HP 3000 Manuals HP 3000 Manuals
FLOCK
INTRINSIC NUMBER 15
Dynamically locks a file.
IV LV
FLOCK(filenum,lockcond);
The FLOCK intrinsic dynamically locks a file and transfers the latest
control information from disc to the buffers. A call to FLOCK is
required before any attempt is made to read or modify a file opened for
shared access.
WARNING Always unlock a locked file (refer to FUNLOCK) after reading or
modifying it. Not following this practice may result in
corruption of the file.
PARAMETERS
filenum integer by value (required)
A word supplying the file number of the file to be
locked.
lockcond logical by value (required)
A word specifying conditional or unconditional locking:
TRUE-- Locking will take place
unconditionally. If the file cannot be
locked immediately, the calling process
suspends until the file can be locked.
Bit 15 = 1
FALSE-- Locking will take place only if the
file's Resource Identification Number
(RIN) is not currently locked. If the
RIN is locked, control returns
immediately to the calling process,
with condition code CCG.
Bit 15 = 0
CONDITION CODES
The condition codes possible when lockcond = TRUE are
CCE Request granted.
CCG Not returned when lockcond = TRUE.
CCL Request denied because this file was not opened with the
dynamic locking aoption specified in the FOPEN
intrinsic, or the request was to lock more than one file
and the calling process does not possess the Multiple
RIN Capability.
The condition codes possible if lockcond = FALSE are
CCE Request granted.
CCG Request denied because the file was locked by another
process.
CCL Request denied because: this file was not opened with
the dynamic locking aoption specified in the FOPEN
intrinsic; or the request was to lock more than one file
and the calling process does not possess the Multiple
RIN Capability.
SPECIAL CONSIDERATIONS
Split stack calls permitted.
Standard Capability sufficient if only one file is to be locked
dynamically.
If more than one file is to be locked dynamically, the Multiple RIN
Capability is required.
USING FLOCK
The dynamic locking and unlocking capability allows you to complete any
update to a file when it is possible for other users to access the file.
When dynamic locking is allowed (bit 10 of FOPEN aoptions parameter is
set to allow dynamic locking); then you must use the FLOCK intrinsic to
lock the file before writing, rewriting, or deleting any records. This
requirement insures that another user does not attempt to change the same
record at the same time. FLOCK also insures that the most recent data is
available in the file. A locked file can be unlocked following the
update with the FUNLOCK intrinsic.
When FLOCK is executed, it clears all the buffers and transfers the
latest control information from the KSAM file to the buffers. This
insures that any subsequent read of the file retrieves the latest
information from disc rather than from the buffers. (Note that FCONTROL
control code 7 also clears the buffers.)
If you use the Multiple RIN capability, a sequence of file locking should
be agreed upon or you should use conditional locking (lockcond = FALSE).
Otherwise, it is possible to lock other users from the file. Consider
the situation where one program unconditionally locks file A and then
attempts to lock file B. If another program unconditionally locks file B
and then attempts to lock file A, both programs will wait indefinitely to
lock the second file in sequence. To avoid this, both programs should
agree to lock the files in the sequence A first, then B; or both programs
should use only conditional locks.
For example, suppose you open a KSAM file called DATA1 for shared access
in update mode and allow dynamic locking and unlocking:
FIL1:=FOPEN(DATA1,7,%345);
The parameters specified are:
filenum File number of DATA1, which is assigned to FIL1 when the
file is opened.
formal- Name identifying the file contained in DATA1.
designator
foptions The value 7 specifies that this is an old user file
(bits 14,15 = 11) and that it is coded in ASCII (bit 13
= 1).
aoptions The octal value 345 indicates that the file was opened
for update (bits 12 through 15 = 0101), that dynamic
locking/unlocking is allowed (bit 10 = 1), and that
access is shared (bits 8 and 9 = 11).
This file can then be locked as follows:
FLOCK(FIL1,1)
The parameters specified are:
filenum File number of file DATA1 contained in the variable
FIL1.
lockcond = 1 which means the file is to be locked
unconditionally. If the file cannot be locked
immediately, the calling process is suspended until the
file can be locked.