---
name: "xcb_query_extension"
section: "3"
platform: "linux"
package: "libxcb"
version: "1.17.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/xcb_query_extension"
---

# xcb\_query\_extension(3)

check if extension is present

## NAME

xcb\_query\_extension - check if extension is present

## SYNOPSIS

**#include \<xcb/xproto.h\>**

### Request function

xcb\_query\_extension\_cookie\_t **xcb\_query\_extension**(xcb\_connection\_t \**conn*, uint16\_t *name\_len*, const char \**name*);

### Reply datastructure

```

typedef struct xcb_query_extension_reply_t {
    uint8_t  response_type;
    uint8_t  pad0;
    uint16_t sequence;
    uint32_t length;
    uint8_t  present;
    uint8_t  major_opcode;
    uint8_t  first_event;
    uint8_t  first_error;
} xcb_query_extension_reply_t;
```

### Reply function

xcb\_query\_extension\_reply\_t \***xcb\_query\_extension\_reply**(xcb\_connection\_t \**conn*, xcb\_query\_extension\_cookie\_t *cookie*, xcb\_generic\_error\_t \*\**e*);\

## REQUEST ARGUMENTS

- *conn*

  The XCB connection to X11.

- *name\_len*

  The length of *name* in bytes.

- *name*

  The name of the extension to query, for example "RANDR". This is case sensitive!

## REPLY FIELDS

- *response\_type*

  The type of this reply, in this case *XCB\_QUERY\_EXTENSION*. This field is also present in the *xcb\_generic\_reply\_t* and can be used to tell replies apart from each other.

- *sequence*

  The sequence number of the last request processed by the X11 server.

- *length*

  The length of the reply, in words (a word is 4 bytes).

- *present*

  Whether the extension is present on this X11 server.

- *major\_opcode*

  The major opcode for requests.

- *first\_event*

  The first event code, if any.

- *first\_error*

  The first error code, if any.

## DESCRIPTION

Determines if the specified extension is present on this X11 server.

Every extension has a unique *major\_opcode* to identify requests, the minor opcodes and request formats are extension-specific. If the extension provides events and errors, the *first\_event* and *first\_error* fields in the reply are set accordingly.

There should rarely be a need to use this request directly, XCB provides the *xcb\_get\_extension\_data* function instead.

## RETURN VALUE

Returns an *xcb\_query\_extension\_cookie\_t*. Errors have to be handled when calling the reply function *xcb\_query\_extension\_reply*.

If you want to handle errors in the event loop instead, use *xcb\_query\_extension\_unchecked*. See **[xcb-requests(3)](/xcb-requests/3.md)** for details.

## ERRORS

This request does never generate any errors.

## SEE ALSO

[**xcb-requests**(3)](/xcb-requests/3.md), **xcb\_get\_extension\_data**(3), [**xdpyinfo**(1)](/xdpyinfo/1.md)

## AUTHOR

Generated from xproto.xml. Contact xcb@lists.freedesktop.org for corrections and improvements.