10.3 Base Filename

The base filename is the name of the file optionally including the director/subdirectory path, and in the case of `ftp', `http', and `root' filetypes, the machine identifier. Examples:

    myfile.fits
    !data.fits
    /data/myfile.fits
    fits.gsfc.nasa.gov/ftp/sampledata/myfile.fits.gz

When creating a new output file on magnetic disk (of type file://) if the base filename begins with an exclamation point (!) then any existing file with that same basename will be deleted prior to creating the new FITS file. Otherwise if the file to be created already exists, then CFITSIO will return an error and will not overwrite the existing file. Note that the exclamation point, '!', is a special UNIX character, so if it is used on the command line rather than entered at a task prompt, it must be preceded by a backslash to force the UNIX shell to pass it verbatim to the application program.

If the output disk file name ends with the suffix '.gz', then CFITSIO will compress the file using the gzip compression algorithm before writing it to disk. This can reduce the amount of disk space used by the file. Note that this feature requires that the uncompressed file be constructed in memory before it is compressed and written to disk, so it can fail if there is insufficient available memory.

An input FITS file may be compressed with the gzip or Unix compress algorithms, in which case CFITSIO will uncompress the file on the fly into a temporary file (in memory or on disk). Compressed files may only be opened with read-only permission. When specifying the name of a compressed FITS file it is not necessary to append the file suffix (e.g., `.gz' or `.Z'). If CFITSIO cannot find the input file name without the suffix, then it will automatically search for a compressed file with the same root name. In the case of reading ftp and http type files, CFITSIO generally looks for a compressed version of the file first, before trying to open the uncompressed file. By default, CFITSIO copies (and uncompressed if necessary) the ftp or http FITS file into memory on the local machine before opening it. This will fail if the local machine does not have enough memory to hold the whole FITS file, so in this case, the output filename specifier (see the next section) can be used to further control how CFITSIO reads ftp and http files.

If the input file is an IRAF image file (*.imh file) then CFITSIO will automatically convert it on the fly into a virtual FITS image before it is opened by the application program. IRAF images can only be opened with READONLY file access.

Similarly, if the input file is a raw binary data array, then CFITSIO will convert it on the fly into a virtual FITS image with the basic set of required header keywords before it is opened by the application program (with READONLY access). In this case the data type and dimensions of the image must be specified in square brackets following the filename (e.g. rawfile.dat[ib512,512]). The first character (case insensitive) defines the datatype of the array:

     b         8-bit unsigned byte
     i        16-bit signed integer
     u        16-bit unsigned integer
     j        32-bit signed integer
     r or f   32-bit floating point
     d        64-bit floating point

An optional second character specifies the byte order of the array values: b or B indicates big endian (as in FITS files and the native format of SUN UNIX workstations and Mac PCs) and l or L indicates little endian (native format of DEC OSF workstations and IBM PCs). If this character is omitted then the array is assumed to have the native byte order of the local machine. These datatype characters are then followed by a series of one or more integer values separated by commas which define the size of each dimension of the raw array. Arrays with up to 5 dimensions are currently supported. Finally, a byte offset to the position of the first pixel in the data file may be specified by separating it with a ':' from the last dimension value. If omitted, it is assumed that the offset = 0. This parameter may be used to skip over any header information in the file that preceeds the binary data. Further examples:

  raw.dat[b10000]           1-dimensional 10000 pixel byte array
  raw.dat[rb400,400,12]     3-dimensional floating point big-endian array
  img.fits[ib512,512:2880]  reads the 512 x 512 short integer array in
                            a FITS file, skipping over the 2880 byte header

One special case of input file is where the filename = `-' (a dash or minus sign) or 'stdin' or 'stