ERROR_PIPE_CONNECTED - 535 (0x217)
There is a process on other end of the pipe.
Updated: Feb 21, 2026
Technical Meaning
The ERROR_PIPE_CONNECTED error code indicates that a process has already established a connection to the other end of an open pipe. This typically occurs when attempting to connect to a named or anonymous pipe where another process is already using it.
Error Details
This error is returned by various Windows API functions, such as CreateFile, ConnectNamedPipe, and others that deal with inter-process communication via pipes. The presence of this error suggests that the operation was successful in connecting to an existing pipe but did not result in a new connection being established.
Usage Context
This error is commonly encountered when working with named or anonymous pipes for inter-process communication (IPC). It can occur in scenarios where multiple processes are trying to communicate through the same pipe, and one process has already initiated the connection.
Developer Interpretation
When encountering ERROR_PIPE_CONNECTED, developers should understand that it signifies a successful connection attempt but not necessarily a new connection. This error is often used as part of a two-step process for connecting to pipes: first, attempting to connect, and then waiting for the pipe to be ready (using functions like ConnectNamedPipe).
Related Errors
ERROR_PIPE_NOT_CONNECTED- Indicates that no process has connected to the other end of an open pipe.ERROR_NO_DATA- May occur if there is no data available on a pipe after a connection is established.
FAQ
Q: What does ERROR_PIPE_CONNECTED mean?
A: It indicates that a process has already connected to the other end of an open pipe, and further action may be required to establish communication.
Q: How can I handle this error in my code?
A: Typically, you should check for ERROR_PIPE_CONNECTED after attempting to connect to a pipe. If it is returned, you should call ConnectNamedPipe or similar functions to wait for the pipe to be ready before proceeding with communication.
Q: Can I ignore this error and continue my operation?
A: No, ignoring this error may lead to unexpected behavior in your application. You should handle it appropriately by waiting for the pipe to become ready using ConnectNamedPipe or similar functions.
Summary
The ERROR_PIPE_CONNECTED error code is a specific indication that a process has already connected to an open pipe. Developers must understand its context and properly manage their operations to ensure successful inter-process communication through pipes.