CreateMailslot
The CreateMailslot function creates a mailslot with the specified name and returns a handle that a mailslot server can use to perform operations on the mailslot. The mailslot is local to the computer that creates it. An error occurs if a mailslot with the specified name already exists.
HANDLE CreateMailslot(
LPCTSTR lpName, // mailslot name
DWORD nMaxMessageSize, // maximum message size
DWORD lReadTimeout, // read time-out interval
LPSECURITY_ATTRIBUTES lpSecurityAttributes // inheritance option
);
Parameters
lpName
[in] Pointer to a null-terminated string specifying the name of the mailslot. This name must have the following form:
//./mailslot/[path]name
The name field must be unique. The name may include multiple levels of pseudo directories separated by backslashes. For example, both //./mailslot/example_mailslot_name and //./mailslot/abc/def/ghi are valid names.
nMaxMessageSize
[in] Specifies the maximum size, in bytes, of a single message that can be written to the mailslots. To specify that the message can be of any size, set this value to zero.
lReadTimeout
[in] Specifies the amount of time, in milliseconds, a read operation can wait for a message to be written to the mailslot before a time-out occurs. The following values have special meanings. Value Meaning
0 Returns immediately if no message is present. (The system does not treat an immediate return as an error.)
MAILSLOT_WAIT_FOREVER Waits forever for a message.
This time-out value applies to all subsequent read operations and all inherited mailslot handles.
lpSecurityAttributes
[in] Pointer to a SECURITY_ATTRIBUTES structure. The bInheritHandle member of the structure determines whether the returned handle can be inherited by child processes. If lpSecurityAttributes is NULL, the handle cannot be inherited. The lpSecurityDescriptor member of the structure is ignored.
Return Values
If the function succeeds, the return value is a handle to the mailslot, for use in server mailslot operations. The server side of the handle is overlapped.
If the function fails, the return value is INVALID_HANDLE_VALUE. To get extended error information, call GetLastError.
Remarks
The mailslot exists until one of the following conditions is true:
The last (possibly inherited or duplicated) handle to it is closed using the CloseHandle function.
The process owning the last (possibly inherited or duplicated) handle exits.
The system uses the second method to destroy mailslots.
To write a message to a mailslot, a process uses the CreateFile function, specifying the mailslot name by using one of the following formats.
Format Usage
//./mailslot/name Retrieves a client handle to a local mailslot.
//computername/mailslot/name Retrieves a client handle to a remote mailslot.
//domainname/mailslot/name Retrieves a client handle to all mailslots with the specified name in the specified domain.
//*/mailslot/name Retrieves a client handle to all mailslots with the specified name in the system's primary domain.
If CreateFile specifies a domain or uses the asterisk format to specify the system's primary domain, the application cannot write more than 424 bytes at a time to the mailslot. If the application attempts to do so, the WriteFile function fails and GetLastError returns ERROR_BAD_NETPATH.
An application must specify the FILE_SHARE_READ flag when using CreateFile to retrieve a client handle to a mailslot.
Windows 95/98/Me: CreateMailslotW is supported by the Microsoft Layer for Unicode. To use this, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems.
Example Code
For an example, see Creating a Mailslot.
Requirements
Windows NT/2000/XP: Included in Windows NT 3.1 and later.
Windows 95/98/Me: Included in Windows 95 and later.
Header: Declared in Winbase.h; include Windows.h.
Library: Use Kernel32.lib.
Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000/XP. Also supported by Microsoft Layer for Unicode.
See Also
Mailslots Overview, Mailslot Functions, CloseHandle, CreateFile, GetMailslotInfo, SECURITY_ATTRIBUTES, SetMailslotInfo, WriteFile
ReadFile
The ReadFile function reads data from a file, starting at the position indicated by the file pointer. After the read operation has been completed, the file pointer is adjusted by the number of bytes actually read, unless the file handle is created with the overlapped attribute. If the file handle is created for overlapped input and output (I/O), the application must adjust the position of the file pointer after the read operation.
