Advanced Programming in the UNIX® Environment

ptg10805159

Section 14.3 RecordLocking 495

Assuming that we are at end of file when we perform the firstwrite,this operation

will extend the file by one byte, and that byte will be locked. The unlock operation that

follows has the effect of removing the locks for futurewrites that append data to the

file, but it leaves a lock on the last byte in the file. When the second write occurs, the

end of file is extended by one byte, but this byte is not locked. The state of the file locks

for this sequence of steps is shown in Figure14.10.

first

byte

written

locked

state of file after firstwrite

first

byte

written

second

byte

written

locked

state of file after secondwrite

Figure 14.10File range lock diagram

When a portion of a file is locked, the kernel converts the offset specified into an

absolute file offset. In addition to specifying an absolute file offset (SEEK_SET),fcntl

allows us to specify this offset relative to a point in the file: current (SEEK_CUR) or end

of file (SEEK_END). The kernel needs to remember the locks independent of the current

file offset or end of file, because the current offset and end of file can change, and

changes to these attributes shouldn’t affect the state of existing locks.

If we intended to remove the lock covering the byte we wrote in the first write, we

could have specified the length as−1. Negative length values represent the bytes before

the specified offset.

Advisor y versus MandatoryLocking

Consider a library of database access routines. If all the functions in the library handle

recordlocking in a consistent way,then we say that any set of processes using these

functions to access a database arecooperating processes.It is feasible for these database

access functions to use advisory locking if they arethe only ones being used to access

the database. But advisory locking doesn’t prevent some other process that has write

permission for the database file from writing whatever it wants to the database file.

This rogue process would be an uncooperating process, since it’s not using the accepted

method (the library of database functions) to access the database.

Mandatory locking causes the kernel to check everyopen,read,andwriteto

verify that the calling process isn’t violating a lock on the file being accessed.

Mandatory locking is sometimes calledenforcement-mode locking.