##### 12.21.2.1 Custom Streams and Cookies

Inside every custom stream is a special object called the cookie. This is an object supplied by you which records where to fetch or store the data read or written. It is up to you to define a data type to use for the cookie. The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type `void *`.

To implement a custom stream, you must specify how to fetch or store the data in the specified place. You do this by defining hook functions to read, write, change “file position”, and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know.

When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type `cookie_io_functions_t`.

These facilities are declared in `stdio.h`.

— Data Type: cookie_io_functions_t

> This is a structure type that holds the functions that define the communications protocol between the stream and its cookie. It has the following members:

> `cookie_read_function_t *read`
>   This is the function that reads data from the cookie. If the value is a null pointer instead of a function, then read operations on this stream always return `EOF`.
> `cookie_write_function_t *write`
>   This is the function that writes data to the cookie. If the value is a null pointer instead of a function, then data written to the stream is discarded.
> `cookie_seek_function_t *seek`
>   This is the function that performs the equivalent of file positioning on the cookie. If the value is a null pointer instead of a function, calls to `fseek` or `fseeko` on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an `ESPIPE` error.
> `cookie_close_function_t *close`
>   This function performs any appropriate cleanup on the cookie when closing the stream. If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed.

— Function: FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io-functions)

> Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem lock | See POSIX Safety Concepts.

> This function actually creates the stream for communicating with the cookie using the functions in the io-functions argument. The opentype argument is interpreted as for `fopen`; see Opening Streams. (But note that the “truncate on open” option is ignored.) The new stream is fully buffered.

> The `fopencookie` function returns the newly created stream, or a null pointer in case of an error.