> For the complete documentation index, see [llms.txt](https://eric-lo.gitbook.io/lab9-filesystem/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://eric-lo.gitbook.io/lab9-filesystem/linux-file-system-calls.md).

# 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`.

![](https://2015623591-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lp-5TQpuuM0wVgRDmqL%2F-Lp-5WZ7mVLwEZ2xvCJW%2F-Lp-5bOK706q_MDczAF-%2Ffd.png?generation=1568738547042825\&alt=media)

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**.