mmap and shared memory

[Matias Surdi]

Suppose I've a process P1, which generates itself a lot of data , for
example 2Mb.
Then, I've a process P2 which must access P1 shared memory and,
probably, modify this data.
To accomplish this, I've been digging around python's mmap module, but I
can't figure how to use it without files.

Could anybody explain me how could this be accomplished? An example will
be very appreciated. Thanks a lot for your help.

[greg]

Carl Banks wrote:

In C you can use the mmap call to request a specific physical location
in memory (whence I presume two different processes can mmap anonymous
memory block in the same location)

Um, no, it lets you specify the *virtual* address in the process's
address space at which the object you specify is to be mapped.

As far as I know, the only way two unrelated processes can share
memory via mmap is by mapping a file. An anonymous block is known
only to the process that creates it -- being anonymous, there's
no way for another process to refer to it.

However, if one process is forked from the other, the parent
can mmap an anonymous block and the child will inherit that
mapping.

(I suppose if both processes had sufficient privileges they
could map physical memory out of /dev/mem, but that would be
*really* dangerous!)

--
Greg

[Jeff Schwab]

greg wrote:

Carl Banks wrote:

>In C you can use the mmap call to request a specific physical location
in memory (whence I presume two different processes can mmap anonymous
memory block in the same location)

Um, no, it lets you specify the *virtual* address in the process's
address space at which the object you specify is to be mapped.

As far as I know, the only way two unrelated processes can share
memory via mmap is by mapping a file. An anonymous block is known
only to the process that creates it -- being anonymous, there's
no way for another process to refer to it.

On POSIX systems, you can create a shared memory object without a file
using shm_open. The function returns a file descriptor.

However, if one process is forked from the other, the parent
can mmap an anonymous block and the child will inherit that
mapping.

(I suppose if both processes had sufficient privileges they
could map physical memory out of /dev/mem, but that would be
*really* dangerous!)

--
Greg

[Nikita the Spider]

In article <Ko******************************@comcast.com>,
Jeff Schwab <je**@schwabcenter.comwrote:

greg wrote:

Carl Banks wrote:

In C you can use the mmap call to request a specific physical location
in memory (whence I presume two different processes can mmap anonymous
memory block in the same location)

Um, no, it lets you specify the *virtual* address in the process's
address space at which the object you specify is to be mapped.

As far as I know, the only way two unrelated processes can share
memory via mmap is by mapping a file. An anonymous block is known
only to the process that creates it -- being anonymous, there's
no way for another process to refer to it.

On POSIX systems, you can create a shared memory object without a file
using shm_open. The function returns a file descriptor.

Sorry I missed the OP, but you might be interested in this shared memory
module for Python:
http://NikitaTheSpider.com/python/shm/

--
Philip
http://NikitaTheSpider.com/
Whole-site HTML validation, link checking and more

[Jeff Schwab]

Nikita the Spider wrote:

In article <Ko******************************@comcast.com>,
Jeff Schwab <je**@schwabcenter.comwrote:

>greg wrote:

>>Carl Banks wrote:
In C you can use the mmap call to request a specific physical location
in memory (whence I presume two different processes can mmap anonymous
memory block in the same location)
Um, no, it lets you specify the *virtual* address in the process's
address space at which the object you specify is to be mapped.

As far as I know, the only way two unrelated processes can share
memory via mmap is by mapping a file. An anonymous block is known
only to the process that creates it -- being anonymous, there's
no way for another process to refer to it.

On POSIX systems, you can create a shared memory object without a file
using shm_open. The function returns a file descriptor.

Sorry I missed the OP, but you might be interested in this shared memory
module for Python:
http://NikitaTheSpider.com/python/shm/

Thanks; I just downloaded it. It seems to be missing the INSTALL file;
any idea where I could find that, or should I write to the author?

[Nikita the Spider]

In article <C5******************************@comcast.com>,
Jeff Schwab <je**@schwabcenter.comwrote:

Nikita the Spider wrote:

In article <Ko******************************@comcast.com>,
Jeff Schwab <je**@schwabcenter.comwrote:

greg wrote:
Carl Banks wrote:
In C you can use the mmap call to request a specific physical location
in memory (whence I presume two different processes can mmap anonymous
memory block in the same location)
Um, no, it lets you specify the *virtual* address in the process's
address space at which the object you specify is to be mapped.

As far as I know, the only way two unrelated processes can share
memory via mmap is by mapping a file. An anonymous block is known
only to the process that creates it -- being anonymous, there's
no way for another process to refer to it.
On POSIX systems, you can create a shared memory object without a file
using shm_open. The function returns a file descriptor.

Sorry I missed the OP, but you might be interested in this shared memory
module for Python:
http://NikitaTheSpider.com/python/shm/

Thanks; I just downloaded it. It seems to be missing the INSTALL file;
any idea where I could find that, or should I write to the author?

The main author has been AWOL for some years now. I'm the current
maintainer. Sorry about the missing INSTALL file. Looks like I made
reference to it in the README but never created it. It installs with the
normal setup.py:

sudo python setup.py

I'll put out an updated package with an INSTALL file one of these days.
Thanks for pointing that out.

Cheers

--
Philip
http://NikitaTheSpider.com/
Whole-site HTML validation, link checking and more