System Calls                                             vfork(2)


NAME

     vfork, vforkx - spawn new process in a virtual memory  effi-
     cient way


SYNOPSIS

     #include <unistd.h>

     pid_t vfork(void);

     #include <sys/fork.h>

     pid_t vforkx(int flags);


DESCRIPTION

     The vfork() and vforkx()  functions  create  a  new  process
     without  fully copying the address space of the old process.
     These functions are useful in instances where the purpose of
     a fork(2) operation is to create a new system context for an
     execve() operation (see exec(2)).

     Unlike with the fork() function, the child  process  borrows
     the  parent's  memory  and thread of control until a call to
     execve() or an exit (either  abnormally  or  by  a  call  to
     _exit()  (see  exit(2)).   Any modification made during this
     time to any part of memory in the child process is reflected
     in  the  parent  process on return from vfork() or vforkx().
     The parent process is suspended while the child is using its
     resources.

     In a multithreaded application, vfork() and vforkx()  borrow
     only  the  thread of control that called vfork() or vforkx()
     in the parent; that is, the child contains only one  thread.
     The  use  of  vfork()  or vforkx() in multithreaded applica-
     tions, however, is unsafe due to race  conditions  that  can
     cause  the  child  process  to  become deadlocked and conse-
     quently block both the child and parent process from  execu-
     tion indefinitely.

     The vfork() and vforkx() functions can normally be used  the
     same  way  as  fork() and forkx(), respectively. The calling
     procedure, however, should not return while running  in  the
     child's  context,  since the eventual return from vfork() or
     vforkx() in the parent would be to a  stack  frame  that  no
     longer  exists. The _exit() function should be used in favor
     of exit(3C) if unable  to  perform  an  execve()  operation,
     since   exit()  will  invoke  all  functions  registered  by
     atexit(3C) and will flush and close standard  I/O  channels,

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System Calls                                             vfork(2)

     thereby  corrupting  the  parent process's standard I/O data
     structures. Care must be taken in the child process  not  to
     modify any global or local data that affects the behavior of
     the parent process  on  return  from  vfork()  or  vforkx(),
     unless such an effect is intentional.

     Unlike fork() and forkx(), fork handlers are  not  run  when
     vfork() and vforkx() are called.

     The vfork() and vforkx()  functions  are  deprecated.  Their
     sole  legitimate  use as a prelude to an immediate call to a
     function from the exec family  can  be  achieved  safely  by
     posix_spawn(3C) or posix_spawnp(3C).

  Fork Extensions
     The vforkx() function accepts a flags argument consisting of
     a  bitwise  inclusive-OR  of  zero  or more of the following
     flags, which are defined in the header <sys/fork.h>:
       FORK_NOSIGCHLD
       FORK_WAITPID

     See fork(2) for descriptions of these flags.  If  the  flags
     argument is 0, vforkx() is identical to vfork().


RETURN VALUES

     Upon successful completion, vfork() and vforkx() return 0 to
     the  child  process  and returns the process ID of the child
     process to the parent process. Otherwise, -1 is returned  to
     the  parent  process, no child process is created, and errno
     is set to indicate the error.


ERRORS

     The vfork() and vforkx() functions will fail if:

     EAGAIN    The system-imposed limit on the  total  number  of
               processes  under  execution (either system-quality
               or by a single user) would be exceeded. This limit
               is determined when the system is generated.

     ENOMEM    There is insufficient swap space for the new  pro-
               cess.

     The vforkx() function will fail if:

     EINVAL    The flags argument is invalid.

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System Calls                                             vfork(2)


ATTRIBUTES

     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | Interface Stability         | Obsolete                    |
    |_____________________________|_____________________________|
    | MT-Level                    | Unsafe                      |
    |_____________________________|_____________________________|


SEE ALSO

     exec(2), exit(2), fork(2), ioctl(2),  atexit(3C),  exit(3C),
     posix_spawn(3C),      posix_spawnp(3C),     signal.h(3HEAD),
     wait(3C), attributes(5), standards(5)


NOTES

     To avoid a possible deadlock situation, processes  that  are
     children  in  the  middle of a vfork() or vforkx() are never
     sent SIGTTOU or SIGTTIN signals; rather,  output  or  ioctls
     are allowed and input attempts result in an EOF indication.

     To forestall parent memory corruption due to race conditions
     with  signal  handling,  vfork()  and  vforkx() treat signal
     handlers in the child process in  the  same  manner  as  the
     exec(2)  functions:  signals  set to be caught by the parent
     process are set to the default action (SIG_DFL) in the child
     process  (see signal.h(3HEAD)).  Any attempt to set a signal
     handler in the child before execve() to anything other  than
     SIG_DFL  or SIG_IGN is disallowed and results in setting the
     handler to SIG_DFL.

     On some systems, the implementation of vfork() and  vforkx()
     cause  the parent to inherit register values from the child.
     This can create problems for certain optimizing compilers if
     <unistd.h>  is not included in the source calling vfork() or
     if <sys/fork.h>  is  not  included  in  the  source  calling
     vforkx().

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