Carnegie Mellon System Level I O 15 213 Introduc0on 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 redirecEon Standard I O Conclusions and examples 2 Carnegie Mellon Unix Files A Unix le is a sequence of m bytes B0 B1 Bk Bm 1 All I O devices are represented as les dev sda2 usr disk par00on dev tty2 terminal Even the kernel is represented as a le dev kmem proc kernel memory image kernel data structures 3 Carnegie Mellon Unix File Types Regular le 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 le A le that contains the names and loca0ons of other les Character special and block special les Terminals character special and disks block special FIFO named pipe A le type used for inter process communica0on Socket A le type used for network communica0on between processes 4 Carnegie Mellon Unix I O Key Features Elegant mapping of les 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 operaEons system calls Opening and closing les open and close Reading and wri0ng a le read and write Changing the current le posi on seek indicates next o set into le to read or write lseek B0 B1 Bk 1 Bk Bk 1 Current le posiEon k 5 Carnegie Mellon Opening Files Opening a le informs the kernel that you are geUng ready to access that le int fd file descriptor if fd open etc hosts O RDONLY 0 perror open exit 1 Returns a small idenEfying integer le descriptor fd 1 indicates that an error occurred Each process created by a Unix shell begins life with three open les associated with a terminal 0 standard input 1 standard output 2 standard error 6 Carnegie Mellon Closing Files Closing a le informs the kernel that you are nished accessing that le int fd file descriptor int retval return value if retval close fd 0 perror close exit 1 Closing an already closed le is a recipe for disaster in threaded programs more on this later Moral Always check return codes even for seemingly benign funcEons such as close 7 Carnegie Mellon Reading Files Reading a le copies bytes from the current le posiEon to memory and then updates le posiEon 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 le 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 WriEng Files WriEng a le copies bytes from memory to the current le posiEon and then updates current le posiEon 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 wriXen from buf to le 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 Eme 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 situaEons Encountering end of le EOF on reads Reading text lines from a terminal Reading and wri0ng network sockets or Unix pipes Short counts never occur in these situaEons Reading from disk les except for EOF Wri0ng to disk les One way to deal with short counts in your code Use the RIO Robust I O package from your textbook s csapp c le Appendix B 11 Carnegie Mellon Today Unix I O RIO robust I O package Metadata sharing and redirecEon Standard I O Conclusions and examples 12 Carnegie Mellon The RIO Package RIO is a set of wrappers that provide e cient and robust I O in apps such as network programs that are subject to short counts RIO provides two di erent kinds of funcEons Unbu ered input and output of binary data rio readn and rio writen Bu ered input of binary data and text lines rio readlineb and rio readnb Bu ered RIO rou0nes are thread safe and can be interleaved arbitrarily on the same descriptor Download from hXp csapp cs cmu edu public code html src csapp c and include csapp h 13 Carnegie Mellon Unbu ered 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 ImplementaEon 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 Bu ered I O MoEvaEon ApplicaEons o en read write one character at a Eme getc putc ungetc gets fgets Read line of text on character at a 0me stopping at newline ImplemenEng as Unix I O calls expensive read and write require Unix kernel calls 10 000 clock cycles SoluEon Bu ered read Use Unix read to grab block of bytes User input func0ons take one byte at a 0me from bu er Re ll bu er when empty Bu er already read unread 16 Carnegie Mellon Bu ered I O ImplementaEon For reading from le File has associated bu er to hold bytes that have been read from le but not yet read by user code rio cnt Bu er already read rio buf unread rio bufptr Layered on Unix le Bu ered PorEon not in bu er already read unread unseen Current File PosiEon 17 Carnegie Mellon Bu ered I O DeclaraEon All informaEon …
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