mmap的用例

Question

I'm currently studying for my OS finals. The teacher in some papers is briefly mentioning the mmap function (memory map).

As I understand it ( correct me if i'm wrong ), mmap is used to load some files from the physical memory to the RAM (after a page default). The problem is that I don't see any practical reason for this other then to make the access time to that file faster.

Am I correct? Is mmap only used for this?

Answer 1

"mmap" has lots of purposes:

1. Mapping a file for faster read/write access is certainly one use

2. Shared memory (eg for interprocess communications) is another

3. mmap is also used to map I/O port addresses for low-level device communications

Answer 2

mmap is used to load some files from the physical memory to the RAM (after a page default)

to load the missing pages . also modifications can be written to the disk the same way!

• Performance (you don't have to load the whole file), works really well if you have random access.
• It can considerably make your code more compact, you don't have to worry about file I/O.
• The OS can handle memory management, decide which pages to keep in memory and which to discard.

Answer 3

In addition to @paulsm4's answer:

1. ...
2. ...
3. ...
4. Most modern malloc(3) implementations use mmap(2) to manage private process memory.
5. Dynamic link-loader ld.so(8) uses it for mapping shared libraries.

Answer 4

mmap takes memory management out of the hands of the programmer to a large extent, and puts it in the hands of the OS.

It's about demand paging using the virtual memory subsystem from disk to physical memory.

So to look at the 11111th byte of a file, instead of seeking and reading, you can mmap and use an array index. The OS will keep surroundiung data in its "buffer cache" (page cache really).

Here's an example: http://stromberg.dnsalias.org/~strombrg/pbmonherc.html

The example's a little messy because it was written at a time when Linux had mmap support in its kernel, but the C library didn't yet have a stub for calling it. But you can pretty much ignore mmap.c. The example uses mmap to set pixels on and off using a monochromatic display adapter.

Another reasonable use is for a bloom filter: http://stromberg.dnsalias.org/~strombrg/drs-bloom-filter/

...but on 32 bit OS's, the maximum size of an mmap'd memory region kinda hurts.