POSIX Files

This is just a short note about the POSIX files API, and it doesn't cover all of the details.

There are 5 operations implemented by the kernel: open, close, read, write, seek. These are made available to userspace via syscalls.

Each process has a list of open files, which might look something like:

struct file {
    int f_type;     // type (regular file? device file?)
    int f_flags;    // flags provided at open()
    int f_pos;      // current position in file, set by lseek()
    int f_inode;    // inode number of open regular file
    ... additional fields (e.g. if device file, the dev ID)
};

struct task {
    // process fields
    ...
    file** file_list;   // pointers to objects in a kmem_cache for file structs
    ...
};

File descriptors are integers referring to an open file, and can be thought of as indices into the running process' file list. open returns a file descriptor, close/read/write/seek takes one. The C library's fopen/fclose/fread/fwrite/fseek are abstractions of this interface, and pass around FILE structs, whose contents are implementation-defined but contain at minimum a file descriptor.

The open() operation takes various flags bitwise OR'd together in an argument, the key ones being:

• O_RDONLY - can only be read

• O_WRONLY - can only be written

• O_RDWR - can be read or written

• O_CREAT - create file if it doesn't exist, or open if it does

• O_EXCL - if O_CREAT and the file exists, open() fails

• O_TRUNC - truncate file to size 0 when opened

• O_APPEND - before each write(), set f_pos to length of the file (1 past EOF)

Note that (O_RDONLY | O_WRONLY) != O_RDWR on most Unix-likes; each flag is defined as a unique bit. You must provide only 1 of the first 3 flags listed above when opening a file.

Writing past the end of a file first expands the file. Reading past the end of a file reads back zeroes but doesn't affect file size.

O_EXCL is necessary to avoid race conditions where you might try opening a file multiple times concurrently, and overwrite contents without noticing (especially if O_TRUNC is specified). open() acts atomically and performs that "file already exists" check that a multithreaded program may desire to do as part of a single operation.