Re: [Discuss-gnuradio] Mac OS X Build

[John Gilmore]

> my shared memory problem in VrBuffer.cc. I still have to sit down and 
> translate the System V shared memory routines to BSD style.

The basic problem is to map a section of RAM into virtual memory
twice, contiguously.  I can think of two ways to do it:

  *  Allocate the memory and map it to a file.  Then map the file
     to the second address range too.  (You can unlink the file then,
     I think, so that it will disappear after the process terminates.)

  *  Allocate the memory anonymously (e.g. from /dev/zero).  Then map
     the second address range from the process's own address space
     (e.g. using file "/proc/MYPID/mem" or "/proc/self/mem" if BSD
     supports this).

Mmap is more flexible than shmat; you can map particular byte ranges
to particular sets of pages.  This should simplify the code.

The current code maps a 2N sized buffer, solely to determine where in its
address space there's room for a 2N sized, page-aligned, buffer.  It then
detaches that buffer and deletes it.

The mmap-based code can map a 2N sized buffer immediately, and then
just unmap the second half of it, and remap that second half from the
first half.

        John