1.. title:: clang-tidy - concurrency-mt-unsafe 2 3concurrency-mt-unsafe 4===================== 5 6Checks for some thread-unsafe functions against a black list of 7known-to-be-unsafe functions. Usually they access static variables without 8synchronization (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 in 12concurrent programming if only a single thread is used (e.g. setenv(3)), 13however, some functions may track a state in global variables which 14would be clobbered by subsequent (non-parallel, but concurrent) calls to 15a related function. E.g. the following code suffers from unprotected 16accesses 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 buffer 32 33 sleep(1); // implementation may use SIGALRM 34 35.. option:: FunctionSet 36 37 Specifies which functions in libc should be considered thread-safe, 38 possible values are `posix`, `glibc`, or `any`. 39 40 `posix` means POSIX defined thread-unsafe functions. POSIX.1-2001 41 in "2.9.1 Thread-Safety" defines that all functions specified in the 42 standard are thread-safe except a predefined list of thread-unsafe 43 functions. 44 45 Glibc defines some of them as thread-safe (e.g. dirname(3)), but adds 46 non-POSIX thread-unsafe ones (e.g. getopt_long(3)). Glibc's list is 47 compiled from GNU web documentation with a search for MT-Safe tag: 48 https://www.gnu.org/software/libc/manual/html_node/POSIX-Safety-Concepts.html 49 50 If you want to identify thread-unsafe API for at least one libc or 51 unsure which libc will be used, use `any` (default). 52 53