Node:Hook Functions, Previous:Streams and Cookies, Up:Custom Streams
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 -1 to indicate an error.
You should define the function to perform seek operations on the cookie
as:
int seeker (void *cookie, fpos_t *position, int whence)
For this function, the position and whence arguments are
interpreted as for fgetpos; see Portable Positioning. In
the GNU library, fpos_t is equivalent to off_t or
long int, and simply represents the number of bytes from the
beginning of the file.
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.
| cookie_read_function | Data Type |
| 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. |
| cookie_write_function | Data Type |
| The data type of the write function for a custom stream. |
| cookie_seek_function | Data Type |
| The data type of the seek function for a custom stream. |
| cookie_close_function | Data Type |
| The data type of the close function for a custom stream. |