The win32 API CreateFile() and CloseFile() Functions

The win32 CreateFile() function opens existing files and creates new ones.

Normally, file and directory names used as API function arguments can be up to 255 characters long, and the pathnames are limited to MAX_PATH characters. A period separates a file’s name from its extension, which typically indicate the file’s type.

The first argument to a win32 CreateFile() function is the lpName, which is a pointer to the null-terminated string that names the file, pipe, or other object to create.  The lpName argument is of type LPCTSTR.

The second argument to CreateFile() is dwAccess, which specifies the read and write access using the GENERIC_READ and GENERIC_WRITE flags, or a combination thereof.

#include <Windows.h>
#include <stdio.h>

int main(void){

    DWORD x = GENERIC_READ;
    DWORD y = GENERIC_WRITE;

    printf("%x\n", GENERIC_READ);
    printf("%x\n", x);
    printf("%x\n", GENERIC_WRITE);
    printf("%x\n", y);
    printf("%x\n", GENERIC_READ | GENERIC_WRITE);

    return 0;

}

The third argument to CreateFile() is dwShareMode, it is a bitwise OR combination of three values, 0, FILE_SHARE_READ, and FILE_SHARE_WRITE.

#include <Windows.h>
#include <stdio.h>

int main(void){

    DWORD read = FILE_SHARE_READ;
    DWORD write = FILE_SHARE_WRITE;
    DWORD readWrite = FILE_SHARE_READ | FILE_SHARE_WRITE;
    DWORD noShare = 0;

    printf("FILE_SHARE_READ: other processes can open this file for concurrent reads (%x)\n\n", read);
    printf("FILE_SHARE_WRITE: other processes can open this file for concurrent writes (%x)\n\n", write);
    printf("FILE_SHARE_READ | FILE_SHARE_WRITE: other processes can open this file for concurrent read/writes (%x)\n\n", readWrite);
    printf("0: no other HANDLEs can be opened on this file, even from within this process (%x)\n\n", noShare);

    return 0;

}

The lpSecurityAttributes parameter is a pointer that points to a SECURITY_ATTRIBUTES structure. If this attribute is set to NULL, the file handle cannot be inherited. A SECURITY_ATTRIBUTES structure has three fields, nLength, of type DWORD, lpSecurityDescriptor, of type LPVOID, and bInheritHandle, of type BOOL. The second field is a pointer to another structure, of type SECURITY_DESCRIPTOR.

For now, we will set the lpSecurityAttributes parameter to NULL.

The dwCreate parameter specifies whether to create a new file, overwrite an existing file, open an existing file, etc.

#include "stdafx.h"
#include <Windows.h>
#include <stdio.h>

int main(void){

    printf("Create a new file; fail if the file already exists (CREATE_NEW %x)\n", CREATE_NEW);
    printf("Create a new file; overwrite the file if it already exists (CREATE_ALWAYS %x)\n", CREATE_ALWAYS);
    printf("Open an existing file; fail if the file does not exist (OPEN_EXISTING %x)\n", OPEN_EXISTING);
    printf("Open the file; create the file if it does not exist (OPEN_ALWAYS %x)\n", OPEN_ALWAYS);
    printf("Set the file length to zero; fail if the file does not exist (TRUNCATE_EXISTING %x)", TRUNCATE_EXISTING);

    return 0;

}

The dwAttrAndFlags parameter specifies the file attributes and flags of the file itself, not the file’s handle; thus, this parameter only comes into play if the file is being created and is ignored if the file already exists and is merely being opened.

#include <Windows.h>
#include <stdio.h>

int main(void){

    DWORD normal = FILE_ATTRIBUTE_NORMAL;
    DWORD readonly = FILE_ATTRIBUTE_READONLY;
    DWORD temp = FILE_FLAG_DELETE_ON_CLOSE;
    DWORD randAccess = FILE_FLAG_RANDOM_ACCESS;
    DWORD seqAccess = FILE_FLAG_SEQUENTIAL_SCAN;

    printf("Normal file %x\n", normal);
    printf("Read only file, cannot be deleted by application %x\n", readonly);
    printf("Temporary file %x\n", temp);
    printf("Random-access file %x\n", randAccess);
    printf("Sequential-access file %x\n", seqAccess);

    return 0;

}

In most cases, the final parameter, hTemplateFile, is set to NULL.

Windows has a single function called CloseHandle() to close handles and release system resources. This function returns a Boolean value indicating the success or failure of the function.

#include <Windows.h>
#include <stdio.h>

int main(void){

    const LPSECURITY_ATTRIBUTES default_security = NULL;

    HANDLE ourHandle = CreateFile(
        _T("theTest.txt"),
        GENERIC_READ | GENERIC_WRITE,
        FILE_SHARE_READ,
        default_security,
        CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL
        );

    CloseHandle(ourHandle);

    return 0;

}

Note the use of the _T() macro for converting the char * string literal to the correct format.

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