15 213 System Level I O November 12 2007 Topics class20 ppt Unix I O Robust reading and writing Reading file metadata Sharing files I O redirection Standard I O 15 213 F 07 Unix I O Key Characteristics Classic Unix Linux I O Mainframe I O I O operates on linear streams of Bytes I O operates on structured records Can reposition insertion point and extend file at end I O tends to be synchronous Read or write operation block until data has been transferred Fine grained I O 2 One key stroke at a time Each I O event is handled by the kernel and an appropriate process Functions to locate insert remove update records I O tends to be asynchronous Overlap I O and computation within a process Coarse grained I O Process writes channel programs to be executed by the I O hardware Many I O operations are performed autonomously with one interrupt at completion 15 213 F 07 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 kernel memory image proc kernel data structures 3 15 213 F 07 Unix File Types Regular file Binary or text file Unix does not know the difference 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 interprocess communication Socket A file type used for network communication between processes 4 15 213 F 07 Unix I O The elegant mapping of files to devices allows kernel to export simple interface called Unix I O Key Unix idea All input and output is handled in a consistent and uniform way Basic Unix I O operations system calls Opening and closing files z open and close Changing the current file position seek z lseek not discussed Reading and writing a file z read and write 5 15 213 F 07 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 6 0 standard input 1 standard output 2 standard error 15 213 F 07 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 15 213 F 07 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 8 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 15 213 F 07 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 Transfers up to 512 bytes from address buf to file fd 9 15 213 F 07 Unix I O Example Copying standard input to standard output 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 Note the use of error handling wrappers for read and write Appendix B 10 15 213 F 07 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 How should you deal with short counts in your code Use the RIO Robust I O package from your textbook s csapp c file Appendix B 11 15 213 F 07 The RIO Package RIO is a set of wrappers that provide efficient and robust I O in applications such as network programs that are subject to short counts RIO provides two different kinds of functions Unbuffered input and output of binary data z rio readn and rio writen Buffered input of binary data and text lines z rio readlineb and rio readnb z Buffered RIO routines are thread safe and can be interleaved arbitrarily on the same descriptor Download from csapp cs cmu edu public ics code src csapp c csapp cs cmu edu public ics code include csapp h 12 15 213 F 07 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 it encounters EOF z Only use it when you know how many bytes to read 13 rio writen never returns a short count Calls to rio readn and rio writen can be interleaved arbitrarily on the same descriptor 15 213 F 07 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 14 15 213 F 07 Buffered I O Motivation I O Applications Read Write One Character at a Time getc putc ungetc gets z Read line of text stopping at newline Implementing as Calls to Unix I O Expensive Read Write involve require Unix kernel calls z 10 000 clock cycles Buffer already read unread Buffered Read 15 Use Unix read to grab block of characters User input functions take one …
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