Support discontinuous memory maps

[willglynn]

I'm working with a file format that scatters its data around on disk. It's composed of multiple logical data streams which are intended to be memory mapped. The segments are all page-aligned, and records within a stream constantly refer to other records based on their positions within the stream entirely oblivious to the physical layout.

Ultimately I want a single continuous read-only &[u8] given a list of offsets and sizes:

0x462C000:   0x1000 bytes
0x473D000: 0x8C4000 bytes
0x5003000: 0xFFE000 bytes
0x6003000: 0x29E000 bytes

In the case of POSIX mmap(), I think this would be done by using mmap(NULL, <total size>, MAP_ANON, PROT_NONE, 0, 0) to ask for a chunk of address space without actually committing resources to it, followed by repeated mmap(<address>, <size>, MAP_FIXED, PROT_READ, <fd>, <offset>) to map each file piece to the appropriate location within that space.

On Windows, I think this is VirtualAllocEx(MEM_RESERVE) to get the address space, then MapViewOfFileEx() to map each piece.

[m4b]

Here's how a unix dynamic linker might do this:

1. first reserve the entire breadth of space: https://github.com/m4b/dryad/blob/585189f3986c284aad52701d6de944a06192ab01/src/loader.rs#L47-L70
2. then mmap chunks with the FIXED flag into the reserved space: https://github.com/m4b/dryad/blob/585189f3986c284aad52701d6de944a06192ab01/src/loader.rs#L141-L156

E.g., the program headers need to be mmap'd into different locations, but the code + data reference each other assuming it's essentially continguous memory, I think this is similar to your requirements.

Some sugar to do this in this lib would be nice though :)

[retep998]

This is not going to happen on Windows. To quote MSDN:

No other memory allocation can take place in the region that is used for mapping, including the use of the VirtualAlloc or VirtualAllocEx function to reserve memory.

[danburkert]

The only thing I think we can really do here is expose MAP_FIXED, which is already being tracked in #21 , so I'm going to go ahead and close this.