Question

I have a producer process, that writes to a mmap’d file and a consumer process that reads from it. This is on Linux.

If the producer makes a change to the mmap and its not instantly flushed, when the consumer access it what happens? Will it get the old version from the disk, or is it clever enough to get the unflushed page?

Answer 1

From the mmap() manual page:

MAP_SHARED

Share this mapping with all other processes that map this object.
Storing to the region is equivalent to
writing to the file. The file may not
actually be updated until msync(2) or
munmap(2) are called.

Bottom line: changes will be immediately visible to other processes if you specify so when calling mmap(). According to the documentation you have to use either MAP_SHARED or MAP_PRIVATE, which controls the behaviour with regard to other processes mapping the same file area.