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RedHat 9 (Linux i386) - man page for vfork (redhat section 2)

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 applica-
       tions 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 memory with its parent, including the stack,
       until execve() is issued by the child.  The child must not return from the  current  func-
       tion 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  sys-
       tems. 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)


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