CreateNamedPipe info  Overview  Group

The CreateNamedPipe function creates an instance of a named pipe and returns a handle for subsequent pipe operations. A named pipe server process uses this function either to create the first instance of a specific named pipe and establish its basic attributes or to create a new instance of an existing named pipe.

HANDLE CreateNamedPipe(

Parameters

lpName

Points to the null-terminated string that uniquely identifies the pipe. The string must have the following form:

\\.\pipe\pipename

The pipename part of the name can include any character other than a backslash, including numbers and special characters. The entire pipe name string can be up to 256 characters long. Pipe names are not case sensitive.

dwOpenMode

Specifies the pipe access mode, the overlapped mode, the write-through mode, and the security access mode of the pipe handle.

This parameter must specify one of the following pipe access mode flags. The same mode must be specified for each instance of the pipe:

Mode	Description
PIPE_ACCESS_DUPLEX	The pipe is bidirectional; both server and client processes can read from and write to the pipe. This mode gives the server the equivalent of GENERIC_READ | GENERIC_WRITE access to the pipe. The client can specify GENERIC_READ or GENERIC_WRITE, or both, when it connects to the pipe using the CreateFile function.
PIPE_ACCESS_INBOUND	The flow of data in the pipe goes from client to server only. This mode gives the server the equivalent of GENERIC_READ access to the pipe. The client must specify GENERIC_WRITE access when connecting to the pipe.
PIPE_ACCESS_OUTBOUND	The flow of data in the pipe goes from server to client only. This mode gives the server the equivalent of GENERIC_WRITE access to the pipe. The client must specify GENERIC_READ access when connecting to the pipe.

This parameter can also include either or both of the following flags, which enable write-through mode and overlapped mode. These modes can be different for different instances of the same pipe.

Mode	Description
FILE_FLAG_WRITE_THROUGH
	Write-through mode is enabled. This mode affects only write operations on byte-type pipes and, then, only when the client and server processes are on different computers. If this mode is enabled, functions writing to a named pipe do not return until the data written is transmitted across the network and is in the pipe’s buffer on the remote computer. If this mode is not enabled, the system enhances the efficiency of network operations by buffering data until a minimum number of bytes accumulate or until a maximum time elapses.
FILE_FLAG_OVERLAPPED
	Overlapped mode is enabled. If this mode is enabled, functions performing read, write, and connect operations that may take a significant time to be completed can return immediately. This mode enables the thread that started the operation to perform other operations while the time-consuming operation executes in the background. For example, in overlapped mode, a thread can handle simultaneous input and output (I/O) operations on multiple instances of a pipe or perform simultaneous read and write operations on the same pipe handle. If overlapped mode is not enabled, functions performing read, write, and connect operations on the pipe handle do not return until the operation is finished. The ReadFileEx and WriteFileEx functions can only be used with a pipe handle in overlapped mode. The ReadFile, WriteFile, ConnectNamedPipe, and TransactNamedPipe functions can execute either synchronously or as overlapped operations.

This parameter can include any combination of the following security access mode flags. These modes can be different for different instances of the same pipe. They can be specified without concern for what other dwOpenMode modes have been specified.

Mode	Description
WRITE_DAC	The caller will have write access to the named pipe’s discretionary access control list (ACL).
WRITE_OWNER	The caller will have write access to the named pipe’s owner.
ACCESS_SYSTEM_SECURITY	The caller will have write access to the named pipe’s system ACL.

dwPipeMode

Specifies the type, read, and wait modes of the pipe handle.

One of the following type mode flags can be specified. The same type mode must be specified for each instance of the pipe. If you specify zero, the parameter defaults to byte-type mode.

Mode	Description
PIPE_TYPE_BYTE	Data is written to the pipe as a stream of bytes. This mode cannot be used with PIPE_READMODE_MESSAGE.
PIPE_TYPE_MESSAGE	Data is written to the pipe as a stream of messages. This mode can be used with either PIPE_READMODE_MESSAGE or PIPE_READMODE_BYTE.

