To activate the OpenACC extensions for C/C++ and Fortran, the compile-time
flag -fopenacc must be specified. This enables the OpenACC directive
#pragma acc in C/C++ and
!$accp directives in free form,
!$acc directives in fixed form,
!$ conditional compilation sentinels in free form and
!$ sentinels in fixed form, for Fortran. The flag also
arranges for automatic linking of the OpenACC runtime library
(OpenACC Runtime Library Routines).
A complete description of all OpenACC directives accepted may be found in the OpenACC Application Programming Interface manual, version 2.0.
Note that this is an experimental feature and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.