Linux File System Calls

To the kernel, all open files are referred to by File descriptors. A file descriptor is a non-negative number. When we open an existing file or create a new file, the kernel returns a file descriptor to the process. When we want to read or write a file, we identify the file with the file descriptor.

Each Linux process (except perhaps a daemon) should expect to have three standard POSIX file descriptors:

POSIX Constants Name	File Descriptors	Description
STDIN_FILENO	0	Standard input
STDOUT_FILENO	1	Standard output
STDERR_FILENO	2	Standard error

There are three "system file tables":

• There is a file descriptor table that maps file descriptors (small integers) to entries in the open file table.

• Each entry in the open file table contains (among other things) a file offset and a pointer to the in-memory inode table.

• In open file table, one file table entry has an open() call, and it is shared if the file descriptor is dup()ed or fork()ed.

Figure: file descriptors for a single process, file table and inode table.

Note that

• multiple file descriptors can refer to the same file table entry (e.g., as a result of the dup system call);

• multiple file table entries can in turn refer to the same inode (if it has been opened multiple times; the table is still simplified because it represents inodes by file names, even though an inode can have multiple names).

• File descriptor 3 does not refer to anything in the file table, signifying that it has been closed.