MPI Fortran 编译错误 #6404: 'omp_get_thread_num()' 函数类型错误

在使用 MPI Fortran 编译器编译代码时，可能会遇到如下错误：

mpiifort -traceback -debug -O2 -static_intel -c -o processes.o processes.f90
processes.f90(147): error #6404: This name does not have a type, and must have an explicit type.   [OMP_GET_THREAD_NUM]
         if(myid==0 .and. omp_get_thread_num()==0) then
--------------------------^
compilation aborted for processes.f90 (code 1)
make: *** [Makefile:24: processes.o] Error 1

这个错误是因为编译器无法识别 'omp_get_thread_num()' 函数的类型，需要在代码中添加 USE OMP_LIB 来指定 OpenMP 库。修改后的代码应该类似于：

program main
  use omp_lib
  implicit none
  integer :: myid, numprocs, thread_num, nthreads

  !$omp parallel private(thread_num, nthreads)
  thread_num = omp_get_thread_num()
  nthreads = omp_get_num_threads()
  !$omp end parallel

  print *, 'Hello world from thread ', thread_num, ' of ', nthreads

end program main

请注意，这里的 USE OMP_LIB 语句应该在程序的开头，而不是在 if 语句中。