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The example code below will open and read a text file. In order to use ReadFile in flat assembler you either have to include ‘WIN32AX.INC’ or handle the imports from kernel32.dll manually. I’ve used the shorter version and included win32ax.in.

After the file has been read, I display the result of ReadFile in a MessageBox. This sample code should work on all operating systems using the Win32 API:

format pe console 4.0
include 'WIN32AX.INC'

.data
        FileTitle db 'file.txt',0
        hFile dd ?
        nSize dd ?
        lpBytesRead dd ?
        lpBuffer rb 8192

        MessageBoxCaption db 'Output:',0
.code
        main:
        invoke  CreateFile, FileTitle, GENERIC_READ, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0 ; Open the file (to get its handle)
        ; TODO: TestApiError
        mov [hFile], eax ; Save the file's handle to hFile
        invoke  GetFileSize, [hFile], 0 ; Determine the file size
        ; TODO: TestApiError
        mov [nSize], eax ; Save the file size given by EAX
        invoke  ReadFile, [hFile], lpBuffer, [nSize], lpBytesRead, 0 ; Now read the full file
        ; TODO: TestApiError
        invoke CloseHandle, [hFile] ; Handle should be closed after the file has been read
        invoke MessageBox, NULL, addr lpBuffer, addr MessageBoxCaption, MB_OK ; Easy way of outputing the text
        invoke  ExitProcess, 0

.end main

Please make sure the file file.txt exists (in the same directory as your application), else it will crash, as I’ve done no error handling in order to keep the code short.