Diving into File Handling with win32

I’d like to review creating and managing files using win32, and maybe go a bit more in depth as well.

The second to last parameter for CreateFile() specifies a range of attributes for the file in the form of flags. These flags are logically organized into three groups; we will cover the first two groups here. The first group represents a set of file attributes. FILE_ATTRIBUTE_HIDDEN keeps the file from being included in an ordinary directory listing. FILE_ATTRIBUTE_ENCRYPTED will cause the file or directory to be encrypted; in the case of a directory, this means that all newly created files and subdirectories will be, by default, encrypted. FILE_ATTRIBUTE_NORMAL is the default; note that this flag is only valid when used by itself. FILE_ATTRIBUTE_READONLY makes the file available for reading, but not writing or deletion. FILE_ATTRIBUTE_TEMPORARY means that the file is being used for temporary storage.

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

int _tmain(int argc, _TCHAR* argv[])
{
    
    printf("FILE_ATTRIBUTE_READONLY \t %d 0x%x\n", FILE_ATTRIBUTE_READONLY, FILE_ATTRIBUTE_READONLY);
    printf("FILE_ATTRIBUTE_HIDDEN \t %d 0x%x\n", FILE_ATTRIBUTE_HIDDEN, FILE_ATTRIBUTE_HIDDEN);
    printf("FILE_ATTRIBUTE_NORMAL \t %d 0x%x\n", FILE_ATTRIBUTE_NORMAL, FILE_ATTRIBUTE_NORMAL);
    printf("FILE_ATTRIBUTE_TEMPORARY \t %d 0x%x\n", FILE_ATTRIBUTE_TEMPORARY, FILE_ATTRIBUTE_TEMPORARY);
    printf("FILE_ATTRIBUTE_ENCRYPTED \t %d 0x%x\n", FILE_ATTRIBUTE_ENCRYPTED, FILE_ATTRIBUTE_ENCRYPTED);

    return 0;

}

The FILE_FLAG_BACKUP_SEMANTICS flag indicates that the file is being opened or created for a backup or restore operation. Note that we must specify this flag if we want to work with a directory. The FILE_FLAG_DELETE_ON_CLOSE flag indicates that the file should be deleted as soon as all of its handles are closed. The FILE_FLAG_OVERLAPPED flag indicates that the file or device is to be opened or created for asynchronous I/O. If this flag is specified, the file can be used for simultaneous read and write operations. The FILE_FLAG_RANDOM_ACCESS is pretty self-explanatory, the file will be accessed randomly. The FILE_FLAG_NO_BUFFERING flag indicates that the file or device is to be opened with no system caching for data reads and writes.

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

int _tmain(int argc, _TCHAR* argv[])
{
    
    printf("FILE_FLAG_DELETE_ON_CLOSE %d.\n", FILE_FLAG_DELETE_ON_CLOSE);
    printf("FILE_FLAG_OVERLAPPED %d.\n", FILE_FLAG_OVERLAPPED);
    printf("FILE_FLAG_BACKUP_SEMANTICS %d.\n", FILE_FLAG_BACKUP_SEMANTICS);
    printf("FILE_FLAG_RANDOM_ACCESS %d.\n", FILE_FLAG_RANDOM_ACCESS);
    printf("FILE_FLAG_NO_BUFFERING %d.\n", FILE_FLAG_NO_BUFFERING);

    return 0;

}

The OPEN_ALWAYS creation disposition will always give us a file handle to the file, unless there has been a grievous error of some sort. If the file doesn’t exist, it will be created, and if it does exist, the function will still succeed but the last-error code will be set to ERROR_ALREADY_EXISTS.

#include <Windows.h>

int _tmain(int argc, _TCHAR* argv[])
{
    char *szFileName = "kilroy.txt";
    HANDLE hFile;
    //create the file!
    //if file already exists, then we will open it
    //otherwise it will be created and then opened
    hFile = CreateFile(
        szFileName,
        GENERIC_WRITE,
        FILE_SHARE_READ,
        NULL,
        OPEN_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL
        );

    if (hFile == INVALID_HANDLE_VALUE){
        printf("Failed to create/open file %s.\n", szFileName);
    }
    else {
        if (GetLastError() == ERROR_ALREADY_EXISTS){
            printf("File already exists, opening...\n");
        }
        else {
            printf("File created, opening...\n");
        }
    }

    //close the handle
    CloseHandle(hFile);

    return 0;

}

The WriteFile() function takes five arguments.The first argument or parameter is a handle to the file or I/O device. The second parameter is a pointer to the buffer that holds the data to be written. The third argument controls the number of bytes to be written. The fourth argument is a pointer to a DWORD value that stores the number of bytes written during a synchronous write operation. The final parameter is a pointer to an OVERLAPPED structure; this parameter is necessary if we are to perform asynchronous I/O.