This function is designed for both synchronous and asynchronous operation. The ReadFileEx function is designed solely for asynchronous operation. It lets an application perform other processing during a file read operation.
BOOL ReadFile(
HANDLE hFile, // handle to file
LPVOID lpBuffer, // data buffer
DWORD nNumberOfBytesToRead, // number of bytes to read
LPDWORD lpNumberOfBytesRead, // number of bytes read
LPOVERLAPPED lpOverlapped // overlapped buffer
);
Parameters
hFile
[in] Handle to the file to be read. The file handle must have been created with GENERIC_READ access to the file.
Windows NT/2000/XP: For asynchronous read operations, hFile can be any handle opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept function.
Windows 95/98/Me: For asynchronous read operations, hFile can be a communications resource opened with the FILE_FLAG_OVERLAPPED flag by CreateFile, or a socket handle returned by socket or accept. You cannot perform asynchronous read operations on mailslots, named pipes, or disk files.
lpBuffer
[out] Pointer to the buffer that receives the data read from the file.
nNumberOfBytesToRead
[in] Specifies the number of bytes to be read from the file.
lpNumberOfBytesRead
[out] Pointer to the variable that receives the number of bytes read. ReadFile sets this value to zero before doing any work or error checking. If this parameter is zero when ReadFile returns TRUE on a named pipe, the other end of the message-mode pipe called the WriteFile function with nNumberOfBytesToWrite set to zero.
Windows NT/2000/XP: If lpOverlapped is NULL, lpNumberOfBytesRead cannot be NULL. If lpOverlapped is not NULL, lpNumberOfBytesRead can be NULL. If this is an overlapped read operation, you can get the number of bytes read by calling GetOverlappedResult. If hFile is associated with an I/O completion port, you can get the number of bytes read by calling GetQueuedCompletionStatus.
If I/O completion ports are used and you are using a callback routine to free the memory allocated to the OVERLAPPED structure pointed to by the lpOverlapped parameter, specify NULL as the value of this parameter to avoid a memory corruption problem during the deallocation. This memory corruption problem will cause an invalid number of bytes to be returned in this parameter.
Windows 95/98/Me: This parameter cannot be NULL.
lpOverlapped
[in] Pointer to an OVERLAPPED structure. This structure is required if hFile was created with FILE_FLAG_OVERLAPPED.
If hFile was opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must not be NULL. It must point to a valid OVERLAPPED structure. If hFile was created with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the function can incorrectly report that the read operation is complete.
If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the read operation starts at the offset specified in the OVERLAPPED structure and ReadFile may return before the read operation has been completed. In this case, ReadFile returns FALSE and the GetLastError function returns ERROR_IO_PENDING. This allows the calling process to continue while the read operation finishes. The event specified in the OVERLAPPED structure is set to the signaled state upon completion of the read operation.
If hFile was not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the read operation starts at the current file position and ReadFile does not return until the operation has been completed.
Windows NT/2000/XP: If hFile is not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the read operation starts at the offset specified in the OVERLAPPED structure. ReadFile does not return until the read operation has been completed.
Windows 95/98/Me: For operations on files, disks, pipes, or mailslots, this parameter must be NULL; a pointer to an OVERLAPPED structure causes the call to fail. However, Windows 95/98/Me supports overlapped I/O on serial and parallel ports.
Return Values
The ReadFile function returns when one of the following is true: a write operation completes on the write end of the pipe, the number of bytes requested has been read, or an error occurs.
If the function succeeds, the return value is nonzero.
If the return value is nonzero and the number of bytes read is zero, the file pointer was beyond the current end of the file at the time of the read operation. However, if the file was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the return value is FALSE and GetLastError returns ERROR_HANDLE_EOF when the file pointer goes beyond the current end of file.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
Remarks
If part of the file is locked by another process and the read operation overlaps the locked portion, this function fails.
