Recitation 11 (Nov. 22) Outline Lab 6: interposition test Error handling I/O practice problem Reminders Lab 6: Due Tuesday Minglong Shao Office hours: Thursdays 5-6PM Wean Hall 1315

Exam2 statistics 118 exams Highest score 74/74 (2 students) Lowest score 24 Average score Median score 49

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

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

Different error handling styles Unix-style e.g. kill, signal, fork, etc. Posix-style e.g. pthread_create DNS-style e.g. gethostbyname

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)) < 0) unix_error(“Error in wait”);

Unix-style error handling cont’d if ((pid = wait(NULL)) < 0) { perror(“Error in wait”); exit (0); }

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”);

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”);

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 void Kill (pid_t pid, int signum) { int rc; if((rc = kill(pid, signum)) <0) unix_error(“Kill error”); } Example: wrappers

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(…)) > 0) printf(“Reaped %d\n”, (int)pid); if(errno != ECHILD) unix_error(“waitpid error”); }

I/O practice problems from Chapter ~ 11.5

Problem 11.1 #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); } What is the output of the following program?

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"

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

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); }

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 = f and not c = o

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); }

Answer to 11.3 Child inherits the parent’s descriptor table  Child & parent share an open file table entry (refcount = 2)  They share file position. c = o

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

Answer to 11.4 dup2(5,0); or dup2(5,STDIN_FILENO);

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); }

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

Have a good thanksgiving break!