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.