Re: vfork()

[Ketan Mukadam <kmukadam@neomagic.com>]

Exactly right...i mistaken wrote that its a *combination* of fork() and 
exec(), the difference between vfork() and fork() is simply that since 
most of the call sequence involving fork() and exec() is like this

if (! fork ())
   exec (...);

It means that as soon as a child is forked, the next step is to replace 
its address space with that of the executable in exec(). So the step of 
actually copying the address spaces of parent into child is useless and 
uneccessary overhead, this extra copying is prevented in the call vfork().

Hence fork() can be used where vfork() is used, with an overhead, but 
simply calling vfork() without an exec is useless.


Ketan


Javier Guerra wrote:
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> MAASK Group wrote:
> 
>>>Vfork is a combination of fork() and exec().
>>
>>I am confused. The programmer explicitly has to call exec() after a
>>vfork(). right?
> 
> 
> right, vfork() isn't a combination of fork() and exec(), more exactly, it's a
> version of fork() to be used only when immediatly followed by a exec()
> so, instead of writing
> 
> 	if (! fork ())
> 		exec (...);
> 
> you could write
> 
> 	if (! vfork ())
> 		exec (...);
> 
> and avoid the copying of the process segment that would be overwritten by
> exec().  in modern OSs, the process is marked copy-on-write, so it's just as
> fast, without the restrictions.  that's why it's man page tells you not to
> use it anymore.
> 
> - ------
> Javier
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