Recitation%2011%20(Nov.%2022)

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Recitation 11 (Nov. 22)

• Outline
• Lab 6 interposition test
• Error handling
• I/O practice problem
• Reminders
• Lab 6
• Due Tuesday

• Minglong Shao
• shaoml213_at_cs.cmu.edu
• Office hours
• Thursdays 5-6PM
• Wean Hall 1315

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Exam2 statistics

• 118 exams
• Highest score 74/74 (2 students)
• Lowest score 24
• Average score 49.94
• Median score 49

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L6 Interposition test

• Build shared libraries mm.so and memlib.so
• make mm.so memlib.so
• gcc fpic shared o mm.so mm-student.c
• gcc fpic shared o memlib.so memlib.c
• Set LD_PRELOAD
• bash
• export LD_PRELOAD/path/to/mm.so
  /path/to/memlib.so
• Remember to restore LD_PRELOAD
• export LD_PRELOAD
• Make sure the heap is initialized

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Error handling

• Always check return code of system calls
• There are subtle ways that things can go wrong
• Use the status info kernel provides us
• Appendix B

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Different error handling styles

• Unix-style
• e.g. kill, signal, fork, etc.
• Posix-style
• e.g. pthread_create
• DNS-style
• e.g. gethostbyname

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Unix-style error handling

• Special return value when encounter error (always
  1)
• Global variable errno set to an error code
• Use strerror function for text description of
  errno
• Or use perror

void unix_error(char msg) fprintf(stderr,
s s\n, msg, strerror(errno))
exit(0) if ((pid wait(NULL)) lt 0)
unix_error(Error in wait)
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Unix-style error handling contd
if ((pid wait(NULL)) lt 0) perror(Error
in wait) exit (0)
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Posix-style error handling

• Return value indicates success (0) or failure
  (nonzero)
• Useful results returned in function arguments

void posix_error(int code, char msg)
fprintf(stderr, s s\n, msg,
strerror(code)) exit(0) if ((retcode
pthread_create()) ! 0) posix_error(retcode,
Error in pthread)
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DNS-style error handling

• Return a NULL pointer on failure
• Set the global h_errno variable

void dns_error(char msg) fprintf(stderr,
s DNS error d\n, msg, h_errno)
exit(0) if ((p gethostbyname(name))
NULL) dns_error(Error in gethostbyname)
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Example wrappers
void Kill (pid_t pid, int signum) int rc
if((rc kill(pid, signum)) lt0) unix_error(Kill
error)

• Appendix B csapp.h and csapp.c
• Unix-Style, for kill function
• Behaves exactly like the base function if no
  error
• Prints informative message and terminates the
  process

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Handle errors gracefully

• The wrappers shown above calls exit()
• In many situations, we want to handle errors more
  gracefully.
• For example web server, etc.

void sigchld_handler(int signum) pid_t pid
while((pid waitpid()) gt 0) printf(Reaped
d\n, (int)pid) if(errno ! ECHILD)
unix_error(waitpid error)
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I/O practice problems from Chapter 11

• 11.1 11.5

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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)
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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"

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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

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Problem 11.2

• foobar.txt has 6 ASCII characters "foobar".
• 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)
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Answer to 11.2

• fd1 and fd2 have different open file table
  entries ? have their own file positions for
  foobar.txt ? fd2 reads the first byte of
  foobar.txt
• The output is c fand not c o

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Problem 11.3

• foobar.txt has 6 ASCII characters "foobar".
• 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)
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Answer to 11.3

• Child inherits the parents descriptor table
• ? Child parent share an open file table entry
  (refcount 2)
• ?They share file position.
• c o

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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

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Answer to 11.4

• dup2(5,0)
• or
• dup2(5,STDIN_FILENO)

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Problem 11.5

• foobar.txt has 6 ASCII characters "foobar".
• 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)
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Answer to 11.5

• Redirect fd1 to fd2
• ? fd1 points to the same open file table entry as
  fd2
• ? The second Read uses the file position offset
  of fd2.
• c o

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Have a good thanksgiving break!