PeekNamedPipe

The PeekNamedPipe function 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.

BOOL PeekNamedPipe(
  HANDLE hNamedPipe,              // handle to pipe
  LPVOID lpBuffer,                // data buffer
  DWORD nBufferSize,              // size of data buffer
  LPDWORD lpBytesRead,            // number of bytes read
  LPDWORD lpTotalBytesAvail,      // number of bytes available
  LPDWORD lpBytesLeftThisMessage  // unread bytes
);

Parameters

hNamedPipe

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

lpBytesRead

[out] 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] 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] 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 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

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 always returns immediately, 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.
• The function can return additional information about the contents of the pipe.

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

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Pipes Overview, Pipe Functions, CreateFile, CreateNamedPipe, CreatePipe, ReadFile, WriteFile