The Linux Programming Interface

55FILE LOCKING

Previous chapters have covered various techniques that processes can use to synchronize their actions, including signals (Chapters 20 to 22) and semaphores (Chapters 47 and 53). In this chapter, we look at further synchronization techniques designed specifically for use with files.

55.1 Overview

A frequent application requirement is to read data from a file, make some change to that data, and then write it back to the file. As long as just one process at a time ever uses a file in this way, then there are no problems. However, problems can arise if multiple processes are simultaneously updating a file. Suppose, for example, that each process performs the following steps to update a file containing a sequence number:

1. Read the sequence ...