IEEE Standards Interpretations for IEEE Std 1003.1b™-1993 IEEE Standard for Information Technology - Portable Operating System Interfaces (POSIX(R)) - Part 1: System Application Program Interface (API) - Amendment 1: Realtime Extension [C language]
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Interpretation Request #3
Topic:mmap Relevant Sections: 188.8.131.52 Classification: (to be assigned)
I'd like to receive clarification on text I read in 1003.1b-1993. Specifically the question has to do with mmap() and what is meant by the following (removed word for word from page 236, lines 213-215): "The mapping established by mmap() replaces any previous mappings for those whole pages containing any part of the process's address space starting at "pa" and continuing for "len" bytes."
There are a couple of ways this can be interpreted and I'd like to find out which is the intended one. a) Does this mean if I map page 3 of a file in one call and page 5 in another call and then map pages 3-5 in a separate call, the call succeeds and the previous mappings of page 3 and page 5 are no longer valid? If so, does that apply to both MAP_PRIVATE and MAP_SHARED? b) Does this apply to MAP_FIXED where lets say the user has page 1 of file A mapped at address 0x40005000 and then a mmap (MAP_FIXED) at address 0x40004000 for three pages (range 0x40004000-0x40006fff) on file B overwrites the other mmap? In other words am I allowed overlay mappings with different files? If so, both MAP_SHARED and MAP_FIXED? c) Does the overwrite only apply to mmap() collisions or any address collision. For instance, can a process mmap() over its data object or other non mmap objects?
For question a), the standard is clear that the mapping is replaced the addresses overlap but not replaced where they don't. Page 238 line 274: "...nor shall it replace an extant mapping." Part one of the question: ... the other mappings are no longer valid? - this is incorrect, unless the addresses are specified in MAP_FIXED and overlap. The standard is clear that this applies to both MAP_PRIVATE and MAP_SHARED.
Question b, the standard is clear that you can replace the existing mappings using MAP_FIXED independent ofMAP_SHARED and MAP_PRIVATE. Question c, the standard is silent on whether you can map over objects other than ones mapped by MMAP. Implementations can allow MAP_FIXED mapping over other kinds of memory or not allow it.
Rationale for Interpretation