Abrir um arquivo para leitura ou gravação
A função CreateFile pode criar um novo arquivo ou abrir um arquivo existente. Você deve especificar o nome do arquivo, as instruções de criação e outros atributos. Quando um aplicativo cria um novo arquivo, o sistema operacional o adiciona ao diretório especificado.
Exemplo: abrir um arquivo para gravação
O exemplo a seguir usa CreateFile para criar um novo arquivo e abri-lo para gravar e WriteFile para gravar uma cadeia de caracteres simples de forma síncrona no arquivo.
Uma chamada subsequente para abrir esse arquivo com CreateFile falhará até que o identificador seja fechado.
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>
void DisplayError(LPTSTR lpszFunction);
int __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]);
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]);
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);
}
Exemplo: abrir um arquivo para leitura
O exemplo a seguir usa CreateFile para abrir um arquivo existente para leitura e ReadFile para ler até 80 caracteres de forma síncrona do arquivo.
Nesse caso, CreateFile só terá êxito se o arquivo especificado já existir no diretório atual. Uma chamada subsequente para abrir esse arquivo com CreateFile terá êxito se a chamada usar os mesmos modos de acesso e compartilhamento.
Dica: você pode usar o arquivo criado com o exemplo writefile anterior para testar este exemplo.
#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.
//
int __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);
}
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