Carnegie Mellon System Level I O 15 213 Introduction to Computer Systems 14th Lecture Oct 12 2010 Instructors Randy Bryant and Dave O Hallaron 1 Carnegie Mellon Today Unix I O RIO robust I O package Metadata sharing and redirection Standard I O Conclusions and examples 2 Carnegie Mellon Unix Files A Unix file is a sequence of m bytes B0 B1 Bk Bm 1 All 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 proc kernel memory image kernel data structures 3 Carnegie Mellon Unix File Types Regular file File containing user app data binary text whatever OS does not know anything about the format other than sequence of bytes akin to main memory Directory file A file that contains the names and locations of other files Character special and block special files Terminals character special and disks block special FIFO named pipe A file type used for inter process communication Socket A file type used for network communication between processes 4 Carnegie Mellon Unix I O Key Features Elegant mapping of files to devices allows kernel to export simple interface called Unix I O Important idea All input and output is handled in a consistent and uniform way Basic Unix I O operations system calls Opening and closing files open and close Reading and writing a file read and write Changing the current file position seek indicates next offset into file to read or write lseek B0 B1 Bk 1 Bk Bk 1 Current file position k 5 Carnegie Mellon Opening Files Opening a file informs the kernel that you are getting ready to access that file int fd file descriptor if fd open etc hosts O RDONLY 0 perror open exit 1 Returns a small identifying integer file descriptor fd 1 indicates that an error occurred Each process created by a Unix shell begins life with three open files associated with a terminal 0 standard input 1 standard output 2 standard error 6 Carnegie Mellon Closing Files Closing a file informs the kernel that you are finished accessing that file int fd file descriptor int retval return value if retval close fd 0 perror close exit 1 Closing an already closed file is a recipe for disaster in threaded programs more on this later Moral Always check return codes even for seemingly benign functions such as close 7 Carnegie Mellon Reading Files Reading a file copies bytes from the current file position to memory and then updates file position char buf 512 int fd file descriptor int nbytes number of bytes read Open file fd Then read up to 512 bytes from file fd if nbytes read fd buf sizeof buf 0 perror read exit 1 Returns number of bytes read from file fd into buf Return type ssize t is signed integer nbytes 0 indicates that an error occurred Short counts nbytes sizeof buf are possible and are not errors 8 Carnegie Mellon Writing Files Writing a file copies bytes from memory to the current file position and then updates current file position char buf 512 int fd file descriptor int nbytes number of bytes read Open the file fd Then write up to 512 bytes from buf to file fd if nbytes write fd buf sizeof buf 0 perror write exit 1 Returns number of bytes written from buf to file fd nbytes 0 indicates that an error occurred As with reads short counts are possible and are not errors 9 Carnegie Mellon Simple Unix I O example Copying standard in to standard out one byte at a time include csapp h int main void char c while Read STDIN FILENO c 1 0 Write STDOUT FILENO c 1 exit 0 cpstdin c Note the use of error handling wrappers for read and write Appendix A 10 Carnegie Mellon Dealing with Short Counts Short counts can occur in these situations Encountering end of file EOF on reads Reading text lines from a terminal Reading and writing network sockets or Unix pipes Short counts never occur in these situations Reading from disk files except for EOF Writing to disk files One way to deal with short counts in your code Use the RIO Robust I O package from your textbook s csapp c file Appendix B 11 Carnegie Mellon Today Unix I O RIO robust I O package Metadata sharing and redirection Standard I O Conclusions and examples 12 Carnegie Mellon The RIO Package RIO is a set of wrappers that provide efficient and robust I O in apps such as network programs that are subject to short counts RIO provides two different kinds of functions Unbuffered input and output of binary data rio readn and rio writen Buffered input of binary data and text lines rio readlineb and rio readnb Buffered RIO routines are thread safe and can be interleaved arbitrarily on the same descriptor Download from http csapp cs cmu edu public code html src csapp c and include csapp h 13 Carnegie Mellon Unbuffered RIO Input and Output Same interface as Unix read and write Especially useful for transferring data on network sockets include csapp h ssize t rio readn int fd void usrbuf size t n ssize t rio writen int fd void usrbuf size t n Return num bytes transferred if OK 0 on EOF rio readn only 1 on error rio readn returns short count only if it encounters EOF Only use it when you know how many bytes to read rio writen never returns a short count Calls to rio readn and rio writen can be interleaved arbitrarily on the same descriptor 14 Carnegie Mellon Implementation of rio readn rio readn robustly read n bytes unbuffered ssize t rio readn int fd void usrbuf size t n size t nleft n ssize t nread char bufp usrbuf while nleft 0 if nread read fd bufp nleft 0 if errno EINTR interrupted by sig handler return nread 0 and call read again else return 1 errno set by read else if nread 0 break EOF nleft nread bufp nread return n nleft return 0 csapp c 15 Carnegie Mellon Buffered I O Motivation Applications often read write one character at a time getc putc ungetc gets fgets Read line of text on character at a time stopping at newline Implementing as Unix I O calls expensive read and write require Unix kernel calls 10 000 clock cycles Solution Buffered read Use Unix read to grab block of bytes User input functions take one byte at a time from buffer Refill buffer when empty Buffer already read unread 16 Carnegie Mellon Buffered I O Implementation For reading from file File has associated buffer to hold bytes that have been read from file but not yet read by user code rio cnt Buffer already read rio buf rio bufptr Layered on Unix file not in buffer unread Buffered Portion already read unread unseen Current File Position 17 Carnegie Mellon Buffered I O Declaration All information contained in struct rio
View Full Document
Unlocking...