I/O Nov 15, 2001A typical hardware systemReading a disk sector (1)Reading a disk sector (2)Reading a disk sector (3)Unix filesUnix file typesI/O functionsStandard I/O vs Unix I/OMeta-data returned by stat()Kernel’s file representationKernel’s file representation (cont)Unix file I/O: open()Unix file I/O: read()Unix file I/O: write()Robustly dealing with read short counts (adapted from Stevens)Robustly dealing with write short counts (adapted from Stevens)Unix file I/O: lseek()Unix file I/O: dup2()dup2() exampleSlide 21Buffering in Standard I/OBuffering in actionUsing buffering to robustly read text lines (Stevens)Robustly reading text lines (cont)mmap() revisitedVisualizing mmap()mmap() exampleFor further informationI/O Nov 15, 2001Topics•Files•Unix I/O•Standard I/Oclass24.ppt15-213“The course that gives CMU its Zip!”CS 213 F’01– 2 –class24.pptA typical hardware systemmainmemoryI/O bridgebus interfaceALUregister fileCPU chipsystem bus memory busdisk controllergraphicsadapterUSBcontrollermouse keyboard monitordiskI/O busExpansion slots forother devices suchas network adapters.CS 213 F’01– 3 –class24.pptReading a disk sector (1) mainmemoryALUregister fileCPU chipdisk controllergraphicsadapterUSBcontrollermouse keyboard monitordiskI/O busbus interfaceCPU initiates a disk read by writing a command, logical block number, and destination memory address to a port (address) associated with disk controller.CS 213 F’01– 4 –class24.pptReading a disk sector (2)mainmemoryALUregister fileCPU chipdisk controllergraphicsadapterUSBcontrollermouse keyboard monitordiskI/O busbus interfaceDisk controller reads the sector and performs a direct memory access (DMA) transfer into main memory.CS 213 F’01– 5 –class24.pptReading a disk sector (3)mainmemoryALUregister fileCPU chipdisk controllergraphicsadapterUSBcontrollermouse keyboard monitordiskI/O busbus interfaceWhen the DMA transfer completes, the disk controller notifies the CPU with an interrupt (i.e., asserts a special “interrupt” pin on the CPU)CS 213 F’01– 6 –class24.pptUnix filesA Unix file is a sequence of n bytes:•B0, B1, .... , Bk , .... , Bn-1All I/O devices are represented as files:•/dev/sda2 (/usr disk partition)•/dev/tty2 (terminal)Even the kernel is represented as a file:•/dev/kmem (kernel memory image) •/proc (kernel data structures)Kernel identifies each file by a small integer descriptor:•0: standard input•1: standard output•2: standard errorCS 213 F’01– 7 –class24.pptUnix file typesRegular file•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 comunicationSocket•A file type used for network communication between processesCS 213 F’01– 8 –class24.pptI/O functionsHardware devicesUnix I/O system calls(unbuffered) Standard I/O functions(buffered)C Application programfopen freadfwrite fprintffscanf fgetsfseek fcloseopen readwrite lseekstat closekernelapplicationprocessCS 213 F’01– 9 –class24.pptStandard I/O vs Unix I/OWhen 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.CS 213 F’01– 10 –class24.pptMeta-data returned by stat()/* * file info returned by the stat function */struct stat { dev_t st_dev; /* device */ ino_t st_ino; /* inode */ mode_t st_mode; /* protection and file type */ nlink_t st_nlink; /* number of hard links */ uid_t st_uid; /* user ID of owner */ gid_t st_gid; /* group ID of owner */ dev_t st_rdev; /* device type (if inode device) */ off_t st_size; /* total size, in bytes */ unsigned long st_blksize; /* blocksize for filesystem I/O */ unsigned long st_blocks; /* number of blocks allocated */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last change */};CS 213 F’01– 11 –class24.pptKernel’s file representationparent’sper processdescriptor tablei-node table entriesfile posrefcnt = 1......st_modest_size...st_modest_sizefile posrefcnt = 1...file Afile Bfile Afile BBefore fork:CS 213 F’01– 12 –class24.pptKernel’s file representation (cont)After fork (processes inherit open file tables):parent’sper processdescriptor tablei-node table entriesfile posrefcnt = 2......st_modest_size...st_modest_sizefile posrefcnt = 2...file Afile Bfile Afile Bchild’sper processdescriptor tableCS 213 F’01– 13 –class24.pptUnix 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 occurredpredefined file descriptors:•0: stdin•1: stdout• 2: stderrint fd; /* file descriptor */if ((fd = open(“/etc/hosts”, O_RDONLY)) < 0) { perror(“open”); exit(1);}CS 213 F’01– 14 –class24.pptUnix file I/O: read()read() allows a program to fetch the contents of a file from the current file position.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 bufchar 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);}CS 213 F’01– 15 –class24.pptUnix file I/O: write()write() allows a program to modify the file contents starting at the current file position.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.char buf[512];int fd; /* file descriptor */int nbytes; /* number
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