윈도우즈 시스템 프로그래밍 - 파일 입출력 예제

윈도우 시스템 프로그래밍에서 파일을 생성하고 읽기/쓰기를 수행해 보자.

 

 

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먼저 아래에 MSDN에서 가져온 예제 코드를 살펴보자.

파일 읽고 쓰기 :

해당 코드는 CreateFile()과 WriteFile() 함수를 이용해 파일을 생성하고 값을 입력하고 있다. 이때, CreateFile()에서 이미 있는 파일을 새로 생성하려고 하면 에러코드를 반환한다(에러코드 80).

만약 log 파일을 만들고 싶은데, 이름이 같은 파일이 있을 시 덮어쓰고 싶다면?
CreateFile() 함수의 CREATE_NEW부분을 바꿔주면 된다! (예제 코드 아래에 상세 설명)

#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>

void DisplayError(LPTSTR lpszFunction);

void __cdecl _tmain(int argc, TCHAR *argv[])
{
    HANDLE hFile;
    char DataBuffer[] = "This is some test data to write to the file.";
    DWORD dwBytesToWrite = (DWORD)strlen(DataBuffer);
    DWORD dwBytesWritten = 0;
    BOOL bErrorFlag = FALSE;

    printf("\n");
    if (argc != 2)
    {
        printf("Usage Error:\tIncorrect number of arguments\n\n");
        _tprintf(TEXT("%s <file_name>\n"), argv[0]);
        system("pause");
        return;
    }

    hFile = CreateFile(argv[1],                // name of the write
        GENERIC_WRITE,          // open for writing
        0,                      // do not share
        NULL,                   // default security
        CREATE_NEW,             // create new file only
        FILE_ATTRIBUTE_NORMAL,  // normal file
        NULL);                  // no attr. template

    if (hFile == INVALID_HANDLE_VALUE)
    {
        DisplayError(TEXT("CreateFile"));
        _tprintf(TEXT("Terminal failure: Unable to open file \"%s\" for write.\n"), argv[1]);
        system("pause");
        return;
    }

    _tprintf(TEXT("Writing %d bytes to %s.\n"), dwBytesToWrite, argv[1]);

    bErrorFlag = WriteFile(
        hFile,           // open file handle
        DataBuffer,      // start of data to write
        dwBytesToWrite,  // number of bytes to write
        &dwBytesWritten, // number of bytes that were written
        NULL);            // no overlapped structure

    if (FALSE == bErrorFlag)
    {
        DisplayError(TEXT("WriteFile"));
        printf("Terminal failure: Unable to write to file.\n");
    }
    else
    {
        if (dwBytesWritten != dwBytesToWrite)
        {
            // This is an error because a synchronous write that results in
            // success (WriteFile returns TRUE) should write all data as
            // requested. This would not necessarily be the case for
            // asynchronous writes.
            printf("Error: dwBytesWritten != dwBytesToWrite\n");
        }
        else
        {
            _tprintf(TEXT("Wrote %d bytes to %s successfully.\n"), dwBytesWritten, argv[1]);
        }
    }

    CloseHandle(hFile);
}

void DisplayError(LPTSTR lpszFunction)
// Routine Description:
// Retrieve and output the system error message for the last-error code
{
    LPVOID lpMsgBuf;
    LPVOID lpDisplayBuf;
    DWORD dw = GetLastError();

    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER |
        FORMAT_MESSAGE_FROM_SYSTEM |
        FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        dw,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR)&lpMsgBuf,
        0,
        NULL);

    lpDisplayBuf =
        (LPVOID)LocalAlloc(LMEM_ZEROINIT,
        (lstrlen((LPCTSTR)lpMsgBuf)
            + lstrlen((LPCTSTR)lpszFunction)
            + 40) // account for format string
            * sizeof(TCHAR));

    if (FAILED(StringCchPrintf((LPTSTR)lpDisplayBuf,
        LocalSize(lpDisplayBuf) / sizeof(TCHAR),
        TEXT("%s failed with error code %d as follows:\n%s"),
        lpszFunction,
        dw,
        lpMsgBuf)))
    {
        printf("FATAL ERROR: Unable to output error code.\n");
    }

    _tprintf(TEXT("ERROR: %s\n"), (LPCTSTR)lpDisplayBuf);

    LocalFree(lpMsgBuf);
    LocalFree(lpDisplayBuf);

}

CREATE_NEW부분의 항목은 dwCreationDisposition이라는 설명이 붙어 있는데, 파일을 생성할 시의 옵션을 부여한다(간단히 말해, fopen("test", "a")에서의 "a"같은 기능이라고 볼 수 있다).

이 부분에 대한 자세한 설명은 다음과 같다.

Value	Meaning
CREATE_ALWAYS (2)	Creates a new file, always. If the specified file exists and is writable, the function overwrites the file, the function succeeds, and last-error code is set to ERROR_ALREADY_EXISTS (183). If the specified file does not exist and is a valid path, a new file is created, the function succeeds, and the last-error code is set to zero. For more information, see the Remarks section of this topic.
CREATE_NEW (1)	Creates a new file, only if it does not already exist. If the specified file exists, the function fails and the last-error code is set to ERROR_FILE_EXISTS (80). If the specified file does not exist and is a valid path to a writable location, a new file is created.
OPEN_ALWAYS (4)	Opens a file, always. If the specified file exists, the function succeeds and the last-error code is set to ERROR_ALREADY_EXISTS (183). If the specified file does not exist and is a valid path to a writable location, the function creates a file and the last-error code is set to zero.
OPEN_EXISTING (3)	Opens a file or device, only if it exists. If the specified file or device does not exist, the function fails and the last-error code is set to ERROR_FILE_NOT_FOUND (2). For more information about devices, see the Remarks section.
TRUNCATE_EXISTING (5)	Opens a file and truncates it so that its size is zero bytes, only if it exists. If the specified file does not exist, the function fails and the last-error code is set to ERROR_FILE_NOT_FOUND (2). The calling process must open the file with the GENERIC_WRITE bit set as part of the dwDesiredAccess parameter.

