5.8 进程控制

基本概念
fork 和 vfork
exit
wait 函数
竞态
exec 函数以及脚本解释器
system 函数
进程记帐
用户和组标识符
进程时间

5.8.1 基本概念

进程
派生和执行
进程标识符

===============================================================================
       #include <unistd.h>

       pid_t getpid (void);
       pid_t getppid (void);
===============================================================================

用户和组标识符

===============================================================================
       #include <unistd.h>
       #include <sys/types.h>

       uid_t getuid (void);
       uid_t geteuid (void);
       uid_t getgid (void);
       uid_t getegid (void);
===============================================================================

5.8.2 fork 和 vfork

fork: 派生子进程

===============================================================================
       #include <unistd.h>
       #include <sys/types.h>

       pid_t fork (void);
-------------------------------------------------------------------------------
EXAMPLE:
       /* 派生子进程 */

        #include <sys/types.h>
        #include <unistd.h>

        void err_sys (const char* info)
        {
            perror (info);
            exit (1);
        }

        int main (void)
        {
            pid_t pid;

            if ( (pid = fork ()) < 0)
                 err_sys ("fork error");
            else if (pid > 0) {
                 // this is parent.
            }
            else {
                // this is child.
            }

            exit (0);
        }
-------------------------------------------------------------------------------
   * fork 之后, 子进程和父进程共享打开的文件. 这是实现管道等 IPC 机制的基础.
   * Linux 利用按需分页 (paging on demand) 和写时复制 (copy-on-write) 方式派生子进程.
===============================================================================

vfork: 用于派生后立即 exec 的情况

===============================================================================
       #include <unistd.h>
       #include <sys/types.h>

       pid_t vfork (void);
-------------------------------------------------------------------------------
   * 不复制父进程的全部地址空间.
   * vfork 保证子进程首先运行, 直到调用 exec 或 exit 之后, 父进程才能得到运行机会.
===============================================================================

5.8.3 exit

exit 调用由 atexit 注册的清除函数, 并关闭所有的标准 I/O 流
exit 调用 _exit 处理与 UNIX 相关的内容, 比如文件描述符, 作业控制等
进程的退出状态非常重要. 一般以 0 值表示正常退出

===============================================================================
       #include <stdlib.h>

       void exit (int status);

       #include <unistd.h>

       void _exit (int status);
===============================================================================

5.8.4 wait 函数

进程的退出状态必须用 wait 函数由父进程获取, 否则形成
僵尸进程
wait/waitpid

===============================================================================
       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait (int *status)
       pid_t waitpid (pid_t pid, int *status, int options);
-------------------------------------------------------------------------------
    * wait 等待某个子进程退出.
    * waitpid 可等待指定的子进程退出, 可指定 WNOHANG 选项而不阻塞调用进程的执行.
-------------------------------------------------------------------------------
用来帮助判断退出状态的宏:

       WIFEXITED (status) 
               is non-zero if the child exited normally.

       WEXITSTATUS (status)
               evaluates to the least significant eight  bits  of
               the  return  code  of  the child which terminated,
               which may have been set as the argument to a  call
               to  exit()  or as the argument for a return state-
               ment in the main program.  This macro can only  be
               evaluated if WIFEXITED returned non-zero.

       WIFSIGNALED (status)
               returns  true  if the child process exited because
               of a signal which was not caught.

       WTERMSIG (status)
               returns the number of the signal that  caused  the
               child process to terminate. This macro can only be
               evaluated if WIFSIGNALED returned non-zero.

       WIFSTOPPED (status)
               returns true if the child process which caused the
               return is currently stopped; this is only possible
               if the call was done using WUNTRACED.

       WSTOPSIG (status)
               returns the number of the signal which caused  the
               child  to  stop.  This macro can only be evaluated
               if WIFSTOPPED returned non-zero.
===============================================================================

wait3/wait4

===============================================================================
       #define _USE_BSD

       #include <sys/types.h>
       #include <sys/resource.h>
       #include <sys/wait.h>

       pid_t wait3 (int *status, int options, struct rusage *rusage)
       pid_t wait4 (pid_t pid, int *status, int options, struct rusage *rusage)
------------------------------------------------------------------------------
    * wait3 和 wait4 分别是 wait 和 waitpid 的扩展调用, 它们可以返回子进程的
      资源使用情况.
===============================================================================

5.8.5 竞态

Race Condition 的概念

5.8.6 exec 函数以及脚本解释器

exec 函数组

===============================================================================
       #include <unistd.h>

       extern char **environ;

       int execl ( const char *path, const char *arg, ...);
       int execlp ( const char *file, const char *arg, ...);
       int execle ( const char *path, const char *arg, ..., char * const envp[]);
       int execv ( const char *path, char *const argv[]);
       int execvp ( const char *file, char *const argv[]);

       int  execve  (const  char  *filename, char *const argv [], char *const envp[]);
------------------------------------------------------------------------------
    * 以可变参数形式传递命令行时, 最后一个参数应该为 NULL 指针.
    * execve 是实际的系统调用, 而其他的函数以库函数实现, 最终要调用 execve.
===============================================================================

脚本解释器

5.8.7 system 函数

#自学#

5.8.8 进程记帐

#自学#

5.8.9 用户和组标识符

#自学#, 注意几个概念: 实际 UID/GID, 有效 UID/GID, 保存的 SUID/SGID
getlogin: 获取 login name
cuserid: 获取进程有效用户 ID 的关联名称

===============================================================================
       #include <unistd.h>

       char * getlogin ( void );

       #include <stdio.h>

       char * cuserid ( char *string );
===============================================================================

5.8.10 进程时间

times: 获取当前进程的时间信息, 包括用户态时间, 核心态时间等等

===============================================================================
       #include <unistd.h>
       #include <sys/times.h>

       clock_t times (struct tms *buf);
-------------------------------------------------------------------------------
       struct  tms  {
           clock_t tms_utime;  /* user time */
           clock_t tms_stime;  /* system time */
           clock_t tms_cutime; /* user time of children */
           clock_t tms_cstime; /* system time of children */
       };
-------------------------------------------------------------------------------
    * times 返回的时间均以系统启动以来的时钟滴答计算.
===============================================================================