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VFORK(2)							      VFORK(2)

NAME
       vfork, vforkx - spawn new process in a virtual memory efficient 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  use‐
       ful  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 modifica‐
       tion  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 applications, however, is unsafe due  to  race	condi‐
       tions  that can cause the child process to become deadlocked and conse‐
       quently block both the child and parent process from execution  indefi‐
       nitely.

       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 even‐
       tual 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, 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	behav‐
       ior  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 legiti‐
       mate 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 process.

       The vforkx() function will fail if:

       EINVAL
		 The flags argument is invalid.

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:

       ┌────────────────────┬─────────────────┐
       │  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 sig‐
       nal 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 prob‐
       lems  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().

				 Dec 13, 2006			      VFORK(2)
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