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.