CREATE_ALWAYS를 사용하면 프로그램이 사용될 때마다 새로운 로그파일을 생성할 수 있다!

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파일 읽기 :

두번째 예제 코드는 ReadFile() 함수를 이용해 파일을 읽는 코드이다.

#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>

#define BUFFERSIZE 5
DWORD g_BytesTransferred = 0;

void DisplayError(LPTSTR lpszFunction);

VOID CALLBACK FileIOCompletionRoutine(
  __in  DWORD dwErrorCode,
  __in  DWORD dwNumberOfBytesTransfered,
  __in  LPOVERLAPPED lpOverlapped
);

VOID CALLBACK FileIOCompletionRoutine(
  __in  DWORD dwErrorCode,
  __in  DWORD dwNumberOfBytesTransfered,
  __in  LPOVERLAPPED lpOverlapped )
 {
  _tprintf(TEXT("Error code:\t%x\n"), dwErrorCode);
  _tprintf(TEXT("Number of bytes:\t%x\n"), dwNumberOfBytesTransfered);
  g_BytesTransferred = dwNumberOfBytesTransfered;
 }

//
// Note: this simplified sample assumes the file to read is an ANSI text file
// only for the purposes of output to the screen. CreateFile and ReadFile
// do not use parameters to differentiate between text and binary file types.
//

void __cdecl _tmain(int argc, TCHAR *argv[])
{
    HANDLE hFile; 
    DWORD  dwBytesRead = 0;
    char   ReadBuffer[BUFFERSIZE] = {0};
    OVERLAPPED ol = {0};

    printf("\n");
    if( argc != 2 )
    {
        printf("Usage Error: Incorrect number of arguments\n\n");
        _tprintf(TEXT("Usage:\n\t%s <text_file_name>\n"), argv[0]);
        return;
    }

    hFile = CreateFile(argv[1],               // file to open
                       GENERIC_READ,          // open for reading
                       FILE_SHARE_READ,       // share for reading
                       NULL,                  // default security
                       OPEN_EXISTING,         // existing file only
                       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, // normal file
                       NULL);                 // no attr. template

    if (hFile == INVALID_HANDLE_VALUE) 
    { 
        DisplayError(TEXT("CreateFile"));
        _tprintf(TEXT("Terminal failure: unable to open file \"%s\" for read.\n"), argv[1]);
        return; 
    }

    // Read one character less than the buffer size to save room for
    // the terminating NULL character. 

    if( FALSE == ReadFileEx(hFile, ReadBuffer, BUFFERSIZE-1, &ol, FileIOCompletionRoutine) )
    {
        DisplayError(TEXT("ReadFile"));
        printf("Terminal failure: Unable to read from file.\n GetLastError=%08x\n", GetLastError());
        CloseHandle(hFile);
        return;
    }
    SleepEx(5000, TRUE);
    dwBytesRead = g_BytesTransferred;
    // This is the section of code that assumes the file is ANSI text. 
    // Modify this block for other data types if needed.

    if (dwBytesRead > 0 && dwBytesRead <= BUFFERSIZE-1)
    {
        ReadBuffer[dwBytesRead]='\0'; // NULL character

        _tprintf(TEXT("Data read from %s (%d bytes): \n"), argv[1], dwBytesRead);
        printf("%s\n", ReadBuffer);
    }
    else if (dwBytesRead == 0)
    {
        _tprintf(TEXT("No data read from file %s\n"), argv[1]);
    }
    else
    {
        printf("\n ** Unexpected value for dwBytesRead ** \n");
    }

    // It is always good practice to close the open file handles even though
    // the app will exit here and clean up open handles anyway.

    CloseHandle(hFile);
}

void DisplayError(LPTSTR lpszFunction) 
// Routine Description:
// Retrieve and output the system error message for the last-error code
{ 
    LPVOID lpMsgBuf;
    LPVOID lpDisplayBuf;
    DWORD dw = GetLastError(); 

    FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | 
        FORMAT_MESSAGE_FROM_SYSTEM |
        FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        dw,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPTSTR) &lpMsgBuf,
        0, 
        NULL );

    lpDisplayBuf = 
        (LPVOID)LocalAlloc( LMEM_ZEROINIT, 
                            ( lstrlen((LPCTSTR)lpMsgBuf)
                              + lstrlen((LPCTSTR)lpszFunction)
                              + 40) // account for format string
                            * sizeof(TCHAR) );

    if (FAILED( StringCchPrintf((LPTSTR)lpDisplayBuf, 
                     LocalSize(lpDisplayBuf) / sizeof(TCHAR),
                     TEXT("%s failed with error code %d as follows:\n%s"), 
                     lpszFunction, 
                     dw, 
                     lpMsgBuf)))
    {
        printf("FATAL ERROR: Unable to output error code.\n");
    }

    _tprintf(TEXT("ERROR: %s\n"), (LPCTSTR)lpDisplayBuf);

    LocalFree(lpMsgBuf);
    LocalFree(lpDisplayBuf);
}

위의 예제 코드들은 이 링크로 가면 확인해 볼 수 있다!