进程和线程是我们平时接触的比较多的两个概念，特别是线程机制，很多语言原生就支持它。前段时间主要演示了下linux下进程和线程的创建，这篇文章对其创建的过程做一个简单的分析，错误之处，还请您斧正。
在linux下，线程其实就是一个轻量级的进程，所以其实现都是通过调用给do_fork函数传入不同的参数实现的。先来看下这几个函数：
1 int sys_fork(struct pt_regs *regs)
2 {
3 return do_fork(SIGCHLD, regs->sp, regs, 0, NULL, NULL);
4 }
2 {
3 return do_fork(SIGCHLD, regs->sp, regs, 0, NULL, NULL);
4 }
1 int sys_vfork(struct pt_regs *regs)
2 {
3 return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs->sp, regs, 0,
4 NULL, NULL);
5 }
2 {
3 return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs->sp, regs, 0,
4 NULL, NULL);
5 }
1 sys_clone(unsigned long clone_flags, unsigned long newsp,
2 void __user *parent_tid, void __user *child_tid, struct pt_regs *regs)
3 {
4 if (!newsp)
5 newsp = regs->sp;
6 return do_fork(clone_flags, newsp, regs, 0, parent_tid, child_tid);
7 }
2 void __user *parent_tid, void __user *child_tid, struct pt_regs *regs)
3 {
4 if (!newsp)
5 newsp = regs->sp;
6 return do_fork(clone_flags, newsp, regs, 0, parent_tid, child_tid);
7 }
上面的代码中，并没有看到fork()函数的实现，其实fork函数的执行过程大致像这样：普通程序调用fork()-->库函数fork()-->系统调用(fork功能号)-->由功能号在 sys_call_table[]中寻到sys_fork()函数地址-->调用sys_fork()，这就完成拉从用户态到内核态的变化过程。所以，实际上，fork函数对应的实现就是sys_fork。
和上面的过程类似，上面的几个函数分别对应与fork，vfork和clone，可以看到，其实际上都是通过一个调用do_fork函数实现的，不同处只是其传入的参数不同。 首先来看看传入的参数的，先看看这些传入的参数分别代表的含义：
/*
* 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 between processes */
#define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */
#define CLONE_PTRACE 0x00002000 /* set if we want to let tracing continue on the child too */
#define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_PARENT 0x00008000 /* set if we want to have the same parent as the 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 if the 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 in stopped state */
#define CLONE_NEWUTS 0x04000000 /* New utsname group? */
#define CLONE_NEWIPC 0x08000000 /* New ipcs */
#define CLONE_NEWUSER 0x10000000 /* New user namespace */
#define CLONE_NEWPID 0x20000000 /* New pid namespace */
#define CLONE_NEWNET 0x40000000 /* New network namespace */
#define CLONE_IO 0x80000000 /* Clone io context */
* 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 between processes */
#define CLONE_SIGHAND 0x00000800 /* set if signal handlers and blocked signals shared */
#define CLONE_PTRACE 0x00002000 /* set if we want to let tracing continue on the child too */
#define CLONE_VFORK 0x00004000 /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_PARENT 0x00008000 /* set if we want to have the same parent as the 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 if the 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 in stopped state */
#define CLONE_NEWUTS 0x04000000 /* New utsname group? */
#define CLONE_NEWIPC 0x08000000 /* New ipcs */
#define CLONE_NEWUSER 0x10000000 /* New user namespace */
#define CLONE_NEWPID 0x20000000 /* New pid namespace */
#define CLONE_NEWNET 0x40000000 /* New network namespace */
#define CLONE_IO 0x80000000 /* Clone io context */