Process Control

• Overview
• Process creation
• Process termination
• Program execution

Process Identifiers

• 每個 process 都有獨特的 id
• pid 是非負整數
• process terminate 後 pid 可被重複使用
• init process
  • 作業系統啟動的第一支程式
  • 是所有 orphaned process 的 parent
  • never die
• ps、top、htop command list running processes

Process Relationships

• pstree command 以 tree structure 列出 process
• init 會在 process tree 的樹根，通常 pid 為 1

Retrieve Process Identifiers

pid_t getpid(void);
pid_t getppid(void);
uid_t getuid(void);
uid_t geteuid(void);
gid_t getgid(void);
gid_t getegid(void);
/* None of these functions has an error return */

The fork function

#include <unistd.h>
 
pid_t fork(void);
/* Returns: 0 in child, process ID of child in parent, -1 on error */

• 建立一個 new(child) process
• 在 child process 中會回傳 0，在 parent process 中會回傳 child pid，error 則回傳 -1
• parent 和 child process 都會從呼叫 fork() 的地方繼續執行
• child gets a copy of parents's data space、heap and stack，這部分是複製，記憶體不共享
• child and parent 共享 text segment
• Copy-on-write (COW): 這是現今多採用的技術，由於完整複製父程序的 data, stack and heap 效能較差，因此 kernel 將 parent 和 child 會修改到的部分複製，而其餘部分則能以 read-only 共享

#include "apue.h"
 
int globvar = 6; /* external variable in initialized data */
char buf[] = "a write to stdout\n";
 
int
main(void)
{
    int var; /* automatic variable on the stack */
    pid_t pid;
 
    var = 88;
    if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1)
        err_sys("write error");
    printf("before fork\n"); /* we don’t flush stdout */
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0) /* child */
    {
        globvar++; /* modify variables */
        var++;
    }
    else /* parent */
    {
        sleep(2);
    }
 
    printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar, var);
    exit(0);
}

$ ./a.out
a write to stdout
before fork
pid = 430, glob = 7, var = 89 # child’s variables were changed
pid = 429, glob = 6, var = 88 # parent’s copy was not changed
$ ./a.out > temp.out
$ cat temp.out
a write to stdout
before fork
pid = 432, glob = 7, var = 89
before fork
pid = 431, glob = 6, var = 88

• “a write to stdout” 字串都只出現一次，因為 write function 是 not buffered，且是在 fork 前被呼叫，所以在呼叫時字串就會被寫到 standard output
• “before fork”: printf from the standard I/O library is buffered
  • 在第一個 case，standard I/O (running the program interactively) 是 line buffered，standard output buffer 會在遇到換行時被 flushed
  • 在第二個 case，standard I/O (redirect stdout to a file) 是 fully buffered，雖然 printf 是在 fork 前被呼叫，但字串保留在 buffer 中，當呼叫 fork 時，parent 的 data space 被複製一份給了 child，直到 exit 時 buffer 才被清空，因此印出了兩次字串

Fork and file Sharing

Handling File Descriptors after fork

• parent waits for child to complete
  • 剛 fork 時 child 的 file descriptor 會和 parent 有一樣的 offset
• Both the parent and the child go their own ways
  • parent 和 child 個別關閉自己不需使用的 file descriptor

Other Properties Inherited by the Child

• real uid、real gid、euid、egid、gid
• controlling terminal
• suid、sgid
• current working directory
• file mode creation mask
• signal mask and dispositions
• the close-on-exec flag for any open file descriptors
• environment variables

Use of fork

• when a process want to duplicate itself
  • parents 和 child 可以執行程式內的不同部分
  • 常用在網路程式，parent 等待 client 連線，當有連線後 fork 讓 child 執行工作，parent 則繼續等待其他連線
• when a process want to execute a different program
  • 在 shell 中常使用，當下了指令時，shell 都是使用 fork 另外去執行

Variants of fork

• vfork:
  • Creates child process 時不會複製 address space 給 child，child 在呼叫 exec 或 exit 前都跑在 parent 的 address space 中
  • 通常用在 child 會立即呼叫 exec 的情況，不需要複製資料，比 fork 更有效率
• clone:
  • 實現 fork 和 vfork 的 Linux system calls，可以決定哪些東西要在 parent 和 child 之間共享

Child process terminate

• zombie process: 當 child 終止時，他的 exit status 應該被 parent 讀取，如果 parent 不理會 child 的 exit status，則 child 會變成 zombie process，直到 parent 呼叫 wait 取得 child 的結束狀態
  • child 佔據的 resource 會被釋放，但是 pid 和 terminate status 會留在 kernel
• 保證 parent process 存在
  • 如果 parent process 在 child 之前終止了， init process 會接手成為那些 child process 的 parent process，child 的 ppid 會改為 1
• 當 child process 終止(正常或異常皆是)時，kernel 會發送 SIGCHLD signal 給 parent process
• child termination 是 asynchronous event，可能發生在 process 執行的任何時候
• kernel 發送給 parent 的 signal 也是 asynchronous
• parent 可以忽略或提供 function 處理這個 signal

wait and waitpid function

#include <sys/types.h>
#include <sys/wait.h>
 
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);

• parent process 可以透過呼叫 wait 和 waitpid functions 來得到 child process 的 terminate status
• 這兩個 function:
  • 如果 children 都還在執行，則會 Block
  • 如果有 child terminate 且正在等待它的 terminate status 被 fetch，則會直接回傳 child 的 termination status
  • 如果沒有 child 則直接回傳 error
• wait 和 waitpid functions 的差異
  • Block or not block
    • wait function always block the caller until a child process terminates
    • waitpid function has an option that prevents it from being blocked
  • Process termination order
    • The waitpid function doesn't wait for the child that terminates first; it has a number of options that control which process it waits for.
• wait function 會等待所有 child process
• waitpid function
  • 參數 pid:
    • < -1: Waits for any child whose process group ID equals the absolute value of pid.
    • == -1: Waits for any child process. In this respect, waitpid is equivalent to wait.
    • == 0: Waits for any child whose process group ID equals that of the calling process.
    • > 0: Waits for the child whose process ID equals pid.
  • 參數 option:
    • WNOHANG: The waitpid function will not block if a child specified by pid is not immediately available. In this case, the return value is 0
    • WUNTRACED:
    • WCONTINUED:

int main(void)
{
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0) /* child */
    {
        exit(7);
    }
 
    if (wait(&status) != pid)
    {
        err_sys("wait error");
    }
    pr_exit(status);
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0) /* child */
    {
        abort(); /* generates SIGABRT */
    }
 
    if (wait(&status) != pid)
    {
        err_sys("wait error");
    }
    pr_exit(status);
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0) /* child */
    {
        status /= 0;
    }
 
    if (wait(&status) != pid)
    {
        err_sys("wait error");
    }
    pr_exit(status);
    exit(0);
}
 
void pr_exit(int status)
{
    if (WIFEXITED(status))
        printf("normal termination, exit status = %d\n", WEXITSTATUS(status));
    else if (WIFSIGNALED(status))
        printf("abnormal termination, signal number=%d%s\n", WTERMSIG(status), WCOREDUMP(status) ? " (core file generated)" : "");
    else if (WIFSTOPPED(status))
        printf("child stopped, signal number=%d\n", WSTOPSIG(status));
}

normal termination, exit status = 7
abnormal termination, signal number = 6
abnormal termination, signal number = 8

Macros to Interpret Exit Status

• WIFEXITED(status): 如果 child 正常終止 (terminated) 則回傳 true，在這個情況下可以使用 WEXITSTATUS(status) 去取得 child 傳入 exit, _exit,or _Exit 的 low-order 8 bits argument
• WIFSIGNALED (status): 如果 child 異常終止則回傳 true，通常是收到一個未能處理的 signal，在這個情況下可以使用 WTERMSIG(status) 去取得導致終止的 signal number，除此之外有些系統也實作了 WCOREDUMP(status)，當 child 終止時若有 coredump file 產生則這個 macro 會回傳 true
• WIFSTOPPED (status): if child currently stopped, return true，在這個情況下可以使用 WSTOPSIG(status) 去取得導致 child process 暫停的 signal number
• WIFCONTINUED (status): True if status was returned for a child that has been continued after a job control stop

Avoid Zombies by Calling fork Twice

fork 時 parent process 可能也有自己的工作，不想呼叫 wait 來等待 child process 而被 block，所以我們可以透過兩次 fork，並立刻結束第一個 child，如此 parent 不需要過多等待，而第二個 child process 會在第一個 child process 結束時被 init 接手，從此與 parent 獨立

#include "apue.h"
#include <sys/wait.h>
 
int
main(void)
{
    pid_t   pid;
 
    if ((pid = fork()) < 0) {
        err_sys("fork error");
    } else if (pid == 0) {      /* first child */
        if ((pid = fork()) < 0)
            err_sys("fork error");
        else if (pid > 0)
            exit(0);    /* parent from second fork == first child */
 