An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING:
File access must begin at byte offsets within the file that are integer multiples of the volume's sector size. To determine a volume's sector size, call the GetDiskFreeSpace function.
File access must be for numbers of bytes that are integer multiples of the volume's sector size. For example, if the sector size is 512 bytes, an application can request reads and writes of 512, 1024, or 2048 bytes, but not of 335, 981, or 7171 bytes.
Buffer addresses for read and write operations must be sector aligned (aligned on addresses in memory that are integer multiples of the volume's sector size). One way to sector align buffers is to use the VirtualAlloc function to allocate the buffers. This function allocates memory that is aligned on addresses that are integer multiples of the system's page size. Because both page and volume sector sizes are powers of 2, memory aligned by multiples of the system's page size is also aligned by multiples of the volume's sector size.
Accessing the input buffer while a read operation is using the buffer may lead to corruption of the data read into that buffer. Applications must not read from, write to, reallocate, or free the input buffer that a read operation is using until the read operation completes.
Characters can be read from the console input buffer by using ReadFile with a handle to console input. The console mode determines the exact behavior of the ReadFile function.
If a named pipe is being read in message mode and the next message is longer than the nNumberOfBytesToRead parameter specifies, ReadFile returns FALSE and GetLastError returns ERROR_MORE_DATA. The remainder of the message may be read by a subsequent call to the ReadFile or PeekNamedPipe function.
When reading from a communications device, the behavior of ReadFile is governed by the current communication time-outs as set and retrieved using the SetCommTimeouts and GetCommTimeouts functions. Unpredictable results can occur if you fail to set the time-out values. For more information about communication time-outs, see COMMTIMEOUTS.
If ReadFile attempts to read from a mailslot whose buffer is too small, the function returns FALSE and GetLastError returns ERROR_INSUFFICIENT_BUFFER.
If the anonymous write pipe handle has been closed and ReadFile attempts to read using the corresponding anonymous read pipe handle, the function returns FALSE and GetLastError returns ERROR_BROKEN_PIPE.
The ReadFile function may fail and return ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY whenever there are too many outstanding asynchronous I/O requests.
The ReadFile code to check for the end-of-file condition (eof) differs for synchronous and asynchronous read operations.
When a synchronous read operation reaches the end of a file, ReadFile returns TRUE and sets *lpNumberOfBytesRead to zero. The following sample code tests for end-of-file for a synchronous read operation:
// Attempt a synchronous read operation.
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL) ;
// Check for end of file.
if (bResult && nBytesRead == 0, )
{
// we're at the end of the file
}
An asynchronous read operation can encounter the end of a file during the initiating call to ReadFile, or during subsequent asynchronous operation.
If EOF is detected at ReadFile time for an asynchronous read operation, ReadFile returns FALSE and GetLastError returns ERROR_HANDLE_EOF.
If EOF is detected during subsequent asynchronous operation, the call to GetOverlappedResult to obtain the results of that operation returns FALSE and GetLastError returns ERROR_HANDLE_EOF.
To cancel all pending asynchronous I/O operations, use the CancelIo function. This function only cancels operations issued by the calling thread for the specified file handle. I/O operations that are canceled complete with the error ERROR_OPERATION_ABORTED.
If you are attempting to read from a floppy drive that does not have a floppy disk, the system displays a message box prompting the user to retry the operation. To prevent the system from displaying this message box, call the SetErrorMode function with SEM_NOOPENFILEERRORBOX.
The following sample code illustrates testing for end-of-file for an asynchronous read operation:
// set up overlapped structure fields
gOverLapped.Offset = 0;
gOverLapped.OffsetHigh = 0;
gOverLapped.hEvent = hEvent;
// attempt an asynchronous read operation
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead,
&gOverlapped) ;
// if there was a problem, or the async. operation's still pending ...
if (!bResult)
{
// deal with the error code
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// we have reached the end of the file
// during the call to ReadFile
// code to handle that
}
case ERROR_IO_PENDING:
{
// asynchronous i/o is still in progress
// do something else for a while
GoDoSomethingElse() ;
// check on the results of the asynchronous read
bResult = GetOverlappedResult(hFile, &gOverlapped,
&nBytesRead, FALSE) ;
// if there was a problem ...
if (!bResult)
{
// deal with the error code
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// we have reached the end of
// the file during asynchronous
// operation
}
// deal with other error cases
}
}
} // end case
// deal with other error cases
} // end switch
} // end if
Example Code
For an example, see Reading, Writing, and Locking Files.
