoryName, bool isOverridden);
288 // XlogLevelInfo objects are always defined with static storage.
289 // This member will always be zero-initialized on program start.
290 std::atomic<LogLevel> level_;
293 template <bool IsInHeaderFile>
294 class XlogCategoryInfo {
296 bool isInitialized() const {
297 return isInitialized_.load(std::memory_order_acquire);
300 LogCategory* init(folly::StringPiece categoryName, bool isOverridden);
302 LogCategory* getCategory(XlogFileScopeInfo*) {
307 * Get a pointer to pass into the LogStreamProcessor constructor,
308 * so that it is able to look up the LogCategory information.
310 XlogCategoryInfo<IsInHeaderFile>* getInfo(XlogFileScopeInfo*) {
315 // These variables will always be zero-initialized on program start.
316 std::atomic<bool> isInitialized_;
317 LogCategory* category_;
320 #ifdef __INCLUDE_LEVEL__
322 * Specialization of XlogLevelInfo for XLOG() statements in the .cpp file being
323 * compiled. In this case we only define a single file-static LogLevel object
324 * for the entire file, rather than defining one for each XLOG() statement.
327 class XlogLevelInfo<false> {
330 LogLevel levelToCheck,
331 folly::StringPiece categoryName,
333 XlogFileScopeInfo* fileScopeInfo) {
334 // As above in the non-specialized XlogFileScopeInfo code, do a simple
335 // relaxed check first.
338 fileScopeInfo->level.load(::std::memory_order_relaxed))) {
342 // If we are still here we the file-scope log level either needs to be
343 // initalized, or the log level check legitimately passed.
345 loadLevelFull(categoryName, isOverridden, fileScopeInfo);
346 return levelToCheck >= currentLevel;
350 static LogLevel loadLevelFull(
351 folly::StringPiece categoryName,
353 XlogFileScopeInfo* fileScopeInfo);
357 * Specialization of XlogCategoryInfo for XLOG() statements in the .cpp file
358 * being compiled. In this case we only define a single file-static LogLevel
359 * object for the entire file, rather than defining one for each XLOG()
363 class XlogCategoryInfo<false> {
366 * Get a pointer to pass into the LogStreamProcessor constructor,
367 * so that it is able to look up the LogCategory information.
369 XlogFileScopeInfo* getInfo(XlogFileScopeInfo* fileScopeInfo) {
370 return fileScopeInfo;
376 * Get the default XLOG() category name for the given filename.
378 * This function returns the category name that will be used by XLOG() if
379 * XLOG_SET_CATEGORY_NAME() has not been used.
381 std::string getXlogCategoryNameForFile(folly::StringPiece filename);
385 * We intentionally use an unnamed namespace inside a header file here.
387 * We want each .cpp file that uses xlog.h to get its own separate
388 * implementation of the following functions and variables.
391 namespace xlog_detail {
393 * The default getXlogCategoryName() function.
395 * By default this simply returns the filename argument passed in.
396 * The default isXlogCategoryOverridden() function returns false, indicating
397 * that the return value from getXlogCategoryName() needs to be converted
398 * using getXlogCategoryNameForFile().
400 * These are two separate steps because getXlogCategoryName() itself needs to
401 * remain constexpr--it is always evaluated in XLOG() statements, but we only
402 * want to call getXlogCategoryNameForFile() the very first time through, when
403 * we have to initialize the LogCategory object.
405 * This is a template function purely so that XLOG_SET_CATEGORY_NAME() can
406 * define a more specific version of this function that will take precedence
409 template <typename T>
410 constexpr inline folly::StringPiece getXlogCategoryName(
411 folly::StringPiece filename,
417 * The default isXlogCategoryOverridden() function.
419 * This returns false indicating that the category name has not been
420 * overridden, so getXlogCategoryName() returns a raw filename that needs
421 * to be translated with getXlogCategoryNameForFile().
423 * This is a template function purely so that XLOG_SET_CATEGORY_NAME() can
424 * define a more specific version of this function that will take precedence
427 template <typename T>
428 constexpr inline bool isXlogCategoryOverridden(T) {
433 * File-scope LogLevel and LogCategory data for XLOG() statements,
434 * if __INCLUDE_LEVEL__ is supported.
436 * This allows us to only have one LogLevel and LogCategory pointer for the
437 * entire .cpp file, rather than needing a separate copy for each XLOG()
440 ::folly::XlogFileScopeInfo xlogFileScopeInfo;