Recitation: Signaling S04, Recitation, Section A Debug Multiple Processes using GDB Debug Multiple Processes using GDB Dup2 Dup2 Signaling Signaling L5 Due: This Wednesday L5 Due: This Wednesday

– 2 – , S’04 Debug Multiple Proc’s Using GDB attach pid attach pid pid: the process id of a running process set follow_fork_mode set follow_fork_mode only work in HP-UX and GNU/Linux (kernel >= ) Fish machines: Linux with kernel

– 3 – , S’04 Attach to a Running Process  Run the parent code and create the process tsh Directly in the shell In GDB  Get the pid $ ps [-e | axu] | grep tsh  Run gdb $ gdb tsh  Attach to the process (gdb) attach $ gdb tsh

– 4 – , S’04 Demo! Really that Easy? The process must be started outside GDB The process must be started outside GDB If you want to debug both the parent process and the child process, you need start another gdb (in another xterm) You have to type in the gdb & attach commands fast enough --- before the process actually finishes You have to type in the gdb & attach commands fast enough --- before the process actually finishes You have to modify the source code to let it wait Two methods sleep(10); int gdbf = 0; while (!gdbf); For Lab 5, it is more troublesome For Lab 5, it is more troublesome sdriver  runtrace  tsh/tshref  mycat

– 5 – , S’04 Attach to a Running Process  Run runtrace and create the process tsh Directly in the shell In GDB  Get the pid $ ps [-e | axu] | grep tsh  Run gdb $ gdb tsh  Attach to the process (gdb) attach $ gdb tsh

– 6 – , S’04 Dup2() Basic concepts on file handler Basic concepts on file handler File descriptor, file table, v-node table File sharing --- dup2() File sharing --- dup2() Practice problems Practice problems

– 7 – , S’04 How the Unix Kernel Represents Open Files Two descriptors referencing two distinct open disk files. Descriptor 1 (stdout) points to terminal, and descriptor 4 points to open disk file. fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table [one table per process] Open file table [shared by all processes] v-node table [shared by all processes] File pos refcnt=1... File pos refcnt=1... stderr stdout stdin File access... File size File type File access... File size File type File A (terminal) File B (disk) Info in stat struct

– 8 – , S’04 How Processes Share Files A child process inherits its parent’s open files. Here is the situation immediately after a fork fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor tables Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=2... File pos refcnt=2... Parent's table fd 0 fd 1 fd 2 fd 3 fd 4 Child's table File access... File size File type File access... File size File type File A File B

– 9 – , S’04 File Sharing Two distinct descriptors sharing the same disk file through two distinct open file table entries E.g., Calling open twice with the same filename argument fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table (one table per process) Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=1... File pos refcnt=1... File access... File size File type File A File B

– 10 – , S’04 I/O Redirection dup2(oldfd, newfd) Copies (per-process) descriptor table entry oldfd to entry newfd a b fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table before dup2(4,1) b b fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table after dup2(4,1)

– 11 – , S’04 I/O Redirection Example Before calling dup2(4,1), stdout (descriptor 1) points to a terminal and descriptor 4 points to an open disk file. Before calling dup2(4,1), stdout (descriptor 1) points to a terminal and descriptor 4 points to an open disk file. fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table (one table per process) Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=1... File pos refcnt=1... stderr stdout stdin File access... File size File type File access... File size File type File A File B

– 12 – , S’04 I/O Redirection Example (cont) After calling dup2(4,1), stdout is now redirected to the disk file pointed at by descriptor 4. fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table (one table per process) Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=0... File pos refcnt=2... File access... File size File type File access... File size File type File A File B

– 13 – , S’04 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

– 14 – , S’04 Problem 11.2 Suppose that consists of the 6 ASCII characters "". Then what is the output of the following program? 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); }

– 15 – , S’04 Answer to 11.2 The descriptors and each have their own open file table entry, so each descriptor has its own file position for. Thus, the read from reads the first byte of, and the output is and not as you might have thought initially. 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.

