三个函数都定义在内核源代码kernel/fork.c文件中，其中
do_fork()的定义如下：
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long do_fork(unsigned long clone_flags,
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unsigned long stack_start,
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unsigned long stack_size,
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int __user *parent_tidptr,
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int __user *child_tidptr)
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{
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struct task_struct *p;
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int trace = 0;
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long nr;
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/*
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* Determine whether and which event to report to ptracer. When
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* called from kernel_thread or CLONE_UNTRACED is explicitly
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* requested, no event is reported; otherwise, report if the event
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* for the type of forking is enabled.
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*/
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if (!(clone_flags & CLONE_UNTRACED)) {
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if (clone_flags & CLONE_VFORK)
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trace = PTRACE_EVENT_VFORK;
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else if ((clone_flags & CSIGNAL) != SIGCHLD)
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trace = PTRACE_EVENT_CLONE;
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else
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trace = PTRACE_EVENT_FORK;
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if (likely(!ptrace_event_enabled(current, trace)))
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trace = 0;
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}
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p = copy_process(clone_flags, stack_start, stack_size,
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child_tidptr, NULL, trace);
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/*
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* Do this prior waking up the new thread - the thread pointer
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* might get invalid after that point, if the thread exits quickly.
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*/
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if (!IS_ERR(p)) {
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struct completion vfork;
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struct pid *pid;
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trace_sched_process_fork(current, p);
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pid = get_task_pid(p, PIDTYPE_PID);
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nr = pid_vnr(pid);
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if (clone_flags & CLONE_PARENT_SETTID)
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put_user(nr, parent_tidptr);
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if (clone_flags & CLONE_VFORK) {
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p->vfork_done = &vfork;
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init_completion(&vfork);
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get_task_struct(p);
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}
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wake_up_new_task(p);
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/* forking complete and child started to run, tell ptracer */
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if (unlikely(trace))
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ptrace_event_pid(trace, pid);
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if (clone_flags & CLONE_VFORK) {
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if (!wait_for_vfork_done(p, &vfork))
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ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
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}
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put_pid(pid);
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} else {
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nr = PTR_ERR(p);
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}
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return nr;
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}
第29行调用copy_process()复制父进程的相关信息创建子进程的进程描述符struct task_struct，包括复制寄存器值和必要的数据结构；
第53行根据进程优先级将创建的子进程放入CPU的调度队列，该进程进入就绪状态。
copy_process()函数代码较多，其中主要调用的函数有：
dup_task_struct()创建新的进程描述符task_struct；
copy_semundo()复制父进程的semaphore undo_list到子进程；
copy_files()和copy_fs()复制父进程文件系统相关的环境到子进程；
copy_sighand()和copy_signal()复制父进程信号处理相关的环境到子进程；
copy_mm()复制父进程内存管理相关的环境到子进程，包括页表、地址空间和代码数据；
copy_thread()设置子进程的执行环境，如子进程运行时各CPU寄存器的值、子进程的kernel栈的起始地址；
sched_fork()设置子进程调度相关的参数，即子进程的运行CPU、初始时间片长度和静态优先级等。
fork系统调用的奇妙之处在于一次调用两次返回，两次返回分别返回到父进程和子进程，在copy_thread()函数中可以一窥玄机：
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*childregs = *current_pt_regs(); //复制内核堆栈
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childregs->ax = 0; //为什么子进程的fork返回0，这里就是原因！
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p->thread.sp = (unsigned long) childregs; //调度到子进程时的内核栈顶
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p->thread.ip = (unsigned long) ret_from_fork; //调度到子进程时的第一条指令地址
从上面的代码可知，子进程的堆栈中保存的寄存器ax值设为0，从而子进程在fork调用中的返回值为0，并且其堆栈指针寄存器sp设置为其内核栈顶，指令指针寄存器ip与父进程相同，因为子进程和父进程共享只读的文本段，系统调用结束后返回到同样的地址继续执行。
dup_task_struct()的定义如下：
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static struct task_struct *dup_task_struct(struct task_struct *orig)
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{
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struct task_struct *tsk;
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struct thread_info *ti;
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int node = tsk_fork_get_node(orig);
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int err;
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tsk = alloc_task_struct_node(node);
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if (!tsk)
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return NULL;
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ti = alloc_thread_info_node(tsk, node);
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if (!ti)
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goto free_tsk;
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-
err = arch_dup_task_struct(tsk, orig);
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if (err)
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goto free_ti;
-
-
tsk->stack = ti;
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#ifdef CONFIG_SECCOMP
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/*
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* We must handle setting up seccomp filters once we're under
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* the sighand lock in case orig has changed between now and
-
* then. Until then, filter must be NULL to avoid messing up
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* the usage counts on the error path calling free_task.
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*/
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tsk->seccomp.filter = NULL;
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#endif
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setup_thread_stack(tsk, orig);
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clear_user_return_notifier(tsk);
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clear_tsk_need_resched(tsk);
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set_task_stack_end_magic(tsk);
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#ifdef CONFIG_CC_STACKPROTECTOR
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tsk->stack_canary = get_random_int();
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#endif
-
-
/*
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* One for us, one for whoever does the "release_task()" (usually
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* parent)
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*/
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atomic_set(&tsk->usage, 2);
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#ifdef CONFIG_BLK_DEV_IO_TRACE
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tsk->btrace_seq = 0;
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#endif
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tsk->splice_pipe = NULL;
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tsk->task_frag.page = NULL;
-
-
account_kernel_stack(ti, 1);
-
-
return tsk;
-
-
free_ti:
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free_thread_info(ti);
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free_tsk:
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free_task_struct(tsk);
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return NULL;
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}
第8行alloc_task_struct_node()用于为新的task_struct分配内存并返回；
第12行alloc_thread_info_node()为thread_info结构分配内存并返回；
第16行arch_dup_task_struct()用于复制父进程的task_struct结构；
