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// Copyright 2019 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// ----------------
// This program demonstrates mmap'ing a ring-buffer's N bytes of physical
// memory three times, to be a contiguous block of 3*N bytes. The three
// pointers (base + 0*N + i), (base + 1*N + i) and (base + 2*N + i), which are
// different addresses in virtual memory, all alias the same physical address.
//
// Reading or writing a chunk of length M <= N is therefore a simple memcpy,
// without having to explicitly wrap around the ring-buffer boundaries.
//
// This is similar to the technique discussed in
// https://lo.calho.st/quick-hacks/employing-black-magic-in-the-linux-page-table/
//
// This program differs from that web page's discussion by mapping the physical
// memory three times, not just two. This lets us read or write, implicitly
// wrapping, both forwards (after the middle mapping's end) and backwards
// (before the middle mapping's start). That web page only considers forwards
// reads or writes. Backwards reads are useful when decoding a Lempel-Ziv style
// compression format, copying from history (recently decoded bytes).
//
// Its output should be:
//
// middle[-8] == 0x00 == 0x00 == middle[131064]
// middle[-7] == 0x00 == 0x00 == middle[131065]
// middle[-6] == 0x00 == 0x00 == middle[131066]
// middle[-5] == 0x00 == 0x00 == middle[131067]
// middle[-4] == 0x00 == 0x00 == middle[131068]
// middle[-3] == 0x00 == 0x00 == middle[131069]
// middle[-2] == 0x20 == 0x20 == middle[131070]
// middle[-1] == 0x21 == 0x21 == middle[131071]
// middle[ 0] == 0x22 == 0x22 == middle[131072]
// middle[ 1] == 0x23 == 0x23 == middle[131073]
// middle[ 2] == 0x12 == 0x12 == middle[131074]
// middle[ 3] == 0x13 == 0x13 == middle[131075]
// middle[ 4] == 0x30 == 0x30 == middle[131076]
// middle[ 5] == 0x31 == 0x31 == middle[131077]
// middle[ 6] == 0x32 == 0x32 == middle[131078]
// middle[ 7] == 0x17 == 0x17 == middle[131079]
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
// We should be able to do:
//
// #include <sys/memfd.h>
//
// to get the memfd_create function signature, but memfd_create is relatively
// recent. For some reason, this #include hits "No such file or directory" on
// Ubuntu 18.04 (linux 4.15, glibc 2.27), and there's also been problems on
// Debian systems. Instead, we explicitly define our own memfd_create.
static int //
my_memfd_create(const char* name, unsigned int flags) {
return syscall(__NR_memfd_create, name, flags);
}
#define N (128 * 1024)
void* //
make_ring_buffer() {
int page_size = getpagesize();
if ((N < page_size) || (page_size <= 0) || ((N % page_size) != 0)) {
return NULL;
}
int memfd = my_memfd_create("ring", 0);
if (memfd == -1) {
return NULL;
}
if (ftruncate(memfd, N) == -1) {
return NULL;
}
// Have the kernel find a contiguous range of unused address space.
void* base = mmap(NULL, 3 * N, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
if (base == MAP_FAILED) {
return NULL;
}
// Map that "ring" file 3 times, filling that range exactly.
for (int i = 0; i < 3; i++) {
void* p = mmap(base + (i * N), N, PROT_READ | PROT_WRITE,
MAP_FIXED | MAP_SHARED, memfd, 0);
if (p == MAP_FAILED) {
return NULL;
}
}
close(memfd);
return base;
}
int //
main(int argc, char** argv) {
uint8_t* base = make_ring_buffer();
if (!base) {
fprintf(stderr, "could not make ring buffer\n");
return 1;
}
for (int i = 0; i < 8; i++) {
base[i] = 0x10 + i;
}
memcpy(base + N - 2, "\x20\x21\x22\x23", 4);
base[(0 * N) + 4] = 0x30;
base[(1 * N) + 5] = 0x31;
base[(2 * N) + 6] = 0x32;
uint8_t* middle = base + N;
for (int i = -8; i < 8; i++) {
int j = N + i;
printf("middle[%2d] == 0x%02X == 0x%02X == middle[%6d]\n", i,
middle[i], middle[j], j);
}
return 0;
}