The difference between fork vfork exec and clone

Question

I was looking to find the difference between these four on Google and I expected there to be a huge amount of information on this  but there really wasn t any solid comparison between the four calls  I set about trying to compile a kind of basic at-a-glance look at the differences between these system calls and here s what I got  Is all this information correct am I missing anything important   Fork   The fork call basically makes a duplicate of the current process  identical in almost every way  not everything is copied over  for example  resource limits in some implementations but the idea is to create as close a copy as possible   The new process  child  gets a different process ID  PID  and has the PID of the old process  parent  as its parent PID  PPID   Because the two processes are now running exactly the same code  they can tell which is which by the return code of fork - the child gets 0  the parent gets the PID of the child  This is all  of course  assuming the fork call works - if not  no child is created and the parent gets an error code  Vfork   The basic difference between vfork and fork is that when a new process is created with vfork    the parent process is temporarily suspended  and the child process might borrow the parent s address space  This strange state of affairs continues until the child process either exits  or calls execve    at which point the parent process continues  This means that the child process of a vfork   must be careful to avoid unexpectedly modifying variables of the parent process  In particular  the child process must not return from the function containing the vfork   call  and it must not call exit    if it needs to exit  it should use  exit    actually  this is also true for the child of a normal fork     Exec   The exec call is a way to basically replace the entire current process with a new program  It loads the program into the current process space and runs it from the entry point  exec   replaces the current process with a the executable pointed by the function  Control never returns to the original program unless there is an exec   error  Clone   Clone  as fork  creates a new process  Unlike fork  these calls allow the child process to share parts of its execution context with the calling process  such as the memory space  the table of file descriptors  and the table of signal handlers  When the child process is created with clone  it executes the function application fn arg    This differs from fork  where execution continues in the child from the point of the original fork call   The fn argument is a pointer to a function that is called by the child process at the beginning of its execution  The arg argument is passed to the fn function  When the fn arg  function application returns  the child process terminates  The integer returned by fn is the exit code for the child process  The child process may also terminate explicitly by calling exit 2  or after receiving a fatal signal  Information gotten form    Differences between fork and exec http   www allinterview com showanswers 59616 html http   www unixguide net unix programming 1 1 2 shtml http   linux about com library cmd blcmdl2 clone htm  Thanks for taking the time to read this

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The fork   vfork   and clone   all call the do fork   to do the real work  but with different parameters   asmlinkage int sys fork struct pt regs regs        return do fork SIGCHLD  regs esp   amp 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   amp regs  0     asmlinkage int sys vfork struct pt regs regs        return do fork CLONE VFORK   CLONE VM   SIGCHLD  regs esp   amp 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    For fork  the child and father has the independent VM page table  but since the efficiency  fork will not really copy any pages  it just set all the writeable pages to readonly for child process  So when child process want to write something on that page  an page exception happen and kernel will alloc a new page cloned from the old page with write permission  That s called  copy on write     For vfork  the virtual memory is exactly by child and father---just because of that  father and child can t be awake concurrently since they will influence each other  So the father will sleep at the end of  do fork    and awake when child call exit   or execve   since then it will own new page table  Here is the code in do fork    that the father sleep   if   clone flags  amp  CLONE VFORK   amp  amp   retval  gt  0   down  amp sem   return retval    Here is the code in mm release   called by exit   and execve    which awake the father   up tsk- gt p opptr- gt vfork sem     For sys clone    it is more flexible since you can input any clone flags to it  So pthread create   call this system call with many clone flags   int clone flags    CLONE VM   CLONE FS   CLONE FILES   CLONE SIGNAL   CLONE SETTLS   CLONE PARENT SETTID   CLONE CHILD CLEARTID   CLONE SYSVSEM    Summary  the fork   vfork   and clone   will create child processes with different mount of sharing resource with the father process  We also can say the vfork   and clone   can create threads actually they are processes since they have independent task struct  since they share the VM page table with father process

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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

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in fork    either child or parent process will execute based on cpu selection    But in vfork    surely child will execute first  after child terminated  parent will execute

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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

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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

[linux] The difference between fork(), vfork(), exec() and clone()

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