How to portably extend a file accessed using mmap()

Question

We\'re experimenting with changing SQLite, an embedded database system, to use mmap() instead of the usual read() and write() calls to access the database file on disk. Usin

Answer 1

2 will work but you don't have to rely on the OS happening to have space available, you can reserve your address space beforehand so your fixed mmapings will always succeed.

For instance, To reserve one gigabyte of address space. Do a

mmap(NULL, 1U << 30, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

Which will reserve one gigabyte of continuous address space without actually allocating any memory or resources. You can then perform future mmapings over this space and they will succeed. So mmap the file into the beginning of the space returned, then mmap further sections of the file as needed using the fixed flag. The mmaps will succeed because your address space is already allocated and reserved by you.

Note: linux also has the MAP_NORESERVE flag which is the behavior you would want for the initial mapping if you were allocating RAM, but in my testing it is ignored as PROT_NONE is sufficient to say you don't want any resources allocated yet.