OS Lab Tutorial 1. Spawning processes Shared memory

Transcription

1 OS Lab Tutorial 1 Spawning processes Shared memory

2 The Spawn exec() family fork()

3 The exec() Functions: Out with the old, in with the new The exec() functions all replace the current program running within the process with another program bring up an xterm: exec sleep 5 #what happens and why? There are two families of exec() functions, the l family (list), and the v family (vector) Each exec() call can choose different ways of finding the executable and whether the environment is delivered in the form of a list or an array (vector) The environment, open file handles, etc. are passed into the exec d program What is the return value of an exec() call?

4 The execl... functions int execl(const char * path, const char * arg0,...); executes the command at path, passing it the environment as a list: arg 0... arg n thus, the execl family breaks down argv into its individual constituents, and then passes them as a lis to the execl? function (the l stands for list) int execlp(const char * path, const char * arg0,...); same as execl, but uses $PATH resolution for locating the program in path, thus an absolute pathname is not necessary int execle(const char * path, const char * arg0,... char * envp[]); allows you to specifically set the new program s environment, which replaces the default current program s environment

5 The execv... functions int execv(const char * path, char *const argv[]); executes the command at path, passing it the environment contained in a single argv[] vector int execvp(const char * path, char *const argv[]); same as execv, but uses $PATH resolution for locating the program in path int execve(const char * path, char *const argv[], char * const envp[]); note that this is the only system call of the lot

6 fork() fork() creates a new child process the OS copies the current program into the new process, resets the program pointer to the start of the new program (child fork location), and both processes continue execution independently as two separate processes The child gets its own copy of the parent s: data segments heap segment stack segment file descriptors

7 fork() Return Values fork() is the one Unix function that is called once but returns twice: If fork() returns 0: you re in the new child process If fork() returns > 1 (i.e., the pid of the new child process) you re back in the parent process

8 Waiting on Our Children Unlike life, parents should always hang around for their children s lives (runtimes) to end, that is to say: Parent processes should always wait for their child processes to end When a child process dies, a SIGCHLD signal is sent to the parent as notification The SIGCHLD signal s default disposition is to ignore the signal A parent can find out the exit status of a child process by calling one of the wait() functions

9 Waiting on Our Children Parent processes find out the exit status of their children by executing a wait() call: pid_t wait(int * status); pid_t waitpid(pid_t pid, int * status, int options); Wait() blocks until it receives the exit status from a child Waitpid can wait on a specific child, and doesn t necessarily block (WNOHANG) Waiting allows the parent to obtain the return value from the child s process

10 waitpid() pid_t waitpid(pid_t pid, int * status, int options); pid can be any of 4 values: < - 1: wait for any child whose gpid is the same as pid == - 1: waits for any child to terminate == 0: waits for a child in the same process group as the current process > 0: waits for process pid to exit The following macros work on status: WIFEXITED(status): WIFSIGNALED(status): signal true if process exited normally true if process was killed by a

11 Sleeping unsigned int sleep (unsigned int seconds) The sleep function is declared in `unistd.h'. The sleep function waits for seconds or until a signal is delivered, whichever happens first. If sleep function returns because the requested interval is over, it returns a value of zero. If it returns because of

12 Working with shared memory There are 4 stages to be executed while using shared memory Tell the OS we want a shared memory object Map the object to the process memory (virtual) Unmap the memory from the process memory Delete the shared memory object Important: IPC objects are never deleted unless explicitly deleted or the computer is rebooted

13 In practice shmget - allocates a shared memory segment SYNOPSIS #include <sys/ipc.h> #include <sys/shm.h> int shmget(key_t key, size_t size, int shmflg); DESCRIPTION shmget() returns the identifier of the shared memory segment associated with the value of the argument key. A new shared memory segment, with size equal to the value of size rounded up to a multiple of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no shared memory segment corresponding to key exists, and IPC_CREAT is asserted in shmflg (i.e. shmflg&ipc_creat isn't zero).

14 Shared memory flags (shmflg) The value shmflg is composed of: IPC_CREAT to create a new segment. If this flag is not used, then shmget() will find the segment associated with key and check to see if the user has permission to access the segment. IPC_EXCL used with IPC_CREAT to ensure failure if the segment already exists. mode_flags (lowest 9 bits) specifying the permissions granted to the owner, group, and world. Presently, the execute permissions are not used by the system.

15 In practice(2) shmat attching a shared memory segment to a process SYNOPSIS #include <sys/types.h> #include <sys/shm.h> void *shmat(int shmid, const void *shmaddr, int shmflg); DESCRIPTION shmid segment id prevousy allocated by shmget shmaddr if is NULL, the system chooses a suitable (unused) address at which to attach the segment. Otherwise, it is used as the (virtual) address for attachment

16 Flags shmflg SHM_RND - the address equal to shmaddr rounded down to the nearest multiple of SHMLBA. Otherwise shmaddr must be a pagealigned address at which the attach occurs. SHM_RDONLY - segment is attached for reading. Otherwise the segment is attached for read and write

17 In practice(3) shmdt detaches a previously attached shared memory SYNOPSIS #include <sys/types.h> #include <sys/shm.h> int shmdt(const void *shmaddr);

18 In practice(4) shmctl - shared memory control SYNOPSIS #include <sys/ipc.h> #include <sys/shm.h> int shmctl(int shmid, int cmd, struct shmid_ds *buf); DESCRIPTION shmctl() allows the user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory segment, or destroy a segment.

19 The shmctl commands IPC_STAT is used to copy the information about the shared memory segment into the buffer buf. IPC_SET is used to apply the changes the user has made to the uid, gid, or mode members of the shm_perms field. Only the lowest 9 bits of mode are used. The shm_ctime member is also updated. The user must be the owner, creator, or the super-user. IPC_RMID is used to mark the segment as destroyed. It will actually be destroyed after the last detach. The user must be the owner, creator, or the super-user.

20 Father-son relationship fork() After a fork() the child inherits the attached shared memory segments. exec() After an exec() all attached shared memory segments are detached (not destroyed). exit() Upon exit() all attached shared memory segments are detached (not destroyed).

21 Assignment 1 Write a program p1 which will accept two arguments: delay and string. It then prints string on screen every delay seconds. Write a program p2 which will prompt the user for delay and string. It will then spawn a copy of p1 to deal with these arguments, and itself continue to prompt the user for another set of arguments (delay and string).

22 Assignment 1 Exiting: p1 exits cleanly with SIGUSR1 p2 exits cleanly when given the input quit. It should call for all its children to exit before quitting. Coding conventions: Use indent command to indent code provide README (if needed) and a Makefile