        /*
         * We're the second child; our parent becomes init as soon
         * as our real parent calls exit() in the statement above.
         * Here's where we'd continue executing, knowing that when
         * we're done, init will reap our status.
         */
        sleep(2);
        printf("second child, parent pid = %ld\n", (long)getppid());
        exit(0);
    }
 
    if (waitpid(pid, NULL, 0) != pid)   /* wait for first child */
        err_sys("waitpid error");
 
    /*
     * We're the parent (the original process); we continue executing,
     * knowing that we're not the parent of the second child.
     */
    exit(0);
}

$ ./a.out
$ second child, parent pid = 1

這邊可以注意到 “second child, parent pid = 1” 前面會先出現提示符，因為在 parent process 結束時提示符就會再出現，而後第二個 child 結束才輸出字串

Race Conditions

在呼叫 fork 後不保證 parent 和 child 誰會先執行

int main(void)
{
    pid_t pid;
    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);
}

解法一:

• 如果 child 要比 parent 早執行，則 parent 可以使用 wait/waitpid 等待 child 執行結束
• 如果是 parent 要比 child 早執行，由於 parent 結束後 child 會被 init 接手，因此可以使用 getppid() 做 Polling

while (getppid() != 1)
    sleep(1);

• 缺點是不管哪一種，parent 或 child 都會被停下來等待另一個，另外 Polling 效率不佳

解法二:

• 透過 IPC 溝通
  • TELL_WAIT(): Initialize
  • WAIT_PARENT(): blocks execution and waits for its parent
  • TELL_CHILD(pid): tell a child that it has finished
  • WAIT_CHILD(): blocks execution and waits for its child
  • TELL_PARENT(ppid): tell its parent that it has finished

#include "apue.h"
 
TELL_WAIT(); /* set things up for TELL_xxx & WAIT_xxx */
 
if ((pid = fork()) < 0) {
    err_sys("fork error");
} else if (pid == 0) { /* child */
 
    /* child does whatever is necessary ... */
 
    TELL_PARENT(getppid()); /* tell parent we’re done */
    WAIT_PARENT(); /* and wait for parent */
 
    /* and the child continues on its way ... */
 
    exit(0);
}
 
/* parent does whatever is necessary ... */
 
TELL_CHILD(pid); /* tell child we’re done */
WAIT_CHILD(); /* and wait for child */
 
/* and the parent continues on its way ... */
exit(0);

parent_go_first

} else if (pid == 0) {
    WAIT_PARENT();      /* parent goes first */
    charatatime("output from child\n");
} else {
    charatatime("output from parent\n");
    TELL_CHILD(pid);
}

child_go_first

} else if (pid == 0) {
    charatatime("output from child\n");
    TELL_PARENT(getppid());
} else {
    WAIT_CHILD(); /* child goes first */
    charatatime("output from parent\n");
}

exec Function

• 當 process 執行 exec function 後， 內容會完全被新的 program 取代，並執行新 program 的 main function
• 執行 exec 後 pid 並不會改變，因為並沒有產生新的 process 只是原來的 process 內容被取代了
  • fork creates new processes
  • exec functions initiates new programs
  • exit handles termination
  • wait functions handle waiting for termination

#include <unistd.h>
 
extern char **environ;
int execl(const char *pathname, const char *arg0, ...,(char *)0);
int execlp(const char *filename, const char *arg0, ...,(char *)0);
int execle(const char *pathname, const char *arg0, ...,(char *)0 , char * constenvp[]);
int execv(const char *pathname, char *const argv[]);
int execvp(const char *filename, char *const argv[]);
int execve(const char *pathname, char *const argv[], char *const envp[]);
int fexecve(int fd, char *const argv[], char *const envp[]);
 
/* All seven return: −1 on error, no return on success */

• pathname 必須要為絕對或相對路徑
• filename 中不含 slash(/)，exec 會自動搜尋在 $PATH 環境變數中的檔案名稱，function 中含有 p
• arg list 是直接將參數一個一個餵給 exec，function 中含有 l
• argv[] 是將參數存在陣列中再餵給 exec，function 中含有 v
• environ 是可以直接使用的外部環境變數
• envp[] 則是將可用環境變數放在陣列傳入 exec， function 中含有 e

