Chapter 3: File I/OFirstWhat is a file?Accessing FilesCreat and openopen()creat()close(int fildes)lseek()Read()Write()I/O Efficiency (section 3.9)File SharingFile Sharing (cont.)Discuss what happens when various operations occurDup and dup2fcntlIoctl()Chapter 3: File I/OCMPS 105: Systems ProgrammingProf. Scott BrandtT Th 2-3:45Soc Sci 2, Rm. 167First Questions? Programming Assignment 1 Programming Assignment 2 Class in general?What is a file? Data storage Byte stream Named Non-volatile Shared Protected …Accessing Files System manages disk Abstraction and manages sharing Protection Can only access via system calls Permissions Abstraction creat(), open(), close(), read(), write(), stat()Creat and open Creat creates a new file Open opens or creates a file Both return a file descriptor Index into kernel table Entry contains relevant info May fail if No file No permission …open() Sys/types.h, sys/stat.h, fcntl.h int open (const char *pathname, int oflag, …, /* mode_t mode */); Pathname may be absolute or relative Oflag = O_RDONLY, O_WRONLY, O_RDRW, O_APPEND, O_CREAT, O_EXCL, O_TRUNC, O_NOCTTY, O_NONBLOCK, O_SYNC Returns error statuscreat() Sys/types.h, sys/stat.h, fcntl.h int creat(const char *pathname, mode_t mode); Pathname Mode specifies permissions (section 4.5) Returns error statusclose(int fildes) Unistd.h Int close(int filedes); Closes an open file Takes a file descriptor as a parameter All open files are closed automatically when a process terminates Returns error statuslseek() Sys/types.h, unistd.h off_t lseek(int fildes, off_t offset, intwhence) Whence: SEEK_SET, SEEK_CUR, SEEK_END Returns current positionRead() Unistd.h Ssize_t read(int filedes, void *buff, size_t n_bytes); Returns number of bytes read or 0 (end of file) or -1 (error)Write() Unistd.h Ssize_t write(int filedes, const void *buff, size_t nbytes) Returns number of bytes written or error (file size or disk full)I/O Efficiency (section 3.9) Different buffer sizes Bigger buffer is better (page 57) Discuss why? Discuss other efficiency concerns What is the problem with big buffers Sparse files, small files, big files What other issues exist?File Sharing Process table Table of open file descriptors File descriptor flags A pointer to the file table entry File table of all open files The file status flags (read, write, append, sync, nonblocking, …); The current file offset A pointer to the v-node table entryFile Sharing (cont.) V-node structure Type of file Pointers to functions that operate on it i-node info File sizeDiscuss what happens when various operations occur Open Close Read Write Lseek Issues: position, size, modification time, creation time, contents, …Dup and dup2 Unistd.h Copy a file descriptor Int dup(int fildes); Uses lowest numbered available file descriptor Int dup2(int fildes, int fildes2); If fildes2 is open, it is closed first Can be used to reassign stdin and stdout Atomicfcntl Sys/types.h, unistd.h, fcntl.h Change the properties of an open file Int fcntl(int fildes, int cmd, … /*intarg*/); Cmds: F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETOWN, F_SETOWNIoctl() Everything else Int