Mapping files into virtual memory in C on Windows

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");
}