Guardian Procedure Calls Reference Manual

suspended and queued in the "locking" queue behind other processes attempting to lock or
read the file or record.
NOTE: A deadlock condition occurs if a call to READ[X] is made by a process having multiple
opens on the same file and the filenum used to lock the file differs from the filenum
supplied to READ[X].
• Read call when alternate locking mode is in effect
If the alternate locking mode is in effect when READ[X] is called, and the file or record is
locked through a file number other than that supplied in the call, the call is rejected with
file-system error 73 (file is locked).
• Locking mode for read
The locking mode is specified by the SETMODE procedure, function 4. If you encounter error
73 (file is locked), you do not need to call SETMODE for every READ[X]. SETMODE stays in
effect indefinitely (for example, until another SETMODE is performed or the file is closed), and
there is no additional overhead involved.
• Considerations for READX only
The buffer and count transferred can be in the user stack or in an extended data segment.
The buffer and count transferred cannot be in the user code space.
◦
◦ The buffer and count transferred address must be relative; they cannot be an absolute
extended address
◦ If the buffer or count transferred is in a selectable extended data segment, the segment
must be in use at the time of the call. Flat segments allocated by a process are always
accessible to the process.
◦ The size of the transfer is subject to current restrictions for the type of file.
◦ If the file is opened for nowait I/O, and the buffer is in an extended data segment, you
must not deallocate or reduce the size of the extended data segment before the I/O
finishes with a call to AWAITIOX or is canceled by a call to CANCEL or CANCELREQ.
◦ If the file is opened for nowait I/O, you must not modify the buffer before the I/O finishes
with a call to AWAITIOX. This also applies to other processes that might be sharing the
segment. It is the application's responsibility to ensure this.
◦ If the file is opened for nowait I/O and the I/O has been initiated with these routines,
the I/O must be completed with a call to AWAITIOX (not AWAITIO).
◦ If the file is opened for nowait I/O, a selectable extended data segment containing the
buffer need not be in use at the time of the call to AWAITIOX.
◦ Nowait I/O initiated with these routines can be canceled with a call to CANCEL or
CANCELREQ. The I/O is canceled if the file is closed before the I/O finishes or AWAITIOX
is called with a positive time limit and specific file number, and the request times out.
◦ A file opened by FILE_OPEN_ uses direct I/O transfers by default; you can use SETMODE
function 72 to force the system to use an intermediate buffer in the process file segment
(PFS) for I/O transfers. A file opened by OPEN uses a PFS buffer for I/O transfers, except
for large transfers to DP2 disks.
◦ If the extended address of the buffer is odd, bounds checking rounds the address to the
next lower word boundary and checks an extra byte as well. The odd address is used
for the transfer.
READ[X] Procedures 1195