4 ms·

But you call mmap only once per file: it gives you a pointer and then you just use the pointer normally. While for read() you have to call once per read() opera

But you call mmap only once per file: it gives you a pointer and then you just use the pointer normally. While for read() you have to call once per read() operation (minus some user space buffering).

correct_horse 6y ago
mmap is more general than memory mapping files. It "establish[es] a mapping between an address space of a process and a memory object". In particular, many allocators (malloc and friends) use mmap to get a block of memory from the OS with mmap(0, size, PROT_READ|PROT_WRITE, MAP_ANONYMOUS, -1, 0); or similar.
- mhh__ 6y ago
  Do any allocators use anything else these days? D's std.experimental.allocator module is helpful for building allocators bit by bit even for eventual use in C++ or C.
  - JoshTriplett 6y ago
    > Do any allocators use anything else these days? Yes, some allocators still support using sbrk, though that's a bad idea for security reasons (ASLR).
    - floatboth 6y ago
      Also sbrk just doesn't exist on many new platforms (e.g. FreeBSD does not have it on arm64 and riscv64)
      - saagarjha 6y ago
        And macOS emulates it. It's a fairly archaic design at this point.
  - jashmatthews 6y ago
    IIUC, glibc malloc still manages one arena with sbrk and then grabs other arenas via mmap.