Copies data from a named or anonymous pipe into a buffer without removing it from the pipe. It also returns information about data in the pipe.

Syntax

BOOL WINAPI PeekNamedPipe(
  __in       HANDLE hNamedPipe,
  __out_opt  LPVOID lpBuffer,
  __in       DWORD nBufferSize,
  __out_opt  LPDWORD lpBytesRead,
  __out_opt  LPDWORD lpTotalBytesAvail,
  __out_opt  LPDWORD lpBytesLeftThisMessage
);

Parameters

hNamedPipe [in]

A handle to the pipe. This parameter can be a handle to a named pipe instance, as returned by the CreateNamedPipe or CreateFile function, or it can be a handle to the read end of an anonymous pipe, as returned by the CreatePipe function. The handle must have GENERIC_READ access to the pipe.

lpBuffer [out, optional]

A pointer to a buffer that receives data read from the pipe. This parameter can be NULL if no data is to be read.

nBufferSize [in]

The size of the buffer specified by the lpBuffer parameter, in bytes. This parameter is ignored if lpBuffer is NULL.

lpBytesRead [out, optional]

A pointer to a variable that receives the number of bytes read from the pipe. This parameter can be NULL if no data is to be read.

lpTotalBytesAvail [out, optional]

A pointer to a variable that receives the total number of bytes available to be read from the pipe. This parameter can be NULL if no data is to be read.

lpBytesLeftThisMessage [out, optional]

A pointer to a variable that receives the number of bytes remaining in this message. This parameter will be zero for byte-type named pipes or for anonymous pipes. This parameter can be NULL if no data is to be read.

Return Value

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

The PeekNamedPipe function is similar to the ReadFile function with the following exceptions:

• The data is read in the mode specified with CreateNamedPipe. For example, create a pipe with PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE. If you change the mode to PIPE_READMODE_BYTE with SetNamedPipeHandleState, ReadFile will read in byte mode, but PeekNamedPipe will continue to read in message mode.
• The data read from the pipe is not removed from the pipe's buffer.
• The function can return additional information about the contents of the pipe.
• The function always returns immediately in a single-threaded application, even if there is no data in the pipe. The wait mode of a named pipe handle (blocking or nonblocking) has no effect on the function.

Note  The PeekNamedPipe function can block thread execution the same way any I/O function can when called on a synchronous handle in a multi-threaded application. To avoid this condition, use a pipe handle created for asynchronous I/O.

If the specified handle is a named pipe handle in byte-read mode, the function reads all available bytes up to the size specified in nBufferSize. For a named pipe handle in message-read mode, the function reads the next message in the pipe. If the message is larger than nBufferSize, the function returns TRUE after reading the specified number of bytes. In this situation, lpBytesLeftThisMessage will receive the number of bytes remaining in the message.

Requirements

See Also

CreateFile

CreateNamedPipe

CreatePipe

Pipe Functions

Pipes Overview

ReadFile

WriteFile

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Build date: 11/12/2009