6.1 The Iterator Work Function

The user-supplied iterator work function must have the following set of input parameters (the function can be given any desired name):

  int user_fn( long totaln, long offset, long firstn, long nvalues,
               int narrays, iteratorCol *data,  void *userPointer )

• totaln - the total number of table rows or image pixels that will be passed to the work function during 1 or more iterations.

• offset - the offset applied to the first table row or image pixel to be passed to the work function. In other words, this is the number of rows or pixels that are skipped over before starting the iterations. If offset = 0, then all the table rows or image pixels will be passed to the work function.

• firstn - the number of the first table row or image pixel (starting with 1) that is being passed in this particular call to the work function.

• nvalues - the number of table rows or image pixels that are being passed in this particular call to the work function. nvalues will always be less than or equal to totaln and will have the same value on each iteration, except possibly on the last call which may have a smaller value.

• narrays - the number of arrays of data that are being passed to the work function. There is one array for each image or table column.

• *data - array of structures, one for each column or image. Each structure contains a pointer to the array of data as well as other descriptive parameters about that array.

• *userPointer - a user supplied pointer that can be used to pass ancillary information from the driver function to the work function. This pointer is passed to the CFITSIO iterator function which then passes it on to the work function without any modification. It may point to a single number, to an array of values, to a structure containing an arbitrary set of parameters of different types, or it may be a null pointer if it is not needed. The work function must cast this pointer to the appropriate data type before using it it.

The totaln, offset, narrays, data, and userPointer parameters are guaranteed to have the same value on each iteration. Only firstn, nvalues, and the arrays of data pointed to by the data structures may change on each iterative call to the work function.

Note that the iterator treats an image as a long 1-D array of pixels regardless of it's intrinsic dimensionality. The total number of pixels is just the product of the size of each dimension, and the order of the pixels is the same as the order that they are stored in the FITS file. If the work function needs to know the number and size of the image dimensions then these parameters can be passed via the userPointer structure.

The iteratorCol structure is currently defined as follows:

typedef struct  /* structure for the iterator function column information */
{
   /* structure elements required as input to fits_iterate_data: */

  fitsfile *fptr;       /* pointer to the HDU containing the column or image */
  int      colnum;      /* column number in the table; ignored for images    */
  char     colname[70]; /* name (TTYPEn) of the column; null for images      */
  int      datatype;    /* output datatype (converted if necessary) */
  int      iotype;      /* type: InputCol, InputOutputCol, or OutputCol */

  /* output structure elements that may be useful for the work function: */

  void     *array;    /* pointer to the array (and the null value) */
  long     repeat;    /* binary table vector repeat value; set     */
                      /*     equal to 1 for images                 */
  long     tlmin;     /* legal minimum data value, if any          */
  long     tlmax;     /* legal maximum data value, if any          */
  char     unit[70];  /* physical unit string (BUNIT or TUNITn)    */
  char     tdisp[70]; /* suggested display format; null if none    */

} iteratorCol;

Instead of directly reading or writing the elements in this structure, it is recommended that programmers use the access functions that are provided for this purpose.

The first five elements in this structure must be initially defined by the driver routine before calling the iterator routine. The CFITSIO iterator routine uses this information to determine what column or array to pass to the work function, and whether the array is to be input to the work function, output from the work function, or both. The CFITSIO iterator function fills in the values of the remaining structure elements before passing it to the work function.

The array structure element is a pointer to the actual data array and it must be cast to the correct data type before it is used. The `repeat' structure element give the number of data values in each row of the table, so that the total number of data values in the array is given by repeat * nvalues. In the case of image arrays and ASCII tables, repeat will always be equal to 1. When the datatype is a character string, the array pointer is actually a pointer to an array of string pointers (i.e., char **array). The other output structure elements are provided for convenience in case that information is needed within the work function. Any other information may be passed from the driver routine to the work function via the userPointer parameter.

Upon completion, the work routine must return an integer status value, with 0 indicating success and any other value indicating an error which will cause the iterator function to immediately exit at that point. Return status values in the range 1 - 1000 should be avoided since these are reserved for use by CFITSIO. A return status value of -1 may be used to force the CFITSIO iterator function to stop at that point and return control to the driver routine after writing any output arrays to the FITS file. CFITSIO does not considered this to be an error condition, so any further processing by the application program will continue normally.