Linux/UNIX Programming APUE (Process Control) 문양세 강원대학교 IT 대학 컴퓨터과학전공

Linux/UNIX Programming APUE (Process Control) 문양세 강원대학교 IT대학 컴퓨터과학전공

강의 내용 APUE (Process Control) 프로세스 ID 프로세스 생성 프로세스 종료 레이스 컨디션 프로그램 실행 기타

Process Identifiers (1/2) APUE (Process Control) • Every process has a unique process ID, a nonnegative integer.(모든 프로세스는 양수의 유일한 식별자(PID)를 가짐) • Process ID 0: swapper • scheduler process (it controls time slots for processes) • system process • part of the kernel • no program on disk corresponds to this process

Process Identifiers (2/2) APUE (Process Control) • Process ID 1: init process • invoked by the kernel at the end of the bootstrap procedure • /etc/init or /sbin/init • reads the system-dependent initialization files (/etc/rc*) • brings the system to a certain state (multi-user) • a normal user process but runs with super-user privileges • PID 1 is immortal • Process ID 2: pagedaemon • supports the paging of the virtual memory system • kernel process

System Process 예제 APUE (Process Control)

PID 관련 함수 APUE (Process Control) #include <sys/types.h> #include <unistd.h> pid_t getpid(void); // returns process ID pid_t getppid(void); // returns parent process ID uid_t getuid(void); // returns real user ID uid_t geteuid(void); // returns effective user ID gid_t getgid(void); // returns real group ID gid_t getegid(void); // returns effective group ID None of these functions has an error return.

fork() (1/3) APUE (Process Control) #include <sys/types.h> #include <unistd.h> pid_t fork(void); • fork() is the ONLY way to create a process in Unix kernel. • Child process is the new process created by fork(). • fork() is called once, but returns twice! • returns 0 in child process. • returns the child process ID in parent process.

fork() (2/3) APUE (Process Control) • Child gets a copy of parent’s data space, heap, and stack • Often, read-only text segment is shared • Parent and child continue executing instructionsfollowing the fork() call • Often, fork() is followed by exec().

fork() (3/3) APUE (Process Control)

예제: fork.c (1/2) APUE (Process Control)

예제: fork.c (2/2) APUE (Process Control) 실행 결과

Properties Inherited to the Child APUE (Process Control) real user and group ID, effective user and group ID supplementary group IDs process group ID, session ID 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 limits

Properties NOT Inherited to the Child APUE (Process Control) the return value from fork() the process IDs are different file locks pending alarms are cleared for the child the set of pending signals for the child is set to the empty set

exit() – Process Termination APUE (Process Control) • A process can terminate in 5 ways: • Normal Termination • return from main() • exit()  w/ cleanup procedure • _exit()  w/o cleanup procedure • Abnormal Termination • calling abort() (generates SIGABRT signal) • process receives signals

wait(), waitpid()– skip APUE (Process Control) • Child terminates • Kernel sends SIGCHLD signal to parent. • an asynchronous event • Default action for signal: ignore it. • Signal handlers can be defined by users • call wait() to fetch the termination status of child. • (무사히, 죽을 수 있도록 배려~)

Race Conditions (1/2) APUE (Process Control) • Multiple processes share some data. • Outcome depends on the order of their execution (i.e. RACE) • (Process A) x = 20; • (Process B) x += 10; • After fork(), we cannot predict if the parent or the child runs first! • The order of execution depends on: • System Load • Kernel’s Scheduling Algorithm

Race Conditions (2/2) APUE (Process Control) • For parent to wait for child, • Call wait(), waitpid(), wait3(), wait4() • Use signals or other IPC methods • For child to wait for parent, • while(getppid() != 1) sleep(1); // Parent가 죽을 때까지 기다림 • Use signals or other IPC methods

예제: race.c (1/2) APUE (Process Control) #include <stdio.h> // race.c #include <sys/types.h> err_sys(char *p) { perror(p); exit(-1); } int main(void) { pid_tpid; if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) charatatime("output from child\n"); else charatatime("output from parent\n"); exit(0); } charatatime(char *str) { char *ptr; int c; for (ptr = str; c = *ptr++; ) { putc(c, stdout); fflush(stdout); usleep(1); } }

