File I/O

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

2. 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

3. Open • Functions defined in <unistd.h> • open functionint open(const char *pathname, int flags);int open(const char *pathname, int flags, mode_t mode);

4. 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

5. Open Example int fd; fd = open(“data.txt”, O_RDONLY | O_CREAT | O_TRUNC, mode); if(fd < 0) { perror(“Unable to open file”); exit(1); }

6. creat • creat function int creat(const char *pathname, mode_t mode); • Same as using open with O_WRONLY | O_CREAT | O_TRUNC, mode)

7. close • close functionint 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

8. lseek • lseek functionoff_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

9. Unbuffered I/O - read • read functionssize_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)

10. Unbuffered I/O - write • write functionssize_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

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

12. 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

13. 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

14. 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

15. 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

16. ioctl • int ioctl(int d, int request, ...); • Catchall for I/O functions that don’t fit well in other headers.