15213 Recitation Section C Shimin Chen Nov 18 2002 Outline Robust I O package Chapter 11 practice problems Important Dates Lab 6 Malloc due on Thursday Nov 21 Lab 7 Proxy due on Thursday Dec 5 Final Exam Tuesday Dec 17 15213 Recitation C 2 Shimin Chen Robust I O RIO csapp c and csapp h Why Handles interrupted system calls Handles short counts Good for network programming Two parts Unbuffered I O Buffered I O 15213 Recitation C 3 Shimin Chen Rio Unbuffered Input Output Use Unix I O No internal buffering Useful for reading writing binary data to from networks ssize t rio readn int fd void usrbuf size t n reads n bytes from fd and put into usrbuf only returns short count on EOF ssize t rio writen int fd void usrbuf size t n writes n bytes from usrbuf to fd never returns short count 15213 Recitation C 4 Shimin Chen RIO Buffered Input Internal buffers define RIO BUFSIZE 8192 typedef struct int rio fd int rio cnt char rio bufptr char rio buf RIO BUFSIZE rio t void rio readinitb rio t rp int fd ssize t rio readlineb rio t rp void usrbuf size t maxlen ssize t rio readnb rio t rp void usrbuf size t n 15213 Recitation C 5 Shimin Chen Rio Buffered Input void rio readinitb rio t rp int fd called only once per open descriptor associate fd with a read buffer rio t structure pointed to by rp ssize t rio readlineb rio t rp void usrbuf size t maxlen for reading text lines read a line until n or maxlen 1 chars from file rp to usrbuf terminate the text line with null zero character returns number of chars read ssize t rio readnb rio t rp void usrbuf size t n reads n bytes from rp into usrbuf Result string is NOT null terminated Returns number of bytes read 15213 Recitation C 6 Shimin Chen rio readlineb ssize t rio readlineb rio t rp void usrbuf size t maxlen int n rc char c bufp usrbuf for n 1 n maxlen n if rc rio read rp c 1 1 bufp c if c n break else if rc 0 if n 1 return 0 EOF no data read else break EOF some data was read else return 1 error bufp 0 return n 15213 Recitation C 7 Shimin Chen Interleaving RIO Read Functions Do not interleave calls on the same fd between the buffered and unbuffered functions Within each set it is ok unbuffered buffered rio readinitb rio readn rio writen rio readlineb rio readnb Why 15213 Recitation C 8 Shimin Chen Rio Error Checking RIO functions handle Short counts interrupted system calls All functions have upper case equivalents Rio readn Rio writen Rio readlineb Rio readnb etc call unix error if the function encounters an error But EPIPE errors for Lab 7 EPIPE should not terminate the process 15213 Recitation C 9 Shimin Chen Problems from Chapter 11 11 1 11 5 15213 Recitation C 10 Shimin Chen Problem 11 1 What is the output of the following program include csapp h int main int fd1 fd2 fd1 Open foo txt O RDONLY 0 Close fd1 fd2 Open baz txt O RDONLY 0 printf fd2 d n fd2 exit 0 15213 Recitation C 11 Shimin Chen Answer to 11 1 stdin descriptor 0 stdout descriptor 1 stderr descriptor 2 open always returns lowest unopened descriptor First open returns 3 close frees it So second open also returns 3 Program prints fd2 3 15213 Recitation C 12 Shimin Chen File Sharing Descriptor table Each process has its own Child inherits from parents File Table set of all open files Shared by all processes Reference count of number of file descriptors pointing to each entry File position V node table Contains information in the stat structure Shared by all processes 15213 Recitation C 13 Shimin Chen Problem 11 2 Suppose that foobar txt consists of the 6 ASCII characters foobar Then what is the output of the following program include csapp h int main int fd1 fd2 char c fd1 Open foobar txt O RDONLY 0 fd2 Open foobar txt O RDONLY 0 Read fd1 c 1 Read fd2 c 1 printf c c n c exit 0 15213 Recitation C 14 Shimin Chen Answer to 11 2 The descriptors fd1 and fd2 each have their own open file table entry so each descriptor has its own file position for foobar txt Thus the read from fd2 reads the first byte of foobar txt and the output is c f and not c o as you might have thought initially 15213 Recitation C 15 Shimin Chen Problem 11 3 As before suppose foobar txt consists of 6 ASCII characters foobar Then what is the output of the following program include csapp h int main int fd char c fd Open foobar txt O RDONLY 0 if Fork 0 Read fd c 1 exit 0 Wait NULL Read fd c 1 printf c c n c exit 0 15213 Recitation C 16 Shimin Chen Answer to 11 3 Child inherit s the parent s descriptor table So child and parent share an open file table entry refcount 2 Hence they share a file position c o 15213 Recitation C 17 Shimin Chen Problem 11 4 How would you use dup2 to redirect standard input to descriptor 5 int dup2 int oldfd int newfd copies descriptor table entry oldfd to descriptor table entry newfd 15213 Recitation C 18 Shimin Chen Answer to 11 4 dup2 5 0 or dup2 5 STDIN FILENO 15213 Recitation C 19 Shimin Chen Problem 11 5 Assuming that foobar txt consists of 6 ASCII characters foobar Then what is the output of the following program include csapp h int main int fd1 fd2 char c fd1 Open foobar txt O RDONLY 0 fd2 Open foobar txt O RDONLY 0 Read fd2 c 1 Dup2 fd2 fd1 Read fd1 c 1 printf c c n c exit 0 15213 Recitation C 20 Shimin Chen Answer to 11 5 We are redirecting fd1 to fd2 fd1 now points to the same open file table entry as fd2 So the second Read uses the file position offset of fd2 c o 15213 Recitation C 21 Shimin Chen
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