One of the following read mode flags can be specified. Different instances of the same pipe can specify different read modes. If you specify zero, the parameter defaults to byte-read mode.

Mode	Description
PIPE_READMODE_BYTE	Data is read from the pipe as a stream of bytes. This mode can be used with either PIPE_TYPE_MESSAGE or PIPE_TYPE_BYTE.
PIPE_READMODE_MESSAGE	Data is read from the pipe as a stream of messages. This mode can be only used if PIPE_TYPE_MESSAGE is also specified.

One of the following wait mode flags can be specified. Different instances of the same pipe can specify different wait modes. If you specify zero, the parameter defaults to blocking mode.

Mode	Description
PIPE_WAIT	Blocking mode is enabled. When the pipe handle is specified in the ReadFile, WriteFile, or ConnectNamedPipe function, the operations are not completed until there is data to read, all data is written, or a client is connected. Use of this mode can mean waiting indefinitely in some situations for a client process to perform an action.
PIPE_NOWAIT	Nonblocking mode is enabled. In this mode, ReadFile, WriteFile, and ConnectNamedPipe always return immediately. Note that nonblocking mode is supported for compatibility with Microsoft LAN Manager version 2.0 and should not be used to achieve asynchronous I/O with named pipes.

nMaxInstances

Specifies the maximum number of instances that can be created for this pipe. The same number must be specified for all instances. Acceptable values are in the range 1 through PIPE_UNLIMITED_INSTANCES. If this parameter is PIPE_UNLIMITED_INSTANCES, the number of pipe instances that can be created is limited only by the availability of system resources.

nOutBufferSize

Specifies the number of bytes to reserve for the output buffer. For a discussion on sizing named pipe buffers, see the following Remarks section.

nInBufferSize

Specifies the number of bytes to reserve for the input buffer. For a discussion on sizing named pipe buffers, see the following Remarks section.

nDefaultTimeOut

Specifies the default time-out value, in milliseconds, if the WaitNamedPipe function specifies NMPWAIT_USE_DEFAULT_WAIT. Each instance of a named pipe must specify the same value.

lpSecurityAttributes

Pointer to a SECURITY_ATTRIBUTES structure that specifies a security descriptor for the new named pipe and determines whether child processes can inherit the returned handle. If lpSecurityAttributes is NULL, the named pipe gets a default security descriptor and the handle cannot be inherited.

Return Values

If the function succeeds, the return value is a handle to the server end of a named pipe instance.

If the function fails, the return value is INVALID_HANDLE_VALUE. To get extended error information, call GetLastError. The return value is ERROR_INVALID_PARAMETER if nMaxInstances is greater than PIPE_UNLIMITED_INSTANCES.

Remarks

To create an instance of a named pipe by using CreateNamedPipe, the user must have FILE_CREATE_PIPE_INSTANCE access to the named pipe object. If a new named pipe is being created, the access control list (ACL) from the security attributes parameter defines the discretionary access control for the named pipe.

All instances of a named pipe must specify the same pipe type (byte-type or message-type), pipe access (duplex, inbound, or outbound), instance count, and time-out value. If different values are used, this function fails and GetLastError returns ERROR_ACCESS_DENIED.

The input and output buffer sizes are advisory. The actual buffer size reserved for each end of the named pipe is either the system default, the system minimum or maximum, or the specified size rounded up to the next allocation boundary.

The pipe server should not perform a blocking read operation until the pipe client has started. Otherwise, a race condition can occur. This typically occurs when initialization code, such as the C run-time, needs to lock and examine inherited handles.

An instance of a named pipe is always deleted when the last handle to the instance of the named pipe is closed.

See Also

ConnectNamedPipe, CreateFile, ReadFile, ReadFileEx, SECURITY_ATTRIBUTES, TransactNamedPipe, WaitNamedPipe, WriteFile, WriteFileEx