Discuss the different techniques with which a file can be shared among different users.

Some popular techniques with which a file can be shared among different users are:

1. Sequential sharing: In this sharing technique, a file can be shared by only one program at a time, that is, file accesses by P1 and P2 are spaced out over time. A lock field can be used to implement this. Setting and resetting of the lock at file open and close ensures that only one program can use the file at any time.
2. Concurrent sharing: Here a number of programs may share a file concurrently. When this is the case, it is essential to avoid mutual interference between them. There are three categories of concurrent sharing:

1. Immutable files: If a file is shared in immutable mode, none of the sharing programs can modify it. This mode has the advantage that sharing programs are independent of one another.
   
2. Single image immutable files: Here the changes made by one program are immediately visible to other programs. The Unix file system uses this file- sharing mode.
   
3. Multiple image mutable files: Here many programs can concurrently update the shared file. Each updating program creates a new version of the file, which is different from the version created by concurrent programs. This sharing mode can only be used in applications where concurrent updates and the existence of multiple versions are meaningful.