5.2 FITS File Access Routines

1

Open an existing data file.  

int fits_open_file / ffopen
    (fitsfile **fptr, char *filename, int iomode, > int *status)

int fits_open_data / ffdopn
    (fitsfile **fptr, char *filename, int iomode, > int *status)

int fits_open_table / fftopn
    (fitsfile **fptr, char *filename, int iomode, > int *status)

int fits_open_image / ffiopn
    (fitsfile **fptr, char *filename, int iomode, > int *status)

The iomode parameter determines the read/write access allowed in the file and can have values of READONLY (0) or READWRITE (1). The filename parameter gives the name of the file to be opened, followed by an optional argument giving the name or index number of the extension within the FITS file that should be moved to and opened (e.g., myfile.fits+3 or myfile.fits[3] moves to the 3rd extension within the file, and myfile.fits[events] moves to the extension with the keyword EXTNAME = 'EVENTS').

The fits_open_data routine is similar to the fits_open_file routine except that it will move to the first HDU containing significant data, if a HDU name or number to open was not explicitly specified as part of the filename. In this case, it will look for the first IMAGE HDU with NAXIS > 0, or the first table that does not contain the strings `GTI' (Good Time Interval extension) or `OBSTABLE' in the EXTNAME keyword value.

The fits_open_table and fits_open_image routines are similar to fits_open_data except they will move to the first significant table HDU or image HDU in the file, respectively, if a HDU name or number is not specified as part of the filename.

IRAF images (.imh format files) and raw binary data arrays may also be opened with READONLY access. CFITSIO will automatically test if the input file is an IRAF image, and if, so will convert it on the fly into a virtual FITS image before it is opened by the application program. If the input file is a raw binary data array of numbers, then the datatype and dimensions of the array must be specified in square brackets following the name of the file (e.g. 'rawfile.dat[i512,512]' opens a 512 x 512 short integer image). See the `Extended File Name Syntax' chapter for more details on how to specify the raw file name. The raw file is converted on the fly into a virtual FITS image in memory which is then opened by the application program with READONLY access.

Programs can read the input file from the 'stdin' file stream if a dash character ('-') is given as the filename. Files can also be opened over the network using FTP or HTTP protocols by supplying the appropriate URL as the filename.

The input file can be modified in various ways to create a virtual file (usually stored in memory) that is then opened by the application program by supplying a filtering or binning specifier in square brackets following the filename. Some of the more common filtering methods are illustrated in the following paragraphs, but users should refer to the 'Extended File Name Syntax' chapter for a complete description of the full file filtering syntax.

When opening an image, a rectangular subset of the physical image may be opened by listing the first and last pixel in each dimension (and optional pixel skipping factor):

myimage.fits[101:200,301:400]

will create and open a 100x100 pixel virtual image of that section of the physical image, and myimage.fits[*,-*] opens a virtual image which is the same size as the physical image but has been flipped in the vertical direction.

When opening a table, the filtering syntax can be used to add or delete columns or keywords in the virtual table: myfile.fits[events][col !time; PI = PHA*1.2] opens a virtual table in which the TIME column has been deleted and a new PI column has been added with a value 1.2 times that of the PHA column. Similarly, one can filter a table to keep only those rows that satisfy a selection criterion: myfile.fits[events][pha > 50] creates and opens a virtual table containing only those rows with a PHA value greater than 50. A large number of boolean and mathematical operators can be used in the selection expression. One can also filter table rows using 'Good Time Interval' extensions, and spatial region filters as in myfile.fits[events][gtifilter()] and myfile.fits[events][regfilter( "stars.rng")].

Finally, table columns may be binned or histogrammed to generate a virtual image. For example, myfile.fits[events][bin (X,Y)=4] will result in a 2-dimensional image calculated by binning the X and Y columns in the event table with a bin size of 4 in each dimension. The TLMINn and TLMAXn keywords will be used by default to determine the range of the image.

A single program can open the same FITS file more than once and then treat the resulting fitsfile pointers as though they were completely independent FITS files. Using this facility, a program can open a FITS file twice, move to 2 different extensions within the file, and then read and write data in those extensions in any order.

2

Create and open a new empty output FITS file.  

int fits_create_file / ffinit
    (fitsfile **fptr, char *filename, > int *status)

An error will be returned if the specified file already exists, unless the filename is prefixed with an exclamation point (!). In that case CFITSIO will overwrite (delete) any existing file with the same name. Note that the exclamation point is a special UNIX character so if it is used on the command line it must be preceded by a backslash to force the UNIX shell to accept the character as part of the filename.

The output file will be written to the 'stdout' file stream if a dash character ('-') or the string 'stdout' is given as the filename. Similarly, '-.gz' or 'stdout.gz' will cause the file to be gzip compressed before it is written out to the stdout stream.

Optionally, the name of a template file which is used to define the structure of the new file may be specified in parentheses following the output file name. The template file may be another FITS file, in which case the new file, at the time it is opened, will be an exact copy of the template file except that the data structures (images and tables) will be filled with zeros. Alternatively, the template file may be an ASCII format text file containing directives that define the keywords to be created in each HDU of the file. See the 'Extended File Name Syntax' section for a complete description of the template file syntax.

3

Close a previously opened FITS file. The first routine simply closes the file, whereas the second one also DELETES THE FILE, which can be useful in cases where a FITS file has been partially created, but then an error occurs which prevents it from being completed.    

  int fits_close_file / ffclos (fitsfile *fptr, > int *status)

  int fits_delete_file / ffdelt (fitsfile *fptr, > int *status)

4

Return the name, I/O mode (READONLY or READWRITE), and/or the file type (e.g. 'file://', 'ftp://') of the opened FITS file.     

  int fits_file_name / ffflnm (fitsfile *fptr, > char *filename, int *status)

  int fits_file_mode / ffflmd (fitsfile *fptr, > int *iomode, int *status)

  int fits_url_type / ffurlt (fitsfile *fptr, > char *urltype, int *status)