do_fork使用的flags

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1. fork, vfork and clone三者最终都会调用do_fork函数，三者的差别就是参数上的不同而已。
fork的实现：
   do_fork(CLONE_SIGCHLD,...)
clone的实现：
     do_fork(CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGCHLD,...)
vfork的实现：
     do_fork(CLONE_VFORK|CLONE_VM|CLONE_SIGCHLD,...)
2. Linux使用copy on wirte的技术，Linux中的fork代价仅仅是创建子进程的页表结构和创建一个task_struct结构。
3. 为了优化那些：fork然后就是exec的程序，Linux提供了vfork。vfork时，父进程会被阻塞，直到子进程调用了exec或exit，因为此时不复制页表结构。
4. clone()系统调用是fork()的推广形式，它允许新进程共享父进程的存储空间、文
   件描述符和信号处理程序

补充：通常Linux是让子进程优先执行的！下面是do_fork使用的flags

41 #define CSIGNAL         0x000000ff      /* signal mask to be sent at exit */
42 #define CLONE_VM        0x00000100      /* set if VM shared between processes */
43 #define CLONE_FS        0x00000200      /* set if fs info shared between processes */
44 #define CLONE_FILES     0x00000400      /* set if open files shared between processes */
45 #define CLONE_SIGHAND   0x00000800      /* set if signal handlers and blocked signals shared */
46 #define CLONE_PTRACE    0x00002000      /* set if we want to let tracing continue on the child too */
47 #define CLONE_VFORK     0x00004000      /* set if the parent wants the child to wake it up on mm_release */
48 #define CLONE_PARENT    0x00008000      /* set if we want to have the same parent as the cloner */
49 #define CLONE_THREAD    0x00010000      /* Same thread group? */
50 #define CLONE_NEWNS     0x00020000      /* New namespace group? */
51 #define CLONE_SYSVSEM   0x00040000      /* share system V SEM_UNDO semantics */
52 #define CLONE_SETTLS    0x00080000      /* create a new TLS for the child */
53 #define CLONE_PARENT_SETTID     0x00100000      /* set the TID in the parent */
54 #define CLONE_CHILD_CLEARTID    0x00200000      /* clear the TID in the child */
55 #define CLONE_DETACHED          0x00400000      /* Unused, ignored */
56 #define CLONE_UNTRACED          0x00800000      /* set if the tracing process can't force CLONE_PTRACE on this clone */
57 #define CLONE_CHILD_SETTID      0x01000000      /* set the TID in the child */
58 #define CLONE_STOPPED           0x02000000      /* Start in stopped state */