File I/O open close lseek read and write – unbuffered I/O dup and dup2

File Descriptors Kernel maintains file descriptors to

reference open files Non-negative integer

The shell defines 3 by convention stdin 0 stdout 1 stderr 2

Open Functions defined in <unistd.h> open function

int open(const char *pathname, int flags);int open(const char *pathname, int flags, mode_t mode);

Open (cont) Required Flags

O_RDONLY O_WRONLY O_RDWR

Useful Optional Flags O_APPEND O_CREAT O_EXCL (used with O_CREAT to cause the

open function to fail if file already exists) O_TRUNC

Open Exampleint fd;fd = open(“data.txt”, O_RDONLY |

O_CREAT | O_TRUNC, mode);if(fd < 0) { perror(“Unable to open file”); exit(1);}

creat creat function

int creat(const char *pathname, mode_t mode);

Same as using open with O_WRONLY | O_CREAT | O_TRUNC, mode)

close close function

int close(int fd); Returns 0 on success or -1 on error Kernel will close any file

descriptors left open by a process when it exits

lseek lseek function

off_t lseek(int fildes, off_t offset, int whence); Returns current file offset if sucessful,

otherwise -1 whence

SEEK_SET – Sets offset from beginning of file SEEK_CUR – Relative offset from current

position SEEK_END – Sets offset past end of file

Unbuffered I/O - read read function

ssize_t read(int fd, void *buf, size_t count); Reads count bytes from the file

indicated by the file descriptor fd into the buffer pointed to by buf

Returns -1 on error, 0 on end of file and number of bytes read otherwise (may be less that the number requested)

Unbuffered I/O - write write function

ssize_t write(int fd, const void *buf, size_t count);

Attempts to write out count bytes from buffer buf to file indicated by file descriptor fd.

Returns -1 on error, number of bytes written otherwise

Buffer Size and Efficiency Program in Fig 3.4 page 69 Table in Fig 3.5 page 70

File Sharing Multiple processes can access a file

simultaneously 3 Kernel level data structures are

used to keep track of file information Process table File table v-node

See Fig 3.6 and 3.7 on pages 72, 73

Atomic Operations Appending to a file

Separate lseek and write operations can result in race conditions

Open file with O_APPEND to force an atomic seek to end of file before each write

Creating a file Separate check for file existence and

creation can result in race condition Use both the O_CREAT and O_EXCL flags

dup and dup2 Both are functions for duplicating a file

descriptor within a process and return -1 on error

dup int dup(int oldfd); Returns a new fd that points to the same

entry in the file table dup2

int dup2(int oldfd, int newfd); Returns newfd which now points to the

same entry in the file table as oldfd

fcntl Allows us to change properties of

an already opened file Several uses. See page 78 for

more info All return -1 on error, but meaning

of other return types depends on specific use

ioctl int ioctl(int d, int request, ...); Catchall for I/O functions that don’t

fit well in other headers.