fd_data_function
fd_data_function — attaches a write-activated callback to a file.
fd_data_function (fd|file, code)
fd|fileA file descriptor as returned from fd_open, or the name of a
file pointer to a file that was opened by a call to open or open_string.
codeAny valid Gamma program, executable code block, or statement.
The return value of the executed code.
This function acts as a callback, causing the code
to execute whenever data is written to the file associated with the
fd or file pointer.
fd_open, fd_eof_function,
fd_write, open, open_string, write