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

int _tmain(int argc, _TCHAR* argv[])
{
    const int Buffer_Size = 256;

    HANDLE hFile;
    char *szFileName = "kilroy.txt";
    char szBuffer[Buffer_Size];
    DWORD dwBytesWritten;
    BOOLEAN boolRVal;

    hFile = CreateFile(szFileName, GENERIC_WRITE, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

    if (hFile == INVALID_HANDLE_VALUE){
        exit(EXIT_FAILURE);
    }

    strncpy_s(szBuffer, "What strength! But don't forget there are many guys like you all over the World.", Buffer_Size);

    //let's write!
    boolRVal = WriteFile(hFile, szBuffer, strlen(szBuffer), &dwBytesWritten, NULL);

    if (boolRVal){
        printf("%s written to successfully!\n", szFileName);
        printf("%d bytes written.\n", dwBytesWritten);
    }

    //close the handle afterwards
    CloseHandle(hFile);

    return 0;

}

The WriteFile() function is designed for both synchronous and asynchronous operations. For asynchronous write operations, the handle must have been opened with CreateFile() using the FILE_FLAG_OVERLAP flag.

The ReadFile() function is a block-reading function. As with WriteFile(), we pass it in a buffer, only this time we indicate the number of bytes to be read, and the function retrieves the specified number of bytes from the file starting at the current offset.

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

int _tmain(int argc, _TCHAR* argv[])
{
    const int Buffer_Size = 256;
    char *szFileName = "kilroy.txt";
    char szInBuffer[Buffer_Size];
    HANDLE hFile;
    DWORD dwBytesRead;

    hFile = CreateFile(szFileName, GENERIC_READ, NULL, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

    if (hFile == INVALID_HANDLE_VALUE){
        exit(EXIT_FAILURE);
    }

    if (ReadFile(hFile, szInBuffer, Buffer_Size, &dwBytesRead, NULL)){
        printf("%d bytes read.\n", dwBytesRead);
        //use dwBytesRead to place a string termination character
        //at the correct location in the buffer.
        szInBuffer[dwBytesRead] = '';
        printf("%s\n", szInBuffer);
    }

    CloseHandle(hFile);

    return 0;

}

The win32 API has several functions that are useful for retrieving file information. Many of these functions require an open file handle rather than the file’s name.

The GetFileTime() function fetches three different pieces of time information concerning a file: the creation time, the last access time, and the last write time. Each of these times is stored in a FILETIME structure. Data from the FILETIME structure should be extracted via win32 function calls, namely FileTimeToSystemTime() , which converts a FILETIME structure to a SYSTEMTIME structure, which is easy to work with.

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

void DisplayTime(FILETIME structFT);

int _tmain(int argc, _TCHAR* argv[])
{
    TCHAR *szFilename = _T("kilroy.txt");
    FILETIME structFTCreate, structFTLastWrite, structFTLastAccess;
    BOOLEAN boolSuccess;

    HANDLE hFile = CreateFile(szFilename, GENERIC_READ, FILE_SHARE_WRITE, 0, OPEN_ALWAYS, 0, 0);

    if (hFile == INVALID_HANDLE_VALUE){
        printf("Error no. %d\n", GetLastError());
        exit(EXIT_FAILURE);
    }
    else {
        printf("Acquiring file times...\n");
    }

    boolSuccess = GetFileTime(hFile, &structFTCreate, &structFTLastAccess, &structFTLastWrite);

    printf("Creation time: \n");
    DisplayTime(structFTCreate);
    printf("\n");
    printf("Last write time: \n");
    DisplayTime(structFTLastWrite);

    return 0;
}

void DisplayTime(FILETIME structFT){
    SYSTEMTIME structST;
    FileTimeToSystemTime(&structFT, &structST);
    printf("Month: %d Day: %d Year: %d ", structST.wMonth, structST.wDay, structST.wYear);
    printf("%d:%d\n", structST.wHour, structST.wMinute);
}

The FILETIME structure contains two 32-bit values. The FileTimeToSystemTime() function convert this amalgamated 64 bit value into a local time suitable for output.

The GetFileSize() function returns the size of the file in bytes; however, a more suitable variant for contemporary programming is GetFileSizeEx(). The GetFileSizeEx() function accepts two arguments, the first being a handle to the file, and the second is a pointer to a LARGE_INTEGER union.

