HP Fortran Programmer's Guide (B3908-90031; September 2011)

Controlling data storage
Sharing data among programs
Chapter 3 113
Sharing data among programs
If you are designing an application that requires multiple threads of control that share the same data, the
design can take either of two forms:
• The program makes calls to the threads library:
/usr/lib/libpthread.sl
which creates multiple threads executing in a single process and therefore all sharing the same address
space.
• The application consists of several programs that run simultaneously in separate processes and that
access an HP-UX shared memory segment.
The first approach is beyond the scope of this manual and requires that you have an understanding of how to
call the threads library.
1
The second approach is described here.
To share data among several HP Fortran programs that are executing simultaneously in separate processes,
use the $HP$ SHARED_COMMON directive. This directive enables you to create a common block that is
accessible by HP Fortran programs executing in different processes.
The $HP$ SHARED_COMMON directive causes the compiler to insert HP-UX system calls to perform shared
memory operations. To the programmer, the programs sharing the memory segment appear as though they
were program units in the same program, accessing a set of common block variables.
Following are two programs to illustrate how the $HP$ SHARED_COMMON directive works:
• The first program, go_to_sleep.f90, must execute first. Because it executes first, it creates the
shared memory segment and then enters a DO loop, where it waits until the second program starts to
execute. You can use the ipcs -m command to confirm that a shared memory segment has been
created.
• When the second program, wake_up.f90, starts to execute, it writes to the shared common block
variables, one of which causes go_to_sleep.f90 to break out of the DO loop and run to completion.
The $HP$ SHARED_COMMON directive must appear at the beginning of the specification part of the main
program unit of each program sharing the memory segment. Also, the common block specified for sharing
must have the same layout in all files in which it is declared.
You can use the ipcs -m command both to determine that HP-UX has created a shared memory segment
and, after the programs complete execution, to confirm that it has been released.
1. Specifying the +Oparallel option causes the compiler to transform eligible loops in an
HP Fortran program for parallel execution on HP 9000 systems. For information about
compiling for parallel execution, see “Compiling for parallel execution” on page 162.