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Vars namevalue strings eg pwdhomesiwspjh execve

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Unformatted text preview: -­‐level diagram of what happens when you run the command ”ls” in a Linux shell: Heap Data Code: /usr/bin/bash fork(): parent child Stack Stack child Stack exec(): Heap Heap Data Code: /usr/bin/bash Data Code: /usr/bin/bash Processes Data Code: /usr/bin/ls 31 University of Washington execve: Loading and Running Programs ¢  ¢  Null-­‐terminated env var strings int execve( char *filename, char *argv, char *envp ) Null-­‐terminated cmd line arg strings Loads and runs in current process: §  Executable filename §  With argument list argv §  And environment variable list envp §  Stack boKom Env. vars: “name=value” strings (e.g. “PWD=/homes/iws/pjh”) ¢  execve does not return (unless error) ¢  Overwrites code, data, and stack §  Keeps pid, open files, a few other items Processes unused envp[n] == NULL envp[n-­‐1] … envp[0] argv[argc] == NULL argv[argc-­‐1] … argv[0] Linker vars envp argv argc Stack frame for main Stack top 32 University of Washington exit: Ending a process ¢  void exit(int status) §  Exits a process Status code: 0 is used for a normal exit, nonzero for abnormal exit §  atexit() registers func:ons to be executed upon exit §  void cleanup(void) { printf("cleaning up\n"); } void fork6() { atexit(cleanup); fork(); exit(0); } Processes 33 University of Washington Zombies ¢  Idea §  When process terminates, it s:ll consumes system resources Various tables maintained by OS §  Called a “zombie” §  A living corpse, half alive and half dead §  ¢  Reaping §  Performed by parent on terminated child §  Parent is given exit status informa:on §  Kernel discards process ¢  What if parent doesn’t reap? §  If any parent terminates without reaping a child, then child will be reaped by init process (pid == 1) §  But in long-­‐running processes we need explicit reaping §  e.g., shells and servers Processes 34 University of Washington wait: Synchronizing with Children ¢  int wait(int *child_status) §  Suspends current process (i.e. the parent) un:l one of its children terminates §  Return value is the pid of the child process that terminated §  On successful return, the child process is reaped §  If child_status != NULL, then the int that it points to will be set to a status indica:ng why the child process terminated §  There are special macros for interpre:ng this status – see wait(2) ¢  If parent process has mul:ple children, wait() will return when any of the children terminates §  waitpid() can be used to wait on a specific child process Processes 35 University of Washington wait Example void fork_wait() { int child_status; pid_t child_pid; if (fork() == 0) { printf("HC: hello from child\n"); } else { child_pid = wait(&child_status); printf("CT: child %d has terminated\n”, child_pid); } printf("Bye\n"); exit(0); HC Bye CT Bye } Processes 36 University of Washington Process management summary ¢  ¢  fork gets us two copies of the same process (but fork() returns different values to the two processes) execve has a new process subsJtute itself for the one that called it §  Two-­‐process program: First fork() §  if (pid == 0) { //child code } else { //parent code } §  Two different programs: §  First fork() §  if (pid == 0) { execve() } else { //parent code } §  Now running two completely different programs §  ¢  wait / waitpi...
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