University of New Mexico1System-Level I/OECE 335: Integrated Software SystemsInstructor:Edward NavaUniversity of New Mexico2Today Unix I/O RIO (robust I/O) package Metadata, sharing, and redirection Standard I/O Conclusions and examplesUniversity of New Mexico3Unix 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)University of New Mexico4Unix 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 processesUniversity of New Mexico5Unix 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()B0B1• • • Bk-1BkBk+1• • •Current file position = kUniversity of New Mexico6Opening Files Opening a file informs the kernel that you are getting ready to access that file 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 errorint fd; /* file descriptor */if ((fd = open("/etc/hosts", O_RDONLY)) < 0) {perror("open");exit(1);}University of New Mexico7Closing Files Closing a file informs the kernel that you are finished accessing that file 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()int fd; /* file descriptor */int retval; /* return value */if ((retval = close(fd)) < 0) {perror("close");exit(1);}University of New Mexico8Reading Files Reading a file copies bytes from the current file position to memory, and then updates file position 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!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);}University of New Mexico9Writing Files Writing a file copies bytes from memory to the current file position, and then updates current file position 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!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);}University of New Mexico10Simple 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);}Note the use of error handling wrappers for read and write (Appendix A).cpstdin.cUniversity of New Mexico11Dealing 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.cfile (Appendix A)University of New Mexico12Today Unix I/O RIO (robust I/O) package Metadata, sharing, and redirection Standard I/O Conclusions and examplesUniversity of New Mexico13The 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.hUniversity of New Mexico14Unbuffered RIO Input and Output Same interface as Unix read and write Especially useful for transferring data on network sockets 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#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 errorUniversity of New Mexico15Implementation 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 */elsereturn -1; /* errno set by read() */ } else if (nread == 0)break; /* EOF */nleft -= nread;bufp += nread;}return (n - nleft); /* return >= 0 */}csapp.cUniversity of New Mexico16Buffered I/O: Motivation Applications often read/write one character at a time getc, putc, ungetc