COMP3891

File Descriptors and Open File Table

File Descriptors

• Each open file has a file descriptor.
• read, write, lseek use them to specify which file to operate on.
• State associated with a file desciptor:
  • File pointer:
    • Determines where in the file the next read or write is performed.
  • Mode:
    • Read only/writeable, etc….

File Descriptor Table Implementations

Single Global Open File Array

• FD is an index into the global open file array.
• Entries contain file pointer and pointer to a vnode.

Issues:

• File descriptor 1 is STDOUT.
  • Console for some processes.
  • File for others.
• Entry 1 needs to be different per process.

Per-process File Descriptor Array

• Each process has its own open file array.
• FD 1 can point to any vnode for each process.

Issues:

• Fork defines that the child shares the file pointer with the parent.
• Per-process table only allows for independent file pointers.

Per-process File Descriptor Table With Global Open File Table

• Each process has own file descriptor array which contains pointers to an open file table entry.
• Global open file table contains entries with a fp and a pointer to a vnode.

Provides:

• Shared file pointers if required.
• Independent file pointers if required.