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Subject: Re: Memory mapping: =?UTF-8?Q?MAP=5FPRIVATE=20and=20msync=28=29?=
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Date: Sun, 07 Apr 24 19:04:27 +0000
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From: pehache <pehache.7@gmail.com>
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As advised, I'm copying this post here

Le 07/04/2024 à 17:18, pehache a écrit :
> Le 07/04/2024 à 15:34, pehache a écrit :
>> Hello,
>> 
>> When memory mapping a file with the MAP_PRIVATE flag, the modifications (writes) 
>> only exist in memory and are not written back to the file.
>> 
>> According to the man pages, calling msync (3) on a such a mapping does NOT 
>> writes the changes back:
>> 
>> "When the msync() function is called on MAP_PRIVATE mappings, any modified data 
>> shall not be written to the underlying object and shall not cause such data to be 
>> made visible to other processes"
>> https://linux.die.net/man/3/msync
>> 
>> So: is there a way to write the changes back to the file? 
>> 
>> An obvious application is:
>> - mapping the file with MAP_PRIVATE
>> - make some modifications in memory only (fast) while keeping the original 
>> version on disk (safe)
>> - at some point (when the user decides, and once the consistency of the changes 
>> have been verified) writing the modifications to the disk
>> 
>> I'm pretty sure it exists some way or another, but I don't know how.
> 
> At the moment what I'm doing is something like:
> 
> ========================================
>     fd = open("aaa",O_RDWR); 
>     p =       mmap ( NULL                        
>                    , n                  
>                    , PROT_READ | PROT_WRITE      
>                    , MAP_PRIVATE | MAP_NORESERVE  
>                    , fd                         
>                    , 0 );
> 
>     // writing to p; the changes exist only in memory
> 
>     void* p2 = mmap( NULL                        
>                    , n                  
>                    , PROT_READ | PROT_WRITE      
>                    , MAP_SHARED | MAP_NORESERVE  
>                    , fd                         
>                    , 0 );
> 
>     memcpy(p2,p,n);  // copying everything from p to p2
>     msync(p2,n);
> 
>     // unmap/remap p so it's ready for new changes
>     munmap(p,n);
>     p =       mmap ( NULL                        
>                    , n                  
>                    , PROT_READ | PROT_WRITE      
>                    , MAP_PRIVATE | MAP_NORESERVE  
>                    , fd                         
>                    , 0 );
>     
> ========================================
> 
> This works, but:
> 
> - the whole content is copied, not only the changed content
> - is this code legal? Is there any potential conflict between the 2 mapping, 
> with an undefined behavior?