问在Linux与Windows上读取大文件(mmap vs CreateFileMapping/MapViewOfFile)

EN

// includes for using a specific platform API
#ifdef _WIN32
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
// using windows handle void*
#define handle_type HANDLE
#else
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>
// using file descriptors
#define handle_type int
#endif

using uint_t = std::size_t;

// open the file -----------------------------------------------------------------------------
handle_type open(const std::string& filename) {
#ifdef _WIN32
  // see windows file mapping api for parameter explanation
  return ::CreateFileA(filename.c_str(), GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL); // private access
#else
  return ::open(filename.c_str(), O_RDONLY);
#endif
}


// get the memory size to later have a bound for reading -------------------------------------
uint_t memory_size(handle_type fid) {
#ifdef _WIN32
  LARGE_INTEGER size{};
  if (!::GetFileSizeEx(fid, &size)) {
    std::cerr << "file not found\n";
    return size.QuadPart;
  }
  return size.QuadPart;
#else
  struct stat sb;
  // get the file stats and check if not zero size
  if (fstat(fid, &sb)) {
    std::cerr << "file not found\n";
    return decltype(sb.st_size){};
  }
  return sb.st_size;
#endif
}

// get the actual char array to access memory ------------------------------------------------
const char* memory_map(handle_type fid, uint_t memory_size) {
#ifdef _WIN32
  HANDLE mapper = ::CreateFileMapping(fid, NULL, PAGE_READONLY, 0, 0, NULL);
  return reinterpret_cast<const char*>(::MapViewOfFile(mapper, FILE_MAP_READ, 0, 0, memory_size));
#else
  return reinterpret_cast<const char*>(::mmap(NULL, memory_size, PROT_READ, MAP_PRIVATE, fid, 0));
#endif
}