File I/O Flashcards
6 basic I/O system calls
- open()
- write()
- read()
- close()
- lseek()
- fcntl()
Flags in open()
Specifies access and creation mode, i.e.
O_WRONLY, O_RDWR, O_CREAT, etc.
Mode in open()
Specifies file permission if file is created using O_CREAT or O_TMPFILE
File descriptor
A small, non-negative integer used as a handle for a particular file to read/write to
Can change each time the file is opened
File offset
The file offset is a pointer to a specific byte in a file we are reading/writing to
read() automatically increments the file offset, write() starts at the end and increments to end each time
lseek()
Used to manually adjust the file offset, 3 parameters: fd, offset, whence - adjusts file offset by offset bytes, specified by ‘whence’ which can be:
SEEK_SET - beginning of file
SEEK_CUR - current offset
SEEK_END - end of the file
Stream vs. File Descriptor
A stream is a “wrapper” around a file descriptor that also has a buffer attached to it
Buffered I/O vs. system calls
Buffered I/O writes to a buffer before making a system call because system calls are costly
fflush()
immediately sends buffered data to the kernel - setbuf(NULL) does the same thing
Blocking vs Nonblocking
Blocking call doesn’t return until the operation can be done, a nonblocking call will return anyway (usually w/ an error)
Everything is a File model
Everything is represented as a file including hardware like keyboard, mouse, and others like the terminal
Disk Partitions
A disk is divided into partitions - can be found at /proc/partitions - usually used as a file system, also used as swap space
File System
a system that manages files/directories, each system has a file tree that starts with root directory /
i-node
An i-node contains metadata for every file, i.e. type, permissions, owner, etc. - identified by a number
Hard link
Giving another name to an existing file - will have the same metadata and content
Hard links can’t be made on directories or outside the same file system
Soft/symbolic link
A soft link is an actual file where the content is the path to the original file
Can be used on directories and between file systems
Setuid Bit
If set, the user that runs the program can act as the owner of the program
Setgid Bit
If set, the user that runs the program can act as if the user belonged to the group of the program
Sticky bit
If set on a directory, a user can delete a file under it only
VFS (virtual file system)
Defines an interface that different file systems can implement such as open, read, write, etc.
Mounting/Unmounting
Different file systems can be mounted to others to create a single file tree
