Mapping files into virtual memory in C on Windows - c

Question

On POSIX systems, I am able to use the mmap function to read the contents of a file faster than getline, getc, etc. This is important in the program that I am developing as it is expected to read very large files into memory; iteratively collecting lines using getline is too costly. Portability is also a requirement of my software, so if I use mmap, I need to find a way to memory map files using the WinApi, as I'd rather not compile through cygwin/msys. From a cursory search I identified this MSDN article which describes very briefly a way to map files into memory, however, from trawling through documentation I can't make head nor tails of how to actually implement it, and I'm stuck on finding example snippets of code, like there are for POSIX mmap.
How do I use the WinApi's memory mapping options to read a file into a char*?

Answer 1

How do I use the WinApi's memory mapping options to read a file into a
char*?
Under Windows, when you map a file in memory, you get a pointer to the memory location where the first byte of the file has been mapped. You can cast that pointer to whatever datatype you like, including char*.
In other words, it is Windows which decide where the mapped data will be in memory. You cannot provide a char* and expect Windows will load data there.
This means that if you already have a char* and want the data from the file in the location pointed by that char*, then you have to copy it. Not a good idea in terms of performances.
Here is a simple program dumping a text file by mapping the file into memory and then displaying all ASCII characters. Tested with MSVC2019.
#include <stdio.h>
#include <Windows.h>
int main(int argc, char *argv[])
{
TCHAR *lpFileName = TEXT("hello.txt");
HANDLE hFile;
HANDLE hMap;
LPVOID lpBasePtr;
LARGE_INTEGER liFileSize;
hFile = CreateFile(lpFileName,
GENERIC_READ, // dwDesiredAccess
0, // dwShareMode
NULL, // lpSecurityAttributes
OPEN_EXISTING, // dwCreationDisposition
FILE_ATTRIBUTE_NORMAL, // dwFlagsAndAttributes
0); // hTemplateFile
if (hFile == INVALID_HANDLE_VALUE) {
fprintf(stderr, "CreateFile failed with error %d\n", GetLastError());
return 1;
}
if (!GetFileSizeEx(hFile, &liFileSize)) {
fprintf(stderr, "GetFileSize failed with error %d\n", GetLastError());
CloseHandle(hFile);
return 1;
}
if (liFileSize.QuadPart == 0) {
fprintf(stderr, "File is empty\n");
CloseHandle(hFile);
return 1;
}
hMap = CreateFileMapping(
hFile,
NULL, // Mapping attributes
PAGE_READONLY, // Protection flags
0, // MaximumSizeHigh
0, // MaximumSizeLow
NULL); // Name
if (hMap == 0) {
fprintf(stderr, "CreateFileMapping failed with error %d\n", GetLastError());
CloseHandle(hFile);
return 1;
}
lpBasePtr = MapViewOfFile(
hMap,
FILE_MAP_READ, // dwDesiredAccess
0, // dwFileOffsetHigh
0, // dwFileOffsetLow
0); // dwNumberOfBytesToMap
if (lpBasePtr == NULL) {
fprintf(stderr, "MapViewOfFile failed with error %d\n", GetLastError());
CloseHandle(hMap);
CloseHandle(hFile);
return 1;
}
// Display file content as ASCII charaters
char *ptr = (char *)lpBasePtr;
LONGLONG i = liFileSize.QuadPart;
while (i-- > 0) {
fputc(*ptr++, stdout);
}
UnmapViewOfFile(lpBasePtr);
CloseHandle(hMap);
CloseHandle(hFile);
printf("\nDone\n");
}