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##### 12.21.2.2 Custom Stream Hook Functions

Here are more details on how you should define the four hook functions that a custom stream needs.

You should define the function to read data from the cookie as:

         ssize_t reader (void *cookie, char *buffer, size_t size)

This is very similar to the `read` function; see I/O Primitives. Your function should transfer up to size bytes into the buffer, and return the number of bytes read, or zero to indicate end-of-file. You can return a value of `-1` to indicate an error.

You should define the function to write data to the cookie as:

         ssize_t writer (void *cookie, const char *buffer, size_t size)

This is very similar to the `write` function; see I/O Primitives. Your function should transfer up to size bytes from the buffer, and return the number of bytes written. You can return a value of `0` to indicate an error. You must not return any negative value.

You should define the function to perform seek operations on the cookie as:

         int seeker (void *cookie, off64_t *position, int whence)

For this function, the position and whence arguments are interpreted as for `fgetpos`; see Portable Positioning.

After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in position. Your function should return a value of `0` on success and `-1` to indicate an error.

You should define the function to do cleanup operations on the cookie appropriate for closing the stream as:

         int cleaner (void *cookie)

Your function should return `-1` to indicate an error, and `0` otherwise.

— Data Type: cookie_read_function

> This is the data type that the read function for a custom stream should have. If you declare the function as shown above, this is the type it will have.

— Data Type: cookie_write_function

> The data type of the write function for a custom stream.

— Data Type: cookie_seek_function

> The data type of the seek function for a custom stream.

— Data Type: cookie_close_function

> The data type of the close function for a custom stream.