Requirements
Windows NT/2000/XP: Included in Windows NT 3.1 and later.
Windows 95/98/Me: Included in Windows 95 and later.
Header: Declared in Winbase.h; include Windows.h.
Library: Use Kernel32.lib.
See Also
WriteFile
The WriteFile function writes data to a file and is designed for both synchronous and asynchronous operation. The function starts writing data to the file at the position indicated by the file pointer. After the write operation has been completed, the file pointer is adjusted by the number of bytes actually written, except when the file is opened with FILE_FLAG_OVERLAPPED. If the file handle was created for overlapped input and output (I/O), the application must adjust the position of the file pointer after the write operation is finished.
This function is designed for both synchronous and asynchronous operation. The WriteFileEx function is designed solely for asynchronous operation. It lets an application perform other processing during a file write operation.
BOOL WriteFile(
HANDLE hFile, // handle to file
LPCVOID lpBuffer, // data buffer
DWORD nNumberOfBytesToWrite, // number of bytes to write
LPDWORD lpNumberOfBytesWritten, // number of bytes written
LPOVERLAPPED lpOverlapped // overlapped buffer
);
Parameters
hFile
[in] Handle to the file to be written to. The file handle must have been created with GENERIC_WRITE access to the file.
Windows NT/2000/XP: For asynchronous write operations, hFile can be any handle opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept function.
Windows 95/98/Me: For asynchronous write operations, hFile can be a communications resource opened with the FILE_FLAG_OVERLAPPED flag by CreateFile, or a socket handle returned by socket or accept. You cannot perform asynchronous write operations on mailslots, named pipes, or disk files.
lpBuffer
[in] Pointer to the buffer containing the data to be written to the file.
nNumberOfBytesToWrite
[in] Specifies the number of bytes to write to the file.
A value of zero specifies a null write operation. The behavior of a null write operation depends on the underlying file system. To truncate or extend a file, use the SetEndOfFile function.
Named pipe write operations across a network are limited to 65,535 bytes.
lpNumberOfBytesWritten
[out] Pointer to the variable that receives the number of bytes written. WriteFile sets this value to zero before doing any work or error checking.
Windows NT/2000/XP: If lpOverlapped is NULL, lpNumberOfBytesWritten cannot be NULL. If lpOverlapped is not NULL, lpNumberOfBytesWritten can be NULL. If this is an overlapped write operation, you can get the number of bytes written by calling GetOverlappedResult. If hFile is associated with an I/O completion port, you can get the number of bytes written by calling GetQueuedCompletionStatus.
If I/O completion ports are used and you are using a callback routine to free the memory allocated to the OVERLAPPED structure pointed to by the lpOverlapped parameter, specify NULL as the value of this parameter to avoid a memory corruption problem during the deallocation. This memory corruption problem will cause an invalid number of bytes to be returned in this parameter.
Windows 95/98/Me: This parameter cannot be NULL.
lpOverlapped
[in] Pointer to an OVERLAPPED structure. This structure is required if hFile was opened with FILE_FLAG_OVERLAPPED.
If hFile was opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must not be NULL. It must point to a valid OVERLAPPED structure. If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the function can incorrectly report that the write operation is complete.
If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the write operation starts at the offset specified in the OVERLAPPED structure and WriteFile may return before the write operation has been completed. In this case, WriteFile returns FALSE and the GetLastError function returns ERROR_IO_PENDING. This allows the calling process to continue processing while the write operation is being completed. The event specified in the OVERLAPPED structure is set to the signaled state upon completion of the write operation.
