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?