We can also write a Sink function, that writes a line of output text
to a file, and use it in basically the same way as the Source function
in the previous section ():
static FILE *output_stream; void Sink( const char *line ) { (void) fprintf( output_stream, "%s\n", line ); }
Note that we must supply the final newline character ourselves.
In this case, our main program would supply a pointer to this Sink function as the second argument to astChannel, as follows:
/* Open the output file. */ output_stream = fopen( "outfile.ast", "w" ); /* Create a Channel and write an Object to it. */ channel = astChannel( Source, Sink, "" ); nobj = astWrite( channel, object ); ... /* Annul the Channel and close the file when done. */ channel = astAnnul( channel ); (void) fclose( output_stream );
Note that we can specify a source and/or a sink function for the Channel, and that these may use either the same file, or different files according to whether we are reading or writing. AST has no knowledge of the underlying file system, nor of file positioning. It just reads and writes sequentially. If you wish, for example, to reposition a file at the beginning in between reads and writes, then this can be done directly (and completely independently of AST) using standard C functions.
If an error occurs in your source or sink function, you can
communicate this to the AST library by setting its error status to any
error value using astSetStatus (). This will
immediately terminate the read or write operation.
AST A Library for Handling World Coordinate Systems in Astronomy