If hFile was not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the write operation starts at the current file position and WriteFile does not return until the operation has been completed.
WriteFile resets the event specified by the hEvent member of the OVERLAPPED structure to a nonsignaled state when it begins the I/O operation. Therefore, there is no need for the caller to do so.
Windows NT/2000/XP: If hFile was not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the write operation starts at the offset specified in the OVERLAPPED structure and WriteFile does not return until the write operation has been completed.
Windows 95/98/Me: For operations on files, disks, pipes, or mailslots, this parameter must be NULL; a pointer to an OVERLAPPED structure causes the call to fail. However, Windows 95/98/Me supports overlapped I/O on serial and parallel ports.
Return Values
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
Remarks
Note that the time stamps may not be updated correctly for a remote file. To ensure consistent results, use unbuffered I/O. An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING:
File access must begin at byte offsets within the file that are integer multiples of the volume's sector size. To determine a volume's sector size, call the GetDiskFreeSpace function.
File access must be for numbers of bytes that are integer multiples of the volume's sector size. For example, if the sector size is 512 bytes, an application can request reads and writes of 512, 1024, or 2048 bytes, but not of 335, 981, or 7171 bytes.
Buffer addresses for read and write operations must be sector aligned (aligned on addresses in memory that are integer multiples of the volume's sector size). One way to sector align buffers is to use the VirtualAlloc function to allocate the buffers. This function allocates memory that is aligned on addresses that are integer multiples of the system's page size. Because both page and volume sector sizes are powers of 2, memory aligned by multiples of the system's page size is also aligned by multiples of the volume's sector size.
If part of the file is locked by another process and the write operation overlaps the locked portion, this function fails.
Accessing the output buffer while a write operation is using the buffer may lead to corruption of the data written from that buffer. Applications must not write to, reallocate, or free the output buffer that a write operation is using until the write operation completes.
Characters can be written to the screen buffer using WriteFile with a handle to console output. The exact behavior of the function is determined by the console mode. The data is written to the current cursor position. The cursor position is updated after the write operation.
The system interprets zero bytes to write as specifying a null write operation and WriteFile does not truncate or extend the file. To truncate or extend a file, use the SetEndOfFile function.
When writing to a nonblocking, byte-mode pipe handle with insufficient buffer space, WriteFile returns TRUE with *lpNumberOfBytesWritten < nNumberOfBytesToWrite.
When an application uses the WriteFile function to write to a pipe, the write operation may not finish if the pipe buffer is full. The write operation is completed when a read operation (using the ReadFile function) makes more buffer space available.
If the anonymous read pipe handle has been closed and WriteFile attempts to write using the corresponding anonymous write pipe handle, the function returns FALSE and GetLastError returns ERROR_BROKEN_PIPE.
The WriteFile function may fail with ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY whenever there are too many outstanding asynchronous I/O requests.
To cancel all pending asynchronous I/O operations, use the CancelIo function. This function only cancels operations issued by the calling thread for the specified file handle. I/O operations that are canceled complete with the error ERROR_OPERATION_ABORTED.
If you are attempting to write to a floppy drive that does not have a floppy disk, the system displays a message box prompting the user to retry the operation. To prevent the system from displaying this message box, call the SetErrorMode function with SEM_NOOPENFILEERRORBOX.
Example Code
For an example, see Reading, Writing, and Locking Files.
Requirements
Windows NT/2000/XP: Included in Windows NT 3.1 and later.
Windows 95/98/Me: Included in Windows 95 and later.
Header: Declared in Winbase.h; include Windows.h.
Library: Use Kernel32.lib.
See Also
File I/O Overview, File I/O Functions, CancelIo, CreateFile, GetLastError, GetOverlappedResult, GetQueuedCompletionStatus, OVERLAPPED, ReadFile, SetEndOfFile, SetErrorMode, WriteFileEx