mmap罢！因为64位地址极大丰富

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <asm-generic/mman-common.h>
int a;
int main () {
  int b;
  char* c = new char[1];
  char* d = (char*)malloc(1);
  printf("%p %p %p %p\n", &a, c, d, &b);
  unsigned long addr = 0x7efffff1e000;
  size_t n = 0x40000000;
  char* m = (char*)mmap((void*)addr, n, PROT_READ | PROT_WRITE,
    MAP_PRIVATE | MAP_ANON | MAP_FIXED | MAP_UNINITIALIZED,
    -1, 0);
  printf("%p\n", m);
  // This flag, MAP_UNINITIALIZED, is honored only if the kernel was configured with the
  // CONFIG_MMAP_ALLOW_UNINITIALIZED option. Because of the security implications, that
  // option is normally enabled only on embedded devices (i.e., devices where one has complete
  // control of the contents of user memory).
  for (int i = 0; i < n; i++) {
    if (m[i]) { printf("i=%d\n", i); break; }
  }
  memset(m, 0x5a, n);
  puts("ok");
}

$ gcc mmap.cpp; a.out
0x55d602f26044 0x55d63cc79eb0 0x55d63cc79ed0 0x7fff841e85cc
0x7efffff1e000
ok

$ clang++ mmap.cpp; a.out
0x557518029044 0x55751b620eb0 0x55751b620ed0 0x7ffcdf084828
0x7efffff1e000
ok

(0x7ffdfdf100ac - 0x7efffff1e000) ≈ 1T

比方说有个set<UserDefinedStruct>，如何save/load，或者说序列化/反序列化？load时如何避免再建一遍树？

以前我用自定义STL容器的allocator和placement new做过个程序，mmap到固定地址。但32位地址空间太小了，不实用——说得好像不实用的原因是地址空间小似的:-)

如何知道0x7efffff1e000这样的地址？一开始我用0x1{11个0}，mmap返回NULL。然后用0，mmap返回个和0x7efffff1e000很接近的地址。

12*4才48位。1 << 48 = 2 ** 48 = 65536G = 64T

1 << 1 = 2 ** 1；上面的算法没错。

2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768 65536应该背住。

2 ** 20 = 1048576也常用。1E6，1E8, 1E9也方便。

头条上有人问为啥没有128位电脑。首先问之前赖好动下脖子上的那玩意:-)，其次就数据宽度而言，avx2算512位的吧。