PeekNamedPipe

The PeekNamedPipe function retrieves a specified number of bytes from a named pipe. It does not consume the bytes read; the bytes are left in the pipe.

DWORD PeekNamedPipe(
                    _In_ HANDLE hPipe,
                    _In_ LPVOID lpBuf,
                    _In_ DWORD nBytes
                );

Parameters

Parameter	Data Type	Description
`hPipe`	`HANDLE`	A handle to the named pipe.
`lpBuf`	`LPVOID`	A pointer to a buffer to receive the data. This buffer must be at least `nBytes` bytes in size.
`nBytes`	`DWORD`	The maximum number of bytes to retrieve from the pipe.

Remarks

The PeekNamedPipe function retrieves data from a named pipe without consuming the data. The data remains in the pipe. This function is useful when you want to inspect the data in a pipe without removing it.

If the PeekNamedPipe function returns 0, the pipe may be in an error state. Call GetLastError to determine the reason for the error.

Example

#include <windows.h>

int main() {
    HANDLE hPipe = CreateNamedPipe(
        "\\\\MyPipe",                 // Pipe name
        PIPE_ACCESS_DUPLEX,           // Access mode
        PIPE_TYPE_BYTE | PIPE_READMODE_BYTE | PIPE_WAIT, // Pipe type and mode
        1,                           // Max instances
        1024,                        // Output buffer size
        1024,                        // Input buffer size
        0,                           // Time-out interval
        NULL                         // Flags
    );

    if (hPipe == INVALID_HANDLE_VALUE) {
        DWORD error = GetLastError();
        printf("CreateNamedPipe failed with error %d\n", error);
        return 1;
    }

    char buffer[10];
    DWORD bytesRead;
    if (PeekNamedPipe(hPipe, buffer, sizeof(buffer), &bytesRead, NULL) == 0) {
        DWORD error = GetLastError();
        printf("PeekNamedPipe failed with error %d\n", error);
    } else {
        printf("Read %d bytes: %s\n", bytesRead, buffer);
    }

    CloseHandle(hPipe);
    return 0;
}

Troubleshooting

If PeekNamedPipe fails, check the following:

The named pipe exists and is accessible.
The pipe is not full.
The pipe is not in an error state. Call GetLastError to determine the reason for the error.