15 213 The course that gives CMU its Zip I O Nov 15 2001 Topics Files Unix I O Standard I O class24 ppt A typical hardware system CPU chip register file ALU system bus memory bus main memory I O bridge bus interface I O bus USB controller mouse keyboard graphics adapter disk controller Expansion slots for other devices such as network adapters monitor disk class24 ppt 2 CS 213 F 01 Reading a disk sector 1 CPU chip register file ALU CPU initiates a disk read by writing a command logical block number and destination memory address to a port address associated with disk controller main memory bus interface I O bus USB controller mouse keyboard graphics adapter disk controller monitor disk class24 ppt 3 CS 213 F 01 Reading a disk sector 2 CPU chip register file ALU Disk controller reads the sector and performs a direct memory access DMA transfer into main memory main memory bus interface I O bus USB controller mouse keyboard graphics adapter disk controller monitor disk class24 ppt 4 CS 213 F 01 Reading a disk sector 3 CPU chip register file ALU When the DMA transfer completes the disk controller notifies the CPU with an interrupt i e asserts a special interrupt pin on the CPU main memory bus interface I O bus USB controller mouse keyboard graphics adapter disk controller monitor disk class24 ppt 5 CS 213 F 01 Unix A Unix file is a sequence files of n bytes B0 B1 Bk Bn 1 All I O devices are represented as files dev sda2 dev tty2 usr disk partition terminal Even the kernel is represented as a file dev kmem kernel memory image kernel data structures proc Kernel identifies each file by a small integer descriptor 0 standard input 1 standard output 2 standard error class24 ppt 6 CS 213 F 01 Regular file Unix file types Binary or text file Unix does not know the difference Directory file A file that contains names and locations of other files Character special file For certain types of streaming devices e g terminals Block special file A file type typically used for disks FIFO named pipe A file type used for interprocess comunication Socket A file type used for network communication between processes class24 ppt 7 CS 213 F 01 I O functions fopen fwrite fscanf fseek fread fprintf fgets fclose open write stat read lseek close class24 ppt C Application program application process Standard I O functions buffered Unix I O system calls unbuffered kernel Hardware devices 8 CS 213 F 01 Standard I O vs Unix I O When should I use the Standard I O functions Whenever possible Many C programmers are able to do all of their work using the standard I O functions Why would I ever need to use the lower level Unix I O functions Fetching file metadata e g size modification date must use lower level stat function Doing network programming there are some bad interactions between sockets and the standard library functions Needing high performance class24 ppt 9 CS 213 F 01 Meta data returned by stat file info returned by the stat struct stat dev t st dev ino t st ino mode t st mode nlink t st nlink uid t st uid gid t st gid dev t st rdev off t st size unsigned long st blksize unsigned long st blocks time t st atime time t st mtime time t st ctime class24 ppt 10 function device inode protection and file type number of hard links user ID of owner group ID of owner device type if inode device total size in bytes blocksize for filesystem I O number of blocks allocated time of last access time of last modification time of last change CS 213 F 01 Kernel s file representation Before fork file A parent s per process descriptor table file pos refcnt 1 i node table entries file A st mode st size file B file B file pos refcnt 1 class24 ppt 11 st mode st size CS 213 F 01 Kernel s file representation cont After fork processes inherit open file tables parent s per process descriptor table file A file pos refcnt 2 i node table entries file A st mode st size file B child s per process descriptor table class24 ppt file B file pos refcnt 2 12 st mode st size CS 213 F 01 Unix file I O open Must open a file before you can do anything else open returns a small integer file descriptor fd 0 indicates that an error occurred predefined file descriptors 0 stdin 1 stdout 2 stderr int fd file descriptor if fd open etc hosts O RDONLY 0 perror open exit 1 class24 ppt 13 CS 213 F 01 Unix file I O read read allows a program to fetch the contents of a file from the current file position char buf 512 int fd file descriptor int nbytes number of bytes read open the file read up to 512 bytes from file fd if nbytes read fd buf sizeof buf 0 perror read exit 1 read returns the number of bytes read from file fd nbytes 0 indicates that an error occurred short counts nbytes sizeof buf are possible and not errors places nbytes bytes into memory address buf class24 ppt 14 CS 213 F 01 Unix file I O write write allows a program to modify the file contents starting at the current file position char buf 512 int fd file descriptor int nbytes number of bytes read open the file write up to 512 bytes from buf to file fd if nbytes write fd buf sizeof buf 0 perror write exit 1 write returns the number of bytes written from buf to file fd nbytes 0 indicates that an error occurred short counts are possible and are not errors class24 ppt 15 CS 213 F 01 Robustly dealing with read short counts ssize t readn int fd void from buf size t count adapted Stevens size t nleft count ssize t nread char ptr buf while nleft 0 if nread read fd ptr nleft 0 if errno EINTR nread 0 and call read again else return 1 errno set by read else if nread 0 break EOF nleft nread ptr nread return count nleft return 0 class24 ppt 16 CS 213 F 01 Robustly dealing with write short counts adapted from Stevens ssize t writen int fd const void buf size t count size t nleft count ssize t nwritten const char ptr buf while nleft 0 if nwritten write fd ptr nleft 0 if errno EINTR nwritten 0 and call write again else return 1 errorno set by write nleft nwritten ptr nwritten return count class24 ppt 17 CS 213 F 01 Unix file I O lseek lseek changes the current file position int fd file descriptor open the file Move the file position to byte offset 100 if lseek fd 100 SEEK SET 0 perror lseek exit 1 lseek changes the current file position class24 ppt 18 CS 213 F 01 Unix file I O dup2 …
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