– 16 – , S’04 Problem 11.3 As before, suppose consists of 6 ASCII characters "". Then what is the output of the following program? 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); }

– 17 – , S’04 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

– 18 – , S’04 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

– 19 – , S’04 Answer to 11.4 dup2(5,0);or dup2(5,STDIN_FILENO);

– 20 – , S’04 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); }

– 21 – , S’04 Answer to 11.5 We are redirecting to. (fd1 now points to the same open file table entry as fd2). So the second uses the file position offset of. 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

– 22 – , S’04 Signaling Busy wait Busy wait waitpid() waitpid() Racing hazard Racing hazard

– 23 – , S’04 Busy Wait if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ /* do nothing */ }

– 24 – , S’04 Pause if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ pause(); } If signal handled before call to pause, then pause will not return when foreground process sends SIGCHLD

– 25 – , S’04 Sleep if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ sleep(1); }

– 26 – , S’04 waitpid () pid_t waitpid(pid_t pid, int *status, int options) pid: wait until child process with pid has terminated -1: wait for any child process status: tells why child terminated options: WNOHANG: return immediately if no children zombied »returns -1 WUNTRACED: report status of stopped children too

– 27 – , S’04 Status in Waitpid int status; waitpid(pid, &status, NULL); Macros to evaluate status: WIFEXITED(status): child exited normally WEXITSTATUS(status): return code when child exits WIFSIGNALED(status): child exited because of a signal not caught WTERMSIG(status): gives the terminating signal number WIFSTOPPED(status): child is currently stopped WSTOPSIG(status): gives the stop signal number

– 28 – , S’04 Race Hazard A data structure is shared by two pieces of code that can run concurrently A data structure is shared by two pieces of code that can run concurrently Different behaviors of program depending upon how the schedule interleaves the execution of code. Different behaviors of program depending upon how the schedule interleaves the execution of code.

– 29 – , S’04 eval & sigchld_handler Race Hazard sigchld_handler() { pid = waitpid(…); deletejob(pid); } eval() { pid = fork(); if(pid == 0) { /* child */ execve(…); } /* parent */ /* signal handler might run BEFORE addjob() */ addjob(…); }

– 30 – , S’04 Shell Signal HandlerChild fork() addjob() execve() exit() sigchld_handler() deletejobs() time An OK Schedule

– 31 – , S’04 Shell Signal HandlerChild fork() execve() exit() sigchld_handler() deletejobs() time addjob() Job added to job list after the signal handler tried to delete it! A Problematic Schedule

– 32 – , S’04 Blocking Signals sigchld_handler() { pid = waitpid(…); deletejob(pid); } eval() { sigprocmask(SIG_BLOCK, …) pid = fork(); if(pid == 0) { /* child */ sigprocmask(SIG_UNBLOCK, …) execve(…); } /* parent */ /* signal handler might run BEFORE addjob() */ addjob(…); sigprocmask(SIG_UNBLOCK, …) } More details (page 633)

– 33 – , S’04 Blocking Signals sigprocmask(SIG_BLOCK, (sigset_t *)SIGCHLD, NULL); sigprocmask(SIG_BLOCK, (sigset_t *)SIGINT, NULL); sigprocmask(SIG_BLOCK, (sigset_t *)SIGTSTP, NULL); sigemptyset(&mask); sigaddset(&mask, SIGCHLD); sigaddset(&mask, SIGINT); sigaddset(&mask, SIGTSTP); sigprocmask(SIG_BLOCK, &mask, NULL); x

– 34 – , S’04 Blocking Signals if (sigemptyset(&mask) < 0) unix_error("sigemptyset error"); if (sigaddset(&mask, SIGCHLD)) unix_error("sigaddset error"); if (sigaddset(&mask, SIGINT)) unix_error("sigaddset error"); if (sigaddset(&mask, SIGTSTP)) unix_error("sigaddset error"); if (sigprocmask(SIG_BLOCK, &mask, NULL) < 0) unix_error("sigprocmask error");

– 35 – , S’04 Summary Debug Multiple Processes using GDB Debug Multiple Processes using GDB Dup2 Dup2 Signaling Signaling