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Date: Thu, 16 Feb 2023 15:59:36 +0100
To: Yeo Kai Wei <yeokaiwei AT hotmail DOT com>
Subject: Re: [QUERY] Help With Semaphore Error on Windows : Cygwin
Message-ID: <Y+5E2Hftco5JMrpl@calimero.vinschen.de>
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Hi Kai,

Apart from what Takashi already wrote, there's another bug in this code:

On Feb 16 12:04, Yeo Kai Wei via Cygwin wrote:
> #define ByteSize 512
> #define BackingFile "/shMemEx"
> #define AccessPerms 0644
> #define SemaphoreName "mysemaphore"
                        ^^^^^^^^^^^^

What Takashi wrote.  The reason that you don't notice that sem_open
actuially failed is...

> 	//Create the semaphore
> 	sem_t* semptr = sem_open(	SemaphoreName,//name 
> 					O_CREAT, //create semaphore
> 					AccessPerms, //protection permissions
> 					0);	//Initial value
> 
> 	//ERROR
> 	if(semptr == (void*) -1)
                     ^^^^^^^^^^
                     This.

Why do you test for -1?  If you read the POSIX man page for sem_open,
you'll see this:

  Upon successful completion, the sem_open() function shall return the
  address of the semaphore. Otherwise, it shall return a value of
  SEM_FAILED [...]
  ^^^^^^^^^^

SEM_FAILED is not necessarily -1.  On Cygwin it's defined as

  #define SEM_FAILED ((sem_t *) 0)

in /usr/include/semaphore.h.

So your code just seems to fail in sem_post, but actually that's
because sem_open failed and your code checks for the wrong return
value.


HTH,
Corinna

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