# UserCB()

This callback function is triggered by including the `$USERCB` token in a member of `KVXMLTemplate`. For example, placing `"$USERCB=my_callback "` in `pszFirstH1Start` results in a callback at the point when `pszFirstH1Start` is processed. The user callback function is identified by the text assigned to `$USERCB`, which in this example is `my_callback`. This identifier is passed to the argument `pszUserCBid`.

## Syntax

    BOOL (pascal *UserCB) (
        void              *pCallingContext,
        char              *pszUserCBid,
        KVOutputStream    *pNewOutput
        void              *pReserved);

## Arguments

`pCallingContext` | A pointer that gets passed back to the caller-provided callback function. This pointer, which can be `NULL`, is specified as the second parameter of the call to `fpConvertStream()`.
`pszUserCBid` | A pointer to a string that identifies the source of the callback. The identifier must be delimited by a trailing white space. For example, `"my_callback "`.
`pNewOutput` | A pointer to a `KVOutputStream` structure that can be used to write data to the current block.
pReserved | Reserved for future use.

## Returns

- If the call is successful, the return value is `TRUE`.

- If the call is unsuccessful, the return value is `FALSE`. Processing is halted.

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