Chapter 8: Process ControlProcess IdentifiersRelated FunctionsForkFork detailsFile Sharing between Parent and ChildNormal casesOther Shared InfoThings that are Not Inherited by the ChildExitProcess Termination DetailsWhen a Process Terminateswait() and waitpid()Race ConditionsRunning a Different Programexec()Variations of exec()Changing User and Group IDsFacts about the three user IDsOther functionsInterpreter FilesSystem()Process Times/procAccessing /procInformation and Control Operations/proc Directory StructureChapter 8: Process ControlCMPS 105: Systems ProgrammingProf. Scott BrandtT Th 2-3:45Soc Sci 2, Rm. 167Process Identifiers Guaranteed to be unique for each currently executing process on a single computer Usually sequentially allocated Some systems services have PIDs as well 0: scheduler/swapper 1: init 2: pagedaemonRelated Functions #include <sys/types.h> #include <unistd.h> pid_t getpid(void); Returns PID of calling process pid_t getppid(void); Returns PID of parent of calling process uid_t getuid(void); uid_t geteuid(void); gid_t getgid(void); gid_t getegid(void) Return real or effective UID or GID for calling processFork #include <sys/types.h> #include <unistd.h> pid_t fork(void); Fork creates a new process The new process is an exact clone of the calling process This is the only way to create a process in Unix Fork returns The PID of the newly created process to the parentprocess 0 to the newly created childprocessFork details The child is a clone of the parent It has a copyof the parent’s Address space (heap, stack, variables, stdio bufs) File descriptors Code (may be shared, since it is read-only) After the fork() call, each process executes as though it was the one that called fork() The only difference is the return value from fork() Usually, different code paths are taken based on a test of the PID returnedFile Sharing between Parent and Child Each process has its own file descriptors The underlying kernel structures for managing the files are shared Specifically, the offsets are shared This means that shared output to the same file will work correctly Important if stdout has been redirected to a fileNormal cases Input and output isn’t redirected, so it doesn’t matter Parent waits for child to finish Parent gets updated file pointers when it resumes executing Child redirects it’s input/output so no shared file pointersOther Shared Info Real & effective user and group IDs Supplementary group IDs Session ID Controlling terminal set-user-ID and set-group-ID flags Current working directory Root directory File mode creation mask Signal mask and dispositions The close-on-exec flag for any open file descriptors Environment Attached shared memory segments Resource limitsThings that are Not Inherited by the Child The return value from fork() The process IDs The process ID of the parent The accumulated CPU time File locks Pending alarms Pending signalsExit Three ways to terminate normally and two ways to terminate abnormally Normal Termination Return from main() Call exit() (C library function) Cleans up standard I/O then calls _exit() Call _exit() (underlying system call) Abnormal Termination Receive certain signals from parent or kernel Call abort (sends SIGABRT to self) Termination status: exit parameter or other kernel-generated statusProcess Termination Details When a process terminates, the parent receives SIGCHLD wait() allows a parent process to wait for a child process to terminate When a process terminates, the kernel maintains a small amount of info until the parent calls wait() Such a process is a zombieuntil the parent calls wait() If the parent terminates first, child is inherited by initWhen a Process Terminates The parent process receives a SIGCHLD Parent can Ignore the signal (the default action), or Set up a signal handler to be called when the signal arrives Use wait() to wait for the child to finish Blocks parent Returns when a child process terminates Returns immediately if any child is a zombie Returns child’s PIDwait() and waitpid() #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *statloc); Wait for any child process to terminate pid_t waitpid(pid_tpid, int *statloc, intoptions); Wait for a specific child process to terminate Statloc contains the child’s termination status (the child’s parameter to exit(), possibly with extra information – see the man page (section 2) for wait())Race Conditions A race condition is any situation where two (or more) processes access shared data, AND The outcome of the processing depends upon the order in which the processes execute Example: two processes do x=x+1, where x is a shared variable Need some form of synchronization Signals File locks Semaphores …Running a Different Program fork() allows us to clone a process The clone is a duplicate of the parent It runs the same program as the parent We want to be able to run different programs, not just clones exec() allows us to run a different program in the current process Often closely follows a call to fork() fork() creates a new process, and exec() makes it run a new program Same PID, new text, data, BSS, stack, heapexec() #include <unistd.h> int execl(const char *pathname, const char *arg0, … /* (char *)0 */); int execv(const char *pathname, char const *argv[]); int execle(const char *pathname, const char *arg0, …/*(char *)0 */, char *const envp[]); int execve(const char *pathname, char const *argv[], char *const envp[]); int execlp(const char *filename, const char *arg0, … /* (char *) 0 */); int execvp(const char *filename, char *const argv[]);Variations of exec() l versions use a list of parameters v versions use an argv[] parameter e versions include an environment parameter p versions search PATH for executable Probably the one you want for assignment 4Changing User and Group IDs #include <sys/types.h> #include <unistd.h> int setuid(uid_tuid); int setgid(gid_tgid); If the process has superuser privileges setuid() sets the real user ID, effective user ID, and saved set-user-ID to uid If the process does not have superuser privileges, but uid=