Cygwin/Cygserver Shared memory

0
votes

I was trying to migrate some shared memory code from CENTOS(3.5) to CYGWIN(2.8.1, win10). the shared memory generally work like this:

  1. Spawn a shared memory at a process by shmget.

  2. Map the shared memory on this process by the shmat and record the location, then fill some information into the memory.

  3. Map the shared memory on another process by the "shmat", pass the location of last process recorded, because we expect that both processes will mapping the shared memory at the same address.

Here are some code to explain:

// one process
size_t size = 1024 * 1024;//1M
int id = shmget(IPC_PRIVATE, size, 0660);
char *madr = 0;
char *location = shmat(id, madr, 0);

// another process
char *location1 = shmat(id, location , 0);
// !!!we hope location1 and location should be the same!!!

On Centos it works well.

On Cygwin one process mapped the shared memory at 0xffd90000, another process is not same with it but mapped at oxffdb0000. we check that the memory 0xffd90000 is available on that process.

1
ccygwinshared-memory

1 Answers

0
votes

Wrong expectation also on Linux, see
https://linux.die.net/man/2/shmat

Be aware that the shared memory segment attached in this way may be attached at different addresses in different processes. Therefore, any pointers maintained within the shared memory must be made relative (typically to the starting address of the segment), rather than absolute.