HP 3000 Manuals HP 3000 Manuals
How to Access a File Mapped
You can access a file mapped by declaring a short (32-bit) or long
(64-bit) pointer variable within a program and passing that variable to
the appropriate HPFOPEN option. The HPFOPEN intrinsic returns the
variable pointing to the beginning of the data area of the opened file.
After HPFOPEN returns the address of the file, you simply reference the
pointer as an array. The machine architecture ensures integrity and
protection of the file.
The following file types are allowed any type of access (Read, Write,
Read/Write, and so forth) when opened using mapped access options:
* standard disk files with fixed-length or undefined-length record
formats
The following file types are allowed read-only access when opened using
mapped access options:
* standard disk files with variable-length record formats
* KSAM files opened with copy mode option enabled
The following file types are not allowed to be opened using mapped access
options:
* relative I/O (RIO) files
* message (MSG) files
* circular (CIR) files
* device files
You can use all applicable file system intrinsics mapped access,
including all data transfer intrinsics; however, when mixing data
transfer intrinsic calls (such as FREAD and FWRITE) with mapped access,
you must take into consideration the data type (ASCII/binary) of the
file, the record format, and the record size. Otherwise, data written to
the file using mapped access may not make sense when read by FREAD.
When you open a file using mapped access and write data to that file, you
must use the FPOINT and FCONTROL intrinsics to reset the EOF before you
close the file. Otherwise, all data you write to the file after the EOF
will be lost when you close the file. In the case of a newly created
file, the EOF initially points to record zero.
NOTE When you access a file with mapped access you are bypassing
file system services that set various file system pointers
automatically, including the EOF and the logical record pointer.
You are responsible for resetting the EOF prior to closing a file
that you have accessed mapped. Also, file system posting is
bypassed; so, if data recovery is needed you should use FCONTROL
controlcodes 2 and/or 6 to post data and update the EOF
periodically. Heavy use of the FCONTROL intrinsic to post data and
set the EOF degrades performance due to the overhead of the extra
posting.
Advantages of mapped access
Mapped access to a file can be much faster than access through normal
file system intrinsics. This is especially the case when you are
accessing a smaller file randomly rather than sequentially. When
accessing a file mapped, there is no file system overhead associated with
a specific reference to the file. The only difference between accessing
a file mapped and accessing normal memory is the locality of the access
and the protection strategy.
NOTE It is possible to show a degradation of performance if an
application that accesses files sequentially is modified to access
those files mapped. Normal system reads prefetch multiple records
per read. Mapped file access has no method of prefetching the
data, consequently, some performance penalty is paid by additional
overhead on page faults.
There are two perspectives you can take on mapped file access:
1. A file is accessible as virtual memory. The advantages from this
perspective are high performance and fast response time from the
file system.
2. Virtual memory is accessed through the file system. The
advantages from this perspective are
* Virtual memory can be easily saved permanently.
* Virtual memory can be checkpointed.
* Virtual memory can be easily shared through a common naming
convention.
Short-mapped access
Item #18 short-mapped option is available in the HPFOPEN intrinsic to
provide you with shared virtual memory. A short pointer is returned in
an optional item parameter. You can use the pointer as a large array of
any type to efficiently access the file.
A file created using the short-mapped option can be up to four megabytes
in size. A process can have open at the same time up to six megabytes of
files that are opened using the short-mapped option. For larger file
needs, a file created using the long-mapped option is required.
An error results if you attempt to open a file using the short-mapped
option that you have previously opened normally or with the long-mapped
option.
You cannot access a loaded program file or a loaded library file using
either mapped access option. In addition, you cannot load a file that is
currently being accessed mapped.
Long-mapped access
Item #21 long-mapped option is available in the HPFOPEN intrinsic to
provide you with access to large amounts of shared virtual memory. You
can use the pointer as a large array of any type to efficiently access
the file.
A file created using the long-mapped option can be up to two gigabytes in
size. There is no practical limit of the number of long-mapped access
files that a process can have open at a time.
You cannot access a loaded program file or a loaded library file using
either mapped access option. In addition, you cannot load a file that is
currently being accessed mapped.
Advantages of long-mapped access over short-mapped access are:
* You can access much larger files than you can using the
short-mapped option.
* You can open files that were opened previously with any options
(as long as the exclusive status of the file is not violated)
The disadvantage of long-mapped access is that it may be slower than
short mapped access because of the need to load a space register to
access the long-pointer space. long-mapped access can be as much as four
times slower than short-mapped access, although long-mapped access still
can be faster than accessing the file through the file system data
transfer intrinsics.
The degradation can be minimized if you make the references to the long
pointer space in a localized part of your code. This way it may be
possible for the system to keep the pointer to the file's virtual space
loaded into a space register rather than to repeatedly load and unload
it.