vfork() - Unix, Linux System Call

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vfork - create a child process and block parent
#include <sys/types.h>
#include <unistd.h> 
pid_t vfork(void); 


pid_t vfork(void); 

(From 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.
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’s memory.
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.
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.
The vfork() system call appeared in 3.0BSD.
In 4.4BSD 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.
4.3BSD, POSIX.1-2001. 
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.