Operating Systems ECE344 Ashvin Goel ECE University of Toronto Unix System Calls and Posix Threads

Overview  Process-related Unix system calls  Posix threads 2

Process-related Unix System Calls  A process in Unix consists of an address space and one thread  Unix provides several process-related system calls: o getpid(), getppid()  Get unique id for process, for parent process  Pid identifies both address space and thread o fork(), execv()  Create new processes o exit(), wait()  Terminate processes and synchronize with terminating process o kill(), sigaction()  Communicate with another process via signals 3

Fork System Call  fork() system call creates a new process o The original process is called the parent process o The new process is called the child of the parent process  Child process is an identical copy of the parent o Thread state (registers) and address space are copied 4 stack text data Address space SP PC Child stack text data Address space SP PC Parent

Fork  After fork system call returns: o Both parent and child process start executing the instruction after fork o To distinguish the two processes, fork returns PID of child in parent process, and 0 in child process o Parent and child run code and modify data completely independently, how?  Why the weird call? int n = 5; int pid = fork(); if (pid == 0) { // run child code n = n + 1; } else { // pid value > 0 // run parent code n = n - 1; } 5

Execv System Call  execv() call allows running a new program o Fork creates a new process that is a copy of its parent, but it doesn’t allow running a new program  When process calls execv, new program replaces current process 6 stack text data Address space SP PC After execve stack text data Address space SP PC Before execve

Execv  On execv system call: o New program is loaded from executable file on disk into current program’s address space o Code and data regions are copied from disk o Stack is initialized with activation frame of main() o Processor registers are reinitialized o New program’s process ID is the same as old program’s process ID o execv() does not return, why? char *cmd = “/bin/ls”; char *arg1 = “-l”; char *args[3]; args[1] = arg1; args[2] = NULL; execv(cmd, args); // code doesn’t execute 7

Exit System Call  A process can terminate itself by calling the exit() system call  On exit(retval) system call: o Address space of the process is destroyed (memory for code, data, etc., regions is reclaimed) o Process-specific OS state, e.g., open files, is destroyed o retval is saved, can be returned to parent process  Why is this useful? o Thread state is destroyed  When is this done? 8

Wait System Call  A parent process can wait for a child process to terminate by calling the wait() system call o When child issues exit(retval), wait() returns retval o What happens if child exits before parent issues wait? o What happens if parent never issues wait()? 9

Wait  Wait needs to handle 4 cases o Parent issues wait before or after child’s exit o Parent doesn’t issue wait, exits before or after child’s exit W: wait, C: continue, E: exit, D: destroy 10 ParentChild W CE D ParentChild W C E D ParentChild E E D ParentChild E E D

Kill and Sigaction System Calls  Unix allows processes to send signals (or messages) to itself or to other processes by calling the kill() system call  Receiver process handles signals similar to interrupts o Receiver process is immediately interrupted o It starts executing a function called a signal handler o When signal handler finishes, normal execution continues  The sigaction() system call allows a process to setup the signal handler function o When no handler is setup, receiver process is forced to exit o Receiver process exits when it is scheduled to run next, why? 11

Code Examples  Read the man pages of the Unix system calls if you have trouble understanding their semantics o On google: man fork  Some code examples are available on the web site  shell.c o A simple shell program in Unix  unix-signal.c o A program that shows how signals are used in Unix 12

Posix Threads  Recall that a Unix process consists of an address space with one thread o Within main(), this thread is running  POSIX Threads or the pthreads system calls allow creating additional threads in a Unix process 13

Posix Threads API  pthread_create(thread, attr, start_routine, arg) o Returns new thread ID in thread o Executes function specified by start_routine ( arg )  pthread_exit(status) o Terminates current thread, returns status to a joining thread  pthread_join(thread_id, status) o Blocks thread until thread specified by thread_id terminates o Return value from pthread_exit is passed in status  pthread_yield() o Thread gives up processor and enters the run queue 14

Posix Threads Synchronization API  Pthreads provides mutex, semaphores, and condition variables  Mutex  Semaphores 15 sem_t sem_name; sem_init(&sem_name, 0, 0); /* 2nd arg is flag, 3rd arg is init value */ sem_wait(&sem_name); /* wait operation */ sem_post(&sem); /* signal operations */ pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_mutex_lock(&mut); pthread_mutex_unlock(&mut);

Posix Monitor Example  Say a thread wishes to wait until x > y  Another thread signals when x > y 16 pthread_mutex_lock(&mut); /* modify x and y */ if (x > y) pthread_cond_signal(&cond); pthread_mutex_unlock(&mut); int x,y; pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; pthread_mutex_lock(&mut); while (x <= y) { pthread_cond_wait(&cond, &mut); } /* operate on x and y */ pthread_mutex_unlock(&mut);

Posix Code Examples  Some posix code examples are available on the web site  pthread-example.c o Several threads are created and run without synchronization  pthread-example-sync.c o Several threads are created and run with synchronization 17

Summary  Unix OS provides various system calls for managing processes o fork creates a new clone process o execv loads a new program in an existing process o exit ends a process o wait allows a parent process to wait for a child’s exit o kill and sigaction are used to send signals to processes, similar to how device controllers interrupt the CPU  Pthreads is a standardized API for creating and managing threads in Unix OSes 18

Think Time  What happens when the various system calls we have discussed fail (due to some error)?  How would you write a Process A that waits for the exit of another arbitrary Process B (not just a child process), using the system calls we have discussed? Can Process A poll to check if Process B has exited? How will you ensure that Process A does not miss the exit event?  Think about how the OS handles the various wait scenarios, i.e., how is the synchronization between the parent and the child implemented in the four cases?  What are the steps involved in sending and receiving signals? 19