Some functions in the C standard library are not guaranteed to be reentrant with respect to threads. Some functions (such as strtok() and asctime()) return a pointer to the result stored in function-allocated memory on a per-process basis. Other functions (such as rand()) store state information in function-allocated memory on a per-process basis. Multiple threads invoking the same function can cause concurrency problems, which often result in abnormal behavior and can cause more serious vulnerabilities, such as abnormal termination, denial-of-service attack, and data integrity violations.
According to the C C11 Standard, the following library functions are not required to avoid data races:
| API | Recommendation |
|---|---|
rand(), srand() | MSC30-C. Do not use the rand() function for generating pseudorandom numbers |
getenv(), getenv_s() | |
strtok() | strtok_s() in C11 Annex K, or strtok_r() in POSIX |
strerror() | strerror_s() in C11 Annex K, or strerror_r() in POSIX |
asctime(), ctime(),localtime(), gmtime() | asctime_s(), ctime_s(), localtime_s(), gmtime_s() in C11 Annex K |
setlocale() | |
ATOMIC_VAR_INIT, atomic_init() | |
tmpnam() |
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tmpnam_s() in C11 Annex K, tmpnam_r() in POSIX | |
mbrtoc16(), c16rtomb(),mbrtoc32(), c32rtomb() |
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| Do not call with a |
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null mbstate_t * argument |
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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This compliant solution uses the strerror_s() function from Annex K of the C Standard, which has the same functionality as strerror() but guarantees thread-safety. Furthermore, in CC11, errno is a thread-local variable, so there is no race condition between the time it is initialized and the time it is read by strerror_s().
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