XL Fortran Enterprise Edition for AIX : User's Guide - IBM

statements from different threads at the same time, you must use this library. In
other words, to perform Fortran I/O in parallel, you must link applications with
this library to get expected results.
However, note that on AIX operating system levels Version 4.3 and higher, a link is
provided from the libxlf90.a library to the libxlf90_r.a library. You do not need to
link with separate libraries depending on whether you are creating a threaded or a
non-threaded application. XL Fortran determines at run time whether your
application is threaded.
Synchronization of I/O Operations
During parallel execution, multiple threads might perform I/O operations on the
same file at the same time. If they are not synchronized, the results of these I/O
operations could be shuffled or merged or both, and the application might produce
incorrect results or even terminate. The XL Fortran thread-safe I/O library
synchronizes I/O operations for parallel applications. It performs the
synchronization within the I/O library, and it is transparent to application
programs. The purpose of the synchronization is to ensure the integrity and
correctness of each individual I/O operation. However, the thread-safe I/O library
does not have control over the order in which threads execute I/O statements.
Therefore, the order of records read in or written out is not predictable under
parallel I/O operations. Refer to “Parallel I/O Issues” for details.
External Files: For external files, the synchronization is performed on a per-unit
basis. The XL Fortran thread-safe I/O library ensures that only one thread can
access a particular logical unit to prevent several threads from interfering with
each other. When a thread is performing an I/O operation on a unit, other threads
attempting to perform I/O operations on the same unit must wait until the first
thread finishes its operation. Therefore, the execution of I/O statements by
multiple threads on the same unit is serialized. However, the thread-safe I/O
library does not prevent threads from operating on different logical units in
parallel. In other words, parallel access to different logical units is not necessarily
serialized.
Functionality of I/O under Synchronization: The XL Fortran thread-safe I/O
library sets its internal locks to synchronize access to logical units. This should not
have any functional impact on the I/O operations performed by a Fortran
program. Also, it will not impose any additional restrictions to the operability of
Fortran I/O statements except for the use of I/O statements in a signal handler
that is invoked asynchronously. Refer to “Use of I/O Statements in Signal
Handlers” on page 343 for details.
The Fortran standard prohibits a function reference from appearing in an
expression anywhere in an I/O statement if such a reference causes another I/O
statement to run. This restriction still applies with the XL Fortran thread-safe I/O
library.
Parallel I/O Issues
The order in which parallel threads perform I/O operations is not predictable. The
XL Fortran thread-safe I/O library does not have control over the ordering. It will
allow whichever thread that executes an I/O statement on a particular logical unit
and obtains the lock on it first to proceed with the operation. Therefore, only use
parallel I/O in cases where at least one of the following is true:
v
v
Each thread performs I/O on a predetermined record in direct-access files.
Each thread performs I/O on a different part of a stream-access file. Different
I/O statements cannot use the same, or overlapping, areas of a file.
Implementation Details of XL Fortran Input/Output 341