VFORK(2)						     Linux Programmer's Manual							  VFORK(2)

NAME

       vfork - create a child process and block parent

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       pid_t vfork(void);

STANDARD DESCRIPTION

       (From  XPG4  /  SUSv2  /  POSIX	draft.)  The vfork() function has the same effect as fork(), except that the behaviour is undefined if the
       process created by vfork() either modifies any data other than a variable of type pid_t used to store the return  value	from  vfork(),	or
       returns	from  the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec
       family of functions.

ERRORS

       EAGAIN Too many processes - try again.

       ENOMEM There is insufficient swap space for the new process.

LINUX DESCRIPTION

       vfork, just like fork(2), creates a child process of the calling process.  For details and return value and errors, see fork(2).

       vfork() is a special case of clone(2).  It is used to create new processes without copying the page tables of the parent process.   It  may
       be useful in performance sensitive applications where a child will be created which then immediately issues an execve().

       vfork() differs from fork in that the parent is suspended until the child makes a call to execve(2) or _exit(2).  The child shares all mem-
       ory with its parent, including the stack, until execve() is issued by the child.  The child must not return from the  current  function	or
       call exit(), but may call _exit().

       Signal handlers are inherited, but not shared.  Signals to the parent arrive after the child releases the parent.

HISTORIC DESCRIPTION

       Under  Linux,  fork()  is  implemented using copy-on-write pages, so the only penalty incurred by fork() is the time and memory required to
       duplicate the parent's page tables, and to create a unique task structure for the child.  However, in the  bad  old  days  a  fork()  would
       require	making a complete copy of the caller's data space, often needlessly, since usually immediately afterwards an exec() is done. Thus,
       for greater efficiency, BSD introduced the vfork system call, that did not fully copy the address space of the parent process, but borrowed
       the  parent's  memory  and thread of control until a call to execve() or an exit occurred. The parent process was suspended while the child
       was using its resources.  The use of vfork was tricky - for example, not modifying data in the parent process  depended	on  knowing  which
       variables are held in a register.

BUGS

       It  is rather unfortunate that Linux revived this spectre from the past.  The BSD manpage states: "This system call will be eliminated when
       proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of vfork  as  it  will,  in  that
       case, be made synonymous to fork."

       Formally  speaking, the standard description given above does not allow one to use vfork() since a following exec might fail, and then what
       happens is undefined.

       Details of the signal handling are obscure and differ between systems.  The BSD manpage states: "To avoid a  possible  deadlock	situation,
       processes  that	are  children in the middle of a vfork are never sent SIGTTOU or SIGTTIN signals; rather, output or ioctls are allowed and
       input attempts result in an end-of-file indication."

       Currently (Linux 2.3.25), strace(1) cannot follow vfork() and requires a kernel patch.

HISTORY

       The vfork() system call appeared in 3.0BSD.  In	BSD  4.4  it  was  made  synonymous  to  fork(),  but  NetBSD  introduced  it  again,  cf.
       http://www.netbsd.org/Documentation/kernel/vfork.html  .   In  Linux,  it  has  been  equivalent  to  fork()  until 2.2.0-pre6 or so. Since
       2.2.0-pre9 (on i386, somewhat later on other architectures) it is an independent system call. Support was added in glibc 2.0.112.

CONFORMING TO

       The vfork call may be a bit similar to calls with the same name in other operating systems. The requirements put on vfork by the  standards
       are  weaker  than  those  put on fork, so an implementation where the two are synonymous is compliant. In particular, the programmer cannot
       rely on the parent remaining blocked until a call of execve() or _exit() and cannot rely on any specific behaviour w.r.t. shared memory.

SEE ALSO

       clone(2), execve(2), fork(2), wait(2)

Linux 2.2.0							    1999-11-01								  VFORK(2)