CMPS 105 Systems Programming. Prof. Darrell Long E2.371

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1 + CMPS 105 Systems Programming Prof. Darrell Long E2.371

2 + Chapter 3: File I/O 2

3 + File I/O 3 n What attributes do files need? n Data storage n Byte stream n Named n Non-volatile n Shared n Protected n... n creat(), open(), close(), read(), write(), stat()

4 + #include <stdio.h> 4 n FILE n A structure containing file stream information n stdin - standard input stream n stdout - standard output stream n stderr - standard error output stream n EOF - End Of File return value n is a special negative integer constant

5 + creat() and open() 5 n A file descriptor n Index into kernel table n Entry contains relevant info n Calls may fail if: n Not permitted (ACL) n Wrong options n Issues: position, size, modification time, creation time, contents, etc...

6 + Accessing files 6 n System manages disk n Abstraction and sharing-management n Protection n Can only access via system calls (kernel) n Permissions (how does the kernel handle this?)

7 + Kernel: File data structures 7 n Process table - every process has an entry n File descriptor flags n A pointer to a file table entry n File table - every open file has an entry n File status flags (read, write, append, sync, etc...) n Current file offset n Pointer to the v-node table entry n V-nodes - open files have v-nodes n Type of file Not true! n Functions that operate on the file n i-node

8 + What is a file descriptor? 8 n A non-negative integer n File descriptors are managed by the kernel n Every process has its own set of file descriptors n Existing descriptors: n 0 - standard input n 1 - standard output n 2 - standard error

9 + creat() 9 n creat() - create a new file or rewrite an existing one n int creat(const char* pathname, mode_t mode); n pathname - path to new file n mode - files permissions n Returns a file descriptor or -1 and error status n Index into kernel table n Entry contains relevant info

10 + open() 10 n open() - opens a file, or creates one if it doesn t exist n int open(const char* pathname, int flags, mode_t mode); n pathname - path to new file n flags - file creation options and file status options n mode - file permissions n Returns a file descriptor or -1 and error status n Index into kernel table n Entry contains relevant info

11 + close() 11 n close() - closes an file descriptor n close(int fildes); n fildes - file descriptor n Returns 0 on success or -1 and error status n All open files are closed automatically when a process terminates n What closes them?

12 + read() 12 n read() - reads bytes from file n ssize_t read(int fildes, void *buff, size_t n_bytes); n fildes - file descriptor n buff - buffer that will hold the bytes read n n_bytes - number of bytes to read n Returns number of bytes read, 0 for end of file (EOF), or -1 and error status

13 + write() 13 n write() - writes to a file n ssize_t write(int fildes, const void* buff, size_t count); n fildes - file descriptor n buff - buffer to be written n count - number of bytes to write from buff n Returns number of bytes written, or -1 and error status

14 + lseek() 14 n lseek() - reposition read/write file offset n off_t lseek(int fildes, off_t offset, int whence); n fildes - file descriptor n offset - offset depending on whence n whence - determines where offset is to start from n SEEK_SET, SEEK_CUR, SEEK_END n Returns: current position or -1 and error status

15 + Example: Holy files Batman! 15 n lseek() can set the current file offset to be greater than the file s current size n What happens when we call read() write() from this offset? n If we can write() from this offset we create a file with a hole

16 + Example: Holy files Batman! 16 cont.

17 + Example: Holy files Batman! 17 cont. n This program outputs a file that is bytes, but takes up 20 blocks n That s 12 more blocks than an similar hole-less file n What about read()?

18 + I/O efficiency 18 n Different buffer sizes n Bigger buffer is better n Why? n Discuss other efficiency concerns: n What is the problem with big buffers? n Sparse files, small files, big files n What other issues exist?

19 + File Sharing 19 n Two processes both sharing a file n Share the same v-node n v-node has EOF n Both processes would like to append to EOF n Processes both have current offset set to EOF with lseek n Process 1 finishes it s time quantum n Process 2 writes to EOF, making file larger n Process 1 resumes to current offset, which is no longer EOF n Process 1 over-writes process 2 s data

20 + Open Files 20

21 + Atomic Operations 21 n pread() and pwrite() combine the lseek() and read/write n Can t be interrupted between lseek() and read()/write() n current offset is not update

22 + sync(), fsync(), and fdatasync() 22 n File I/O is buffered by kernel before writing to disk n Kernel will eventually write buffer to disk n sync() - queues buffer for writing and returns n Does not wait for disk n update - a daemon that calls sync() ~30sec n fsync() - sync() for specified file n fdatasync() - fsync() for file data n

23 + dup() and dup2() 23 n dup2() - duplicate a file descriptor n int dup2(int oldfd, int newfd); n oldfd - old file descriptor n newfd - new file descriptor n Returns new descriptor or -1 and error status

24 + fcntl() 24 n fcntl() - manipulate file descriptor n int fcntl(int fildes, int cmd,...); n fildes - file descriptor n cmd - operation n... - optional third argument (determined by cmd) n Returns - Depends on the cmd chosen, or -1 and error status

25 + fnctl() example 25

26 + ioctl() 26 n ioctl() - control device n int ioctl(int fildes, int request,...); n fildes - file descriptor n request - device-dependent request code n... - untyped pointer to memory n Returns 0 on success or -1 and error status

27 + Recap 27 n Basic I/O functions provided in UNIX n read(),write(), open(), creat(), lseek(), close() n Files n How they are managed by the kernel and processes n File sharing and file properties n dup() & dup2(), fnctl(), ioctl()

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