#include "apue.h"
#include <sys/wait.h>
 
char    *env_init[] = { "USER=unknown", "PATH=/tmp", NULL };
 
int
main(void)
{
    pid_t   pid;
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0)   /* specify pathname, specify environment */
    {
        if (execle("/home/sar/bin/echoall", "echoall", "myarg1", "MY ARG2", (char *)0, env_init) < 0)
            err_sys("execle error");
    }
 
    if (waitpid(pid, NULL, 0) < 0)
        err_sys("wait error");
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0)   /* specify filename, inherit environment */
    {
        if (execlp("echoall", "echoall", "only 1 arg", (char *)0) < 0)
            err_sys("execlp error");
    }
 
    exit(0);
}

echoall.c

#include "apue.h"
 
int
main(int argc, char *argv[])
{
    int         i;
    char        **ptr;
    extern char **environ;
 
    for (i = 0; i < argc; i++)      /* echo all command-line args */
        printf("argv[%d]: %s\n", i, argv[i]);
 
    for (ptr = environ; *ptr != 0; ptr++)   /* and all env strings */
        printf("%s\n", *ptr);
 
    exit(0);
}

$ ./a.out
argv[0]: echoall
argv[1]: myarg1
argv[2]: MY ARG2
USER=unknown
PATH=/tmp
$ argv[0]: echoall
argv[1]: only 1 arg
USER=sar
LOGNAME=sar
SHELL=/bin/bash
...
HOME=/home/sar

Interpreter Files

幾乎現今的 UNIX 系統都支援 Interpreter Files，而 exec 也可執行 Interpreter Files

 #! pathname [ optional-argument ]

#include "apue.h"
#include <sys/wait.h>
 
int
main(void)
{
    pid_t pid;
 
    if ((pid = fork()) < 0)
    {
        err_sys("fork error");
    }
    else if (pid == 0)  /* child */
    {
        if (execl("/home/sar/bin/testinterp", "testinterp", "myarg1", "MY ARG2", (char *)0) < 0)
            err_sys("execl error");
    }
    if (waitpid(pid, NULL, 0) < 0) /* parent */
        err_sys("waitpid error");
    exit(0);
}

$ cat /home/sar/bin/testinterp
#!/home/sar/bin/echoarg foo
$ ./a.out
argv[0]: /home/sar/bin/echoarg
argv[1]: foo
argv[2]: /home/sar/bin/testinterp
argv[3]: myarg1
argv[4]: MY ARG2

system Function

#include <stdlib.h>
 
int system(const char *cmdstring);

system function 的簡易實作，不含 signal handle

• 主要包含 fork(), exec(), and waitpid()

#include    <sys/wait.h>
#include    <errno.h>
#include    <unistd.h>
 
int
system(const char *cmdstring)   /* version without signal handling */
{
    pid_t   pid;
    int     status;
 
    if (cmdstring == NULL)
        return(1);      /* always a command processor with UNIX */
 
    if ((pid = fork()) < 0)
    {
        status = -1;    /* probably out of processes */
    }
    else if (pid == 0)  /* child */
    {
        execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
        _exit(127);     /* execl error */
    }
    else               /* parent */
    {
        while (waitpid(pid, &status, 0) < 0)
        {
            if (errno != EINTR)
            {
                status = -1; /* error other than EINTR from waitpid() */
                break;
            }
        }
    }
 
    return(status);
}

system and suid/sgid Programs

如果一個 process 有 suid/sgid 則又在這個 process 中呼叫了 system，則 system 執行的這個 command 擁有和呼叫他的 process 一樣的權限，所以一般不建議在 suid/sgid 的程式直接使用 system

建議使用 fork 產生新 process 後再以 seteuid and setegid 重新調整權限，確認權限無誤後才呼叫 exec 執行指令

User Identification

#include <sys/types.h>
#include <pwd.h>
 
struct passwd *getpwnam(const char *name);
 
struct passwd *getpwuid(uid_t uid);
 
int getpwnam_r(const char *name, struct passwd *pwd, char *buf, size_t buflen, struct passwd **result);
 
int getpwuid_r(uid_t uid, struct passwd *pwd, char *buf, size_t buflen, struct passwd **result);
 
struct passwd {
    char   *pw_name;       /* username */
    char   *pw_passwd;     /* user password */
    uid_t   pw_uid;        /* user ID */
    gid_t   pw_gid;        /* group ID */
    char   *pw_gecos;      /* user information */
    char   *pw_dir;        /* home directory */
    char   *pw_shell;      /* shell program */
};

• 回傳符合 uid 的 passwd database(/etc/passwd)

#include <unistd.h>
 
char *getlogin(void);
int getlogin_r(char *buf, size_t bufsize);

• getlogin() returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a NULL pointer if this information cannot be determined

• Shichao's note-CH8