Guardian Programming Reference Summary for C

SETMODE Operations
Guardian Programming Reference Summary for C—522630-001
11-22
141,
continued
If the file is audited and structured, the file must be opened with an access mode
of read-only. The exclusion mode can be shared, protected, or exclusive. If the
exclusion mode is shared, updates done by other openers of the file might not be
seen by this opener.
If the file is audited and unstructured, only read operations can be performed on
the file after this SETMODE is issued with param1 set to 1. Write operations
must not be issued until the SETMODE is reissued with param1 set to 0.
If the file is not audited, the file can be opened with any access mode and any
exclusion mode. If the exclusion mode is shared, updates done by other openers
of the file might not be seen by this opener. To issue write requests after the
SETMODE is issued with param1 set to 1, the exclusion mode must be
exclusive.
Once the large transfer mode is enabled, only the data transfer operations of
READX, READUPDATEX, WRITEX, and WRITEUPDATEX are allowed. No
record locks are supported. READX and WRITEX use the record address in
NEXTREC. READUPDATEX and WRITEUPDATEX use the record address in
CURREC. POSITION can be used to set CURREC and NEXTREC. Relative
byte addressing is used for positioning.
The operation is done nowait if the file is opened for nowait I/O. The operation
must then be completed by a call to AWAITIOX. The operation can be canceled
by CANCEL or CANCELREQ.
With the large transfer mode enabled, data is read or written directly from the
user's buffer. The user's buffer is locked in memory until completion of the
operation. The data is not moved to or from the PFS. If the I/O is done nowait,
the user should not modify or examine the data in the buffer until the I/O has
finished. This also applies to other processes sharing the segment containing
the buffer. If the I/O is done in a nowait manner and the buffer is in the stack,
the buffer must be in the high end of the stack.
The record address (in NEXTREC or CURREC) must be on a page boundary (a
multiple of 2K bytes) or error 550, "illegal position," will be returned. Note that,
as usual, when performing successive READXs without intervening
POSITIONs, NEXTREC is incremented by the count actually read, and that just
before the EOF, the count read will probably not be a multiple of 2K bytes,
hence violating the record address constraint for the next READX call. A
POSITION before each READX call ensures that the record address meets the
page boundary constraint.
If -1D positioning is used, the current EOF must be on a page boundary,
otherwise the disk process returns an error. If -1D positioning is used or writes
are done that change the EOF, some performance gains are lost. When the EOF
is changed, DP2 must do extra checkpoints. To avoid the extra checkpoints,
issue a CONTROL 2 to set the EOF to a high value before starting a series of
large writes. When the writes are complete, another CONTROL 2 can be issued
to set EOF to the correct value.