VFORK (2)
create a child process and block parent
SYNOPSIS
#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 memory 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 .\c
"
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
ioctl s
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.
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