kernel学习之sys_fork,sys_vfork,sys_clone和kernel_thread

上面进程的创建最终依赖于：do_fork，只是向其传递了不同的参数。

longdo_fork(unsigned long clone_flags,
          unsigned long stack_start,
          struct pt_regs *regs,
          unsigned long stack_size,
          int __user *parent_tidptr,
          int __user *child_tidptr)

参数clone_flags非常重要，fork把其设置为SIGCHLD,vfork把其设置为CLONE_VFORK|CLONE_VM|SIGCHLD,clone由用户调用时传递。总的来说，do_fork由clone_flags决定。其值可以自由组合决定。include/linux/sched.h中宏定义：

/*
 *cloning flags:
 */
#define CSIGNAL             0x000000ff    /*signal mask to be sent at exit */
#define CLONE_VM            0x00000100    /* set if VM shared between processes */
#define CLONE_FS            0x00000200    /* set if fs info shared between processes*/
#define CLONE_FILES         0x00000400    /* set if open files shared betweenprocesses */
#define CLONE_SIGHAND       0x00000800    /* set if signal handlers and blockedsignals shared */
#define CLONE_PTRACE        0x00002000    /* set if we want to let tracing continue onthe child too */
#define CLONE_VFORK         0x00004000    /* set if the parent wants the child to wakeit up on mm_release */
#define CLONE_PARENT        0x00008000    /* set if we want to have the same parent asthe cloner */
#define CLONE_THREAD        0x00010000    /* Same thread group? */
#define CLONE_NEWNS         0x00020000    /* New namespace group? */
#define CLONE_SYSVSEM       0x00040000    /* share system V SEM_UNDO semantics */
#define CLONE_SETTLS        0x00080000    /* create a new TLS for the child */
#define CLONE_PARENT_SETTID 0x00100000    /*set the TID in the parent */
#define CLONE_CHILD_CLEARTID     0x00200000    /*clear the TID in the child */
#define CLONE_DETACHED      0x00400000    /* Unused,ignored */
#define CLONE_UNTRACED      0x00800000    /* set ifthe tracing process can't force CLONE_PTRACE on this clone */
#define CLONE_CHILD_SETTID  0x01000000    /*set the TID in the child */
#define CLONE_STOPPED       0x02000000    /* Start instopped state */
#define CLONE_NEWUTS        0x04000000    /* Newutsname group? */
#define CLONE_NEWIPC        0x08000000    /* Newipcs */
#defineCLONE_NEWUSER       0x10000000    /* New user namespace */
#define CLONE_NEWPID        0x20000000    /* New pidnamespace */
#define CLONE_NEWNET        0x40000000    /* Newnetwork namespace */
#define CLONE_IO            0x80000000    /* Clone io context */

上面的宏定义都占用了独立的bit，所以能或|组合使用。其低八位没有使用，是为了能和信号量组合使用。

内核线程创建接口：

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