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

int _tmain(int argc, _TCHAR* argv[])
{
    HANDLE hFile;
    TCHAR *szFilename = _TEXT("kilroy.txt");
    LARGE_INTEGER structLargeInt;

    hFile = CreateFile(szFilename, GENERIC_READ, FILE_SHARE_WRITE, NULL, OPEN_EXISTING, NULL, NULL);

    if (hFile == INVALID_HANDLE_VALUE){
        printf("Error no. %d\n", GetLastError());
        exit(EXIT_FAILURE);
    }
    else {
        GetFileSizeEx(hFile, &structLargeInt);
        //only the QuadPart member of the LARGE_INTEGER structure concerns us
        printf("Size: %d bytes.\n", structLargeInt.QuadPart);
    }

    return 0;
}
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Writing Files and Reading Files with WriteFile() and ReadFile()

Most of the time when we want to open a file, we’ll use a high-level system call like fopen(). However, sometimes we would like to bypass higher-level processing.

The system call we use to open a file is CreateFile(), which is an odd name, since a lot of times we are opening a file, not creating one! We can think of it as CreateFileHandle, since the function after all returns a HANDLE to a file, and may, or may not, create a file if one is not already there.

The first argument to CreateFile() is the file name. In the second argument we specify the type of access to the object using GENERIC_READ and GENERIC_WRITE. The third argument sets a variety of options that specify how the object can be shared. The fourth option is a pointer to a SECURITY_ATTRIBUTES structure, which is beyond the ken of this post, so we will putting NULL in for this value. The fifth argument specifies whether or not to create a file if one does or does not already exist; for this argument we can specify CREATE_NEW, CREATE_ALWAYS, OPEN_EXISTING, OPEN_ALWAYS, or TRUNCATE_EXISTING. The sixth argument specifies the file attributes and flags for the file. The final argument specifies a HANDLE with GENERIC_READ access to a template file that supplies file attributes and extended attributes for the file being created. We will supply NULL for this argument.

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

     
int    main(void){

        HANDLE tmpHandle = CreateFile(
            _T("testfile.txt"), //lpFileName
            GENERIC_WRITE, //dwDesiredAccess
            0,//dwShareMode
            NULL, //lpSecurityAttributes
            CREATE_ALWAYS, //dwCreationDistribution
            FILE_FLAG_WRITE_THROUGH, //dwFlagsAndAttributes
            NULL //hTemplateFile
            );

        if(tmpHandle == INVALID_HANDLE_VALUE){
            printf("Failed to create file handle.\n");
        } else {
            printf("File handle created.\n");
        }

        if((CloseHandle(tmpHandle))==false){
            printf("Failed to close file handle.\n") ;
        }
        else {
            printf("File handle closed.\n");

        }

        return 0;

}

The WriteFile() system call takes a file handle, a buffer, the length of the buffer, and a pointer where you want to store the number of bytes written to the file.The final argument is an OVERLAPPED structure, we will be setting this to NULL. The WriteFile() function returns a Boolean value of True on success and False on failure.

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

int main(void){

    char buffer[256] = "You have failed us, Torgo. For this, you must die!";
    DWORD bytesWritten;

    HANDLE fh = CreateFile(_T("newfile.txt"), GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_ALWAYS, NULL, NULL);
    if(fh == INVALID_HANDLE_VALUE){
        printf("File handle not opened.\n");
        exit(EXIT_FAILURE);
    } else {
        printf("File handle opened.\n");
    }

    //WriteFile() returns Boolean value
    if(WriteFile(fh, buffer, sizeof(buffer), &bytesWritten, NULL)){
        printf("File written to.\n");
    } else {
        printf("File not written to.\n");
    }


    if(CloseHandle(fh)){
        printf("File closed.\n");
    } else {
        printf("File not closed.\n");
    }
    return 0;

}

The ReadFile() function is similar to the WriteFile() function. The first argument is a file handle that must have FILE_READ_DATA access, which is a subset of GENERIC_READ access. The second argument is the buffer that is to receive the data. The third argument is the number of bytes to read from the file. The fourth argument points to the actual number of bytes the call processed. The final argument should be NULL. Like WriteFile(), the ReadFile() system call returns a Boolean value indicating the success or failure of the call.

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

int main(void){

    char readBuffer[256];
    DWORD bytesRead;

    HANDLE fh = CreateFile(_T("newfile.txt"), GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

    if(fh!=INVALID_HANDLE_VALUE){
        printf("File opened for reading!\n");
    } else {
        printf("File not opened for reading!\n");
        exit(EXIT_FAILURE);
    }

    if(ReadFile(fh, readBuffer, sizeof(readBuffer), &bytesRead, NULL)){
        printf("File successfully read!\n");
        printf("%d bytes read.\n", bytesRead);
        printf("%s\n", readBuffer);
    }
    

    return 0;

}

We will look at file I/O in the Windows context in greater depth in later posts.