Operating Systemshard

What is memory-mapped I/O (mmap) and when would you use it?

Tags

#mmap#memory#io

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Answer

mmap maps a file or device into a process’s address space so it can be accessed like memory. It’s useful for large files, random access, and zero-copy sharing.

Advanced answer

Deep dive

mmap trades explicit I/O for VM mechanisms:

The OS loads pages on-demand via page faults.
Can reduce syscalls and copy overhead.
Useful for shared memory between processes.
Requires careful handling of page faults and alignment.

Examples

Random access to a large file:

mmap file -> pointer arithmetic -> OS pages in data as needed

Common pitfalls

Assuming mmap is always faster than read/write.
Not handling SIGBUS on truncated files.
Poor access patterns causing thrashing.

Interview follow-ups

When would you prefer read/write over mmap?
How does mmap interact with page cache?
What are the risks for concurrent writers?

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