예제: race.c (2/2) APUE (Process Control) 실행 결과

How to Avoid Race Condition? APUE (Process Control) #include <stdio.h> #include <sys/types.h> int main(void) { pid_t pid; TELL_WAIT(); if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) { WAIT_PARENT(); // parent goes first charatatime("output from child\n"); } else { charatatime("output from parent\n"); TELL_CHILD(pid); } exit(0); } • How to implement TELL_WAIT(), WAIT_PARENT(), and TELL_CHILD()? • Use signals (강의노트 15) • Use IPC methods (강의노트 16)

Program Execution: exec() (1/2) APUE (Process Control) • When a process calls one of the exec()functions • that process is completely replaced by the new program (새로운 프로그램으로 대체)(text, data, heap, and stack segments) • and the new program starts at its main function • 함수 exec()를 호출하여 완전히 다른 프로그램으로 실행된다.

Program Execution: exec() (2/2) APUE (Process Control)

exec() Functions APUE (Process Control) #include <unistd.h> intexecl(const char *pathname, const char *arg0, … , (char *)0); intexecv(const char *pathname, const char *argv[]); intexecle(const char *pathname, const char *arg0, … /* (char*) 0, char *const envp[] */); intexecve(const char *pathname, const char *argv[],char *const envp[]); intexeclp(const char *filename, const char *arg0, … , (char *)0); intexecvp(const char *filename, const char *argv[]); All six return: -1 on error, no return on success • exec? (p, l, v, e) • p: filename (not pathname) • l: takes a list of arguments (the last argument should be a null pointer) • v: takes argv[] vector • e: takes envp[] array without ‘e’, the environment variables of the calling process are copied

Properties inherited to the new program APUE (Process Control) • same ID • process ID, parent process ID, real user ID, real group ID, supplementary group IDs, process group ID, session ID • controlling terminal • time left until alarm clock • current working directory • root directory • file mode creation mask • file locks • process signal mask • pending signals • resource limits, …

예제: nexec.c, echoall.c (1/4) APUE (Process Control)

예제: nexec.c, echoall.c(2/4) APUE (Process Control)

예제: nexec.c, echoall.c(3/4) APUE (Process Control)

예제: nexec.c, echoall.c(4/4) APUE (Process Control) 실행 결과

system() APUE (Process Control) #include <stdlib.h> int system(const char *cmdstring); • 주어진 스트링(cmdstring)을 Shell 상에서 수행시킨다. • e.g.) system(“date > file”); • system() is implemented by calling fork, exec, and waitpid. • Return values: • -1 with errno: fork or waitpid fails • 127: exec fails • Termination status of shell: all 3 functions succeed

예제: myls.c (1/2) APUE (Process Control) #include <stdio.h> // myls.c main(int ac, char *av[]) { inti; char cmdstr[1024]; strcpy(cmdstr, "/bin/ls "); for(i=1;i < ac;i++) { strcat(cmdstr, av[i]); strcat(cmdstr, " "); } fprintf(stdout, "cmdstr = \"%s\"\n", cmdstr); system(cmdstr); exit(0); }

예제: myls.c (2/2) APUE (Process Control) 실행 결과

Process Times – skip APUE (Process Control) #include <sys/times.h> clock_t times(structtms *buf); Returns: elapsed wall clock time in clock ticks if OK, -1 on error structtms ( clock_ttms_utime; /* user cpu time */ clock_ttms_stime; /* system cpu time */ clock_ttms_cutime; /* child user cpu time */ clock_ttms_cstime; /* child system cpu time */ } Wall clock time: the amount of time the process takes to run and depends on the system loads. (실제 수행된 시간) User CPU time: attributed to user instructions(사용자 코드에 의해 CPU를 점유한 시간) System CPU time: attributed to the kernel, when it executes on behalf of the process (시스템 코드에 의해 CPU를 점유한 시간)

Linux/UNIX Programming APUE (Process Control) 문양세 강원대학교 IT 대학 컴퓨터과학전공