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### 5.3 Thread-safety of the runtime library

GNU Fortran can be used in programs with multiple threads, e.g. by using OpenMP, by calling OS thread handling functions via the `ISO_C_BINDING` facility, or by GNU Fortran compiled library code being called from a multi-threaded program.

The GNU Fortran runtime library, (`libgfortran`), supports being called concurrently from multiple threads with the following exceptions.

During library initialization, the C `getenv` function is used, which need not be thread-safe. Similarly, the `getenv` function is used to implement the `GET_ENVIRONMENT_VARIABLE` and `GETENV` intrinsics. It is the responsibility of the user to ensure that the environment is not being updated concurrently when any of these actions are taking place.

The `EXECUTE_COMMAND_LINE` and `SYSTEM` intrinsics are implemented with the `system` function, which need not be thread-safe. It is the responsibility of the user to ensure that `system` is not called concurrently.

For platforms not supporting thread-safe POSIX functions, further functionality might not be thread-safe. For details, please consult the documentation for your operating system.

The GNU Fortran runtime library uses various C library functions that depend on the locale, such as `strtod` and `snprintf`. In order to work correctly in locale-aware programs that set the locale using `setlocale`, the locale is reset to the default “C” locale while executing a formatted `READ` or `WRITE` statement. On targets supporting the POSIX 2008 per-thread locale functions (e.g. `newlocale`, `uselocale`, `freelocale`), these are used and thus the global locale set using `setlocale` or the per-thread locales in other threads are not affected. However, on targets lacking this functionality, the global LC_NUMERIC locale is set to “C” during the formatted I/O. Thus, on such targets it’s not safe to call `setlocale` concurrently from another thread while a Fortran formatted I/O operation is in progress. Also, other threads doing something dependent on the LC_NUMERIC locale might not work correctly if a formatted I/O operation is in progress in another thread.

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