User`s guide

may contain either variable-length or fixed-length records.
Random-Access Files
In some applications, disk files need to be read or written in a nonsequential or random order.
V+ supports random access only for files with fixed-length records. Records are numbered
starting with 1. The position of the first byte in a random-access record can be computed by:
byte_position = 1 + (record_number -1) * record_length
Random access is selected by setting the random-access bit in the mode parameter of the
FOPEN_ instruction. A nonzero record length must also be specified.
A specific record is accessed by specifying the record number in a READ or WRITE instruction.
If the record number is omitted, or is zero, the record following the one last accessed is used.
(See the FOPEN documentation.)
Buffering and I/O Overlapping
All physical disk I/O occurs as 512-byte sector reads and writes. Records are unpacked from
the sector buffer on input, and additional sectors are read as needed to complete a record. To
speed up read operations, V+ automatically issues a read request for the next sector while it
is processing the current sector. This request is called a preread. Preread is selected by default
for both sequential-access and random-access modes. It can be disabled by setting a bit in the
mode parameter of the FOPEN_ instruction. If prereads are enabled, opening a file for read
access immediately issues a read for the first sector in the file.
Preread operations may actually degrade system performance if records are accessed in truly
random order, since sectors would be read that would never be used. In this case, prereads
should be disabled and the FSEEK instruction should be used to initiate a preread of the next
record to be used.
The function IOSTAT(lun, 1) returns the completion status for a pending preread or FSEEK
operation.
On output, records are packed into sector buffers and written after the buffers are filled. If no-
wait mode is selected for a write operation by using the /N format control, the WRITE
instruction does not wait for a sector to be written before allowing program execution to
continue.
In random-access mode, a sector buffer is not normally written to disk until a record not
contained in that buffer is accessed. The FEMPTY instruction empties the current sector
buffer by immediately writing it to the disk.
A file may be opened in nonbuffered mode, which is much slower than normal buffered mode,
but it guarantees that information that is written will not be lost due to a system crash or
power failure. This mode was intended primarily for use with log files that are left opened over
an extended period of time and intermittently updated. For these types of files, the additional
(significant) overhead of this mode is not as important as the benefit.
Advanced Disk Operations
V+Language User's Guide, v17.0
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