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According to the C Standard, the following library functions are not required to avoid data races:
rand(),srand()getenv()strtok()strerror()asctime(),ctime(),localtime()setlocale()ATOMIC_VAR_INIT,atomic_init()tmpnam()ctime()when called with a null pointer argumentmbrtoc16(),c16rtomb(),mbrtoc32(),c32rtomb()when called with a nullmbstate_t *
Section 2.9.1 of the System Interfaces volume of POSIX.1-2008 has a much longer list of functions that are not required to be thread-safe.
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