Thread safe initialization of string from getenv (environment variable)

Question

I have a project with tight level of clang-tidy static analysis.

In order to check some consistencies in the environment I am probing the value of a few environment variables.

int main() {
    char const* ompi_size_cstr = std::getenv("OMPI_COMM_WORLD_SIZE");
    ...
}

(nevermind the meaning of the variable).

But now I get this warning:

/builds/user/mpi3/environment.hpp:49:51: error: function is not thread safe [concurrency-mt-unsafe,-warnings-as-errors]
        const char* ompi_size_cstr = std::getenv("OMPI_COMM_WORLD_SIZE");

First, if I look at the documentation, https://en.cppreference.com/w/cpp/utility/program/getenv, it says that the function is thread-safe since C++11 (I am using C++17). But maybe the warning is referring to the fact that the environment variable itself could change behind the scenes.

Second, even if this is not thread-safe in some sense: What can I do about it? Is there a canonical solution to use getenv?,

for example, by locking a manually introduced get_env_mtx mutex, or perhaps making my variable declaration static? As in,

   static const char* ompi_size_cstr = std::getenv("OMPI_COMM_WORLD_SIZE");

or

   static std::string ompi_size_cstr = std::getenv("OMPI_COMM_WORLD_SIZE");

Of course, I can also introduce an exception to the rule, // NOLINT(concurrency-mt-unsafe) but I wanted to know if there is a fix that I can express as better code instead. It would be also good to improve the code even if at the end I have to add a NOLINT anyway.

Note: I am using clang-tidy version 14.0.6, and in C++17.

Answer 1

It is probably an outdated warning or some confusion with C's getenv which is not thread-safe at all.

There's getenv_s (comes with C11) that stores the environment variable locally. You can use that instead.