Is there any reasons to not use "mmap" to read files?

[H. S. Teoh]

On Sat, Feb 12, 2022 at 07:01:09PM -0800, Ali Çehreli via Digitalmars-d wrote:
> On 2/12/22 05:17, rempas wrote:
> 
> > a system call every single time
> 
> I have a related experience: I realized that very many ftell() calls
> that I were making were very costly. I saved a lot of time after
> realizing that I did not need to make the calls because I could
> maintain a 'long' variable to keep track of where I was in the file.
> 
> I assumed ftell() would do the same but apparently not.
[...]

I think the reason is the ftell involves an OS API call, because fread()
uses the underlying read() syscall which reads from where it left off
last, and there could be multiple threads reading from the same file
descriptor, so the only way for fseek/ftell to work correctly is via a
syscall into the kernel.  Obviously, this would be expensive, as it
would involve a kernel context-switch as well as acquiring and releasing
a lock on the file descriptor.


T

-- 
Too many people have open minds but closed eyes.