Sharing a semaphore between users
in [MFC]
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From: Alexander Grigoriev on 25 Jun 2008 10:55 "Joseph M. Newcomer" <newcomer(a)flounder.com> wrote in message news:d3e3645od2dfcbs5dcjh5aohclr7ghabn5(a)4ax.com... > Some questions, see below... > >>> SecAtt.nLength=sizeof(SECURITY_DESCRIPTOR); >>> if ((g_hSem=OpenSemaphore(SEMAPHORE_ALL_ACCESS,TRUE,"Global\ >>> \MyApp"))==NULL) >>> { >>> g_hSem = CreateSemaphore(&SecAtt,3,3,"Global\\MyApp"); >>> } >>> > **** > In the above sequence, suppose I have two sequences in thread A and B that > can be > represented as o (open) and c (create), where ~ means the operation fails > > > > if the sequences are > > Ao~Bo~AcBc > Ao~Bo~BcAc > > Why the OpenSemaphore call at all? Why not just CreateSemaphore? > CreateSemaphore says if > the named semaphore already exists, it applies SEMAPHORE_ALL_ACCESS, which > is all that is > happening here. > > So what has been gained here? Note the window between open and create > allows the other > thread to create it before the first thread gets to create it. There is a subtle problem with Create<object> (addressed in Vista, IIRC), that if the object already exists, the OS will try to open it with ALL_ACCESS, which may not be granted, though it doesn't apply to this example. This is why it makes sense to try Open<object> first.
From: Joseph M. Newcomer on 25 Jun 2008 14:24
See below... On Wed, 25 Jun 2008 07:55:41 -0700, "Alexander Grigoriev" <alegr(a)earthlink.net> wrote: > >"Joseph M. Newcomer" <newcomer(a)flounder.com> wrote in message >news:d3e3645od2dfcbs5dcjh5aohclr7ghabn5(a)4ax.com... >> Some questions, see below... >> >>>> SecAtt.nLength=sizeof(SECURITY_DESCRIPTOR); >>>> if ((g_hSem=OpenSemaphore(SEMAPHORE_ALL_ACCESS,TRUE,"Global\ >>>> \MyApp"))==NULL) >>>> { >>>> g_hSem = CreateSemaphore(&SecAtt,3,3,"Global\\MyApp"); >>>> } >>>> >> **** >> In the above sequence, suppose I have two sequences in thread A and B that >> can be >> represented as o (open) and c (create), where ~ means the operation fails >> >> >> >> if the sequences are >> >> Ao~Bo~AcBc >> Ao~Bo~BcAc >> >> Why the OpenSemaphore call at all? Why not just CreateSemaphore? >> CreateSemaphore says if >> the named semaphore already exists, it applies SEMAPHORE_ALL_ACCESS, which >> is all that is >> happening here. >> >> So what has been gained here? Note the window between open and create >> allows the other >> thread to create it before the first thread gets to create it. > >There is a subtle problem with Create<object> (addressed in Vista, IIRC), >that if the object already exists, the OS will try to open it with >ALL_ACCESS, which may not be granted, though it doesn't apply to this >example. This is why it makes sense to try Open<object> first. > **** But, under the scenario I show above, the Create *would* be called twice, because both Opens would fail. joe Joseph M. Newcomer [MVP] email: newcomer(a)flounder.com Web: http://www.flounder.com MVP Tips: http://www.flounder.com/mvp_tips.htm |