Using KSAM/XL and KSAM 64 (32650-90886)

54 Chapter5
Reading File Data
Shared File Access
Shared File Access
If only one process is accessing a file, setting a pointer and reading a record in a two-step
process does not present a problem. Shared file access, however, presents potential
retrieval contention. If a pointer is positioned to retrieve a particular record by one
process, another process could modify or delete the record before the original process reads
it. The FLOCK and FUNLOCK intrinsics should be used to ensure proper record retrieval in
any program that allows shared access to its file.
NOTE File locking keeps the file inaccessible to other users until the file is unlocked.
This could be a potential source of performance problems. A different file
structure may be more suitable for applications in a shared environment,
such as IMAGE/3000, etc.
An FLOCK intrinsic call should be made prior to a pointer positioning and record reading
procedure to ensure that the proper retrieval is executed. The FUNLOCK intrinsic restores
shared access once the retrieval is completed. Once the file is unlocked, do not assume that
the pointer is still valid. Before using the pointer again, reposition it. The following
sequence shows the appropriate locking procedure to ensure the proper sequence of
records.
FLOCK
FFINDBYKEY (sets the logical pointer)
FREAD loop (reads records in key sequence)
FUNLOCK