ADC Home > Reference Library > Technical Q&As > Carbon > File Management >

Locking an Area Using PBLockRange

Q I'm locking a small area of a shared file, using PBLockRange, so others cannot write to it. When I try to read the locked area from another machine (by calling FSRead), it returns a -45 (file locked) error. If I try to read over an unlocked area, it works fine.

I've never used PBLockRange before, but all of the documentation on it (that I've found, anyway) says that the locking is for writing not reading, and the documentation on FSRead never suggests that it might return a "file locked" error code.

A PBLockRange is behaving properly in this situation, and has always behaved this way. The idea is that by locking a section of a file you want to control all access to the section. While the section is locked and you are making changes to it, other users should not be able to read the portion of the file that is changing. The information might not be properly synched with the changes made, so readers might be reading old information. By denying readers access to the locked portion of the file, you guarantee the integrity of the data to which readers have access.

As page 2-50 of Inside Macintosh: Files states:

"When a user needs to modify a portion of a file that has been opened with shared read/write permission, it is usually desirable to make that portion of the file unavailable to other users while the changes are made. You can call the PBLockRange function to lock a range of bytes before modifying the file and then PBUnlockRange to unlock that range after your changes are safely recorded in the file.

"Locking a range of bytes in a file gives the user exclusive read/write access to that range and makes it inaccessible to other users. Other users can neither write nor read the bytes in that range until you unlock it. If other users attempt to read data from a portion of a file that you have locked, they receive the fLckdErr result code."
For a thorough description of working with files in a shared environment, see Technical Note FL 37- "Want Permission to do What."

Updated: 14-May-96

Did this document help you?
Yes: Tell us what works for you.
It’s good, but: Report typos, inaccuracies, and so forth.
It wasn’t helpful: Tell us what would have helped.