Is the file position per inode?

Question

I am confused with the concept of file position as used in lseek. Is this file position maintained at inode level or a simple variable which could have different values for different process working on the same file?

Answer 1

Per the lseek docs , the file position is associated with the open file pointed to by a file descriptor, ie the thing that is handed to your by open . Because of functions like dup and fork , multiple descriptors can point to a single description , but it's the description that holds the location cursor.

Think about it: if it were associated with the inode, then you would not be able to have multiple processes accessing a file in a sensible manner, since all accesses to that file by one process would affect other processes.

Thus, a single process could have track many different file positions as it has file descriptors for a given file.

Answer 2

It's not an 'inode', but FILEHANDLE inside kernel.

Inode is a part of file description of the *nix specific file system on the disk. FAT32, for example, has no inodes, but supported by Linux.

Answer 3

For know the relation between file descriptors and open files, we need to examine three data structures.

• the per-process file descriptor table
• the system wide table of open file descriptors
• the file system i-node table.

For each process kernel maintains a table of open file descriptors. Each entry in this table records information about a single file descriptor including

• a set of flags controlling the operation of the file descriptor.
• a reference to the open file description

The kernel maintains a system wide table of all open file descriptors. An open file description stores all information related to an open file including:

• the current file offset(as updated by read() and write(),or explicitly modified using lseek())
• status flags specified when opening the file.
• the file access mode (read-only,write only,or read-write,as specified in open())
• setting relating to signal-driven I/O and
• a reference to i-node object for this file.

Reference-Page 94,The Linux Programming Interface by Michael Kerrisk