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1.. title:: clang-tidy - concurrency-mt-unsafe2 3concurrency-mt-unsafe4=====================5 6Checks for some thread-unsafe functions against a black list of7known-to-be-unsafe functions. Usually they access static variables without8synchronization (e.g. gmtime(3)) or utilize signals in a racy way.9The set of functions to check is specified with the `FunctionSet` option.10 11Note that using some thread-unsafe functions may be still valid in12concurrent programming if only a single thread is used (e.g. setenv(3)),13however, some functions may track a state in global variables which14would be clobbered by subsequent (non-parallel, but concurrent) calls to15a related function. E.g. the following code suffers from unprotected16accesses to a global state:17 18.. code-block:: c++19 20 // getnetent(3) maintains global state with DB connection, etc.21 // If a concurrent green thread calls getnetent(3), the global state is corrupted.22 netent = getnetent();23 yield();24 netent = getnetent();25 26 27Examples:28 29.. code-block:: c++30 31 tm = gmtime(timep); // uses a global buffer32 33 sleep(1); // implementation may use SIGALRM34 35Options36-------37 38.. option:: FunctionSet39 40 Specifies which functions in libc should be considered thread-safe,41 possible values are `posix`, `glibc`, or `any`.42 43 `posix` means POSIX defined thread-unsafe functions. POSIX.1-200144 in "2.9.1 Thread-Safety" defines that all functions specified in the45 standard are thread-safe except a predefined list of thread-unsafe46 functions.47 48 Glibc defines some of them as thread-safe (e.g. dirname(3)), but adds49 non-POSIX thread-unsafe ones (e.g. getopt_long(3)). Glibc's list is50 compiled from GNU web documentation with a search for MT-Safe tag:51 https://www.gnu.org/software/libc/manual/html_node/POSIX-Safety-Concepts.html52 53 If you want to identify thread-unsafe API for at least one libc or54 unsure which libc will be used, use `any` (default).55 56