Unix: sharing already-mapped memory between processes

I have a pre-built userspace library that has an API along the lines of

void getBuffer (void **ppBuf, unsigned long *pSize);
void bufferFilled (void *pBuf, unsigned long size);

The idea being that my code requests a buffer from the lib, fills it with stuff, then hands it back to the lib.

I want another process to be able to fill this buffer. I can do this by creating some new shared buffer via shm*/shm_* APIs, have the other process fill that, then copy it to the lib's buffer in the lib's local process, but this has the overhead of an extra (potentially large) copy.

Is there a way to share memory that has ALREADY been mapped for a process? eg something like:

[local lib process]
getBuffer (&myLocalBuf, &mySize);
shmName = shareThisMemory (myLocalBuf, mySize);

[other process]
myLocalBuf = openTheSharedMemory (shmName);

That way the other process could write directly into the lib's buffer. (Synchronization between the processes is already taken care of so no problems there).