vfork() is an obsolete optimization. Before good memory management, fork() made a full copy of the parent's memory, so it was pretty expensive. since in many cases a fork() was followed by exec(), which discards the current memory map and creates a new one, it was a needless expense. Nowadays, fork() doesn't copy the memory; it's simply set as "copy on write", so fork()+exec() is just as efficient as vfork()+exec(). clone() is the syscall used by fork(). with some parameters, it creates a new process, with others, it creates a thread. the difference between them is just which data structures (memory space, processor state, stack, PID, open files, etc) are shared or not.

execve() replaces the current executable image with another one loaded from an executable file. fork() creates a child process. vfork() is a historical optimized version of fork(), meant to be used when execve() is called directly after fork(). It turned out to work well in non-MMU systems (where fork() cannot work in an efficient manner) and when fork()ing processes with a huge memory footprint to run some small program (think Java's Runtime.exec()). POSIX has standardized the posix_spawn() to replace these latter two more modern uses of vfork(). posix_spawn() does the equivalent of a fork()/execve(), and also allows some fd juggling in between. It's supposed to replace fork()/execve(), mainly for non-MMU platforms. pthread_create() creates a new thread. clone() is a Linux-specific call, which can be used to implement anything from fork() to pthread_create(). It gives a lot of control. Inspired on rfork(). rfork() is a Plan-9 specific call. It's supposed to be a generic call, allowing several degrees of sharing, between full processes and threads.

在fork()中，子进程或父进程将根据CPU选择执行。 但是在vfork()中，child肯定会先执行。子程序终止后，父程序将执行。

fork()，vfork()和clone()都调用do_fork()来完成实际工作，但是使用不同的参数。

asmlinkage int sys_fork(struct pt_regs regs)
{
    return do_fork(SIGCHLD, regs.esp, &regs, 0);
}

asmlinkage int sys_clone(struct pt_regs regs)
{
    unsigned long clone_flags;
    unsigned long newsp;

    clone_flags = regs.ebx;
    newsp = regs.ecx;
    if (!newsp)
        newsp = regs.esp;
    return do_fork(clone_flags, newsp, &regs, 0);
}
asmlinkage int sys_vfork(struct pt_regs regs)
{
    return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, regs.esp, &regs, 0);
}
#define CLONE_VFORK 0x00004000  /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_VM    0x00000100  /* set if VM shared between processes */

SIGCHLD means the child should send this signal to its father when exit.

对于fork来说，子进程和父进程拥有独立的VM页表，但出于效率考虑，fork并不会真正复制任何页，它只是将所有可写的页设置为只读。因此，当子进程想要在该页上写一些东西时，页面异常发生，内核将分配一个从旧页面克隆的具有写权限的新页面。这就是所谓的“写后复制”。

对于vfork，虚拟内存完全是由子节点和父节点分配的——正因为如此，父节点和子节点不能同时唤醒，因为它们会相互影响。因此父进程将在“do_fork()”结束时睡觉，当子进程调用exit()或execve()时醒来，因为它将拥有新的页表。下面是父线程睡觉的代码(在do_fork()中)。

if ((clone_flags & CLONE_VFORK) && (retval > 0))
down(&sem);
return retval;

下面是在mm_release()中由exit()和execve()调用的唤醒父进程的代码。

up(tsk->p_opptr->vfork_sem);

对于sys_clone()，它更加灵活，因为您可以向它输入任何clone_flags。所以pthread_create()用许多clone_flags调用这个系统调用:

int clone_flags = (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGNAL | CLONE_SETTLS | CLONE_PARENT_SETTID | clone_child_cleard | CLONE_SYSVSEM);

总结:fork()，vfork()和clone()将创建与父进程共享资源的不同挂载的子进程。我们也可以说vfork()和clone()可以创建线程(实际上它们是进程，因为它们有独立的task_struct)，因为它们与父进程共享VM页表。

fork() - creates a new child process, which is a complete copy of the parent process. Child and parent processes use different virtual address spaces, which is initially populated by the same memory pages. Then, as both processes are executed, the virtual address spaces begin to differ more and more, because the operating system performs a lazy copying of memory pages that are being written by either of these two processes and assigns an independent copies of the modified pages of memory for each process. This technique is called Copy-On-Write (COW). vfork() - creates a new child process, which is a "quick" copy of the parent process. In contrast to the system call fork(), child and parent processes share the same virtual address space. NOTE! Using the same virtual address space, both the parent and child use the same stack, the stack pointer and the instruction pointer, as in the case of the classic fork()! To prevent unwanted interference between parent and child, which use the same stack, execution of the parent process is frozen until the child will call either exec() (create a new virtual address space and a transition to a different stack) or _exit() (termination of the process execution). vfork() is the optimization of fork() for "fork-and-exec" model. It can be performed 4-5 times faster than the fork(), because unlike the fork() (even with COW kept in the mind), implementation of vfork() system call does not include the creation of a new address space (the allocation and setting up of new page directories). clone() - creates a new child process. Various parameters of this system call, specify which parts of the parent process must be copied into the child process and which parts will be shared between them. As a result, this system call can be used to create all kinds of execution entities, starting from threads and finishing by completely independent processes. In fact, clone() system call is the base which is used for the implementation of pthread_create() and all the family of the fork() system calls. exec() - resets all the memory of the process, loads and parses specified executable binary, sets up new stack and passes control to the entry point of the loaded executable. This system call never return control to the caller and serves for loading of a new program to the already existing process. This system call with fork() system call together form a classical UNIX process management model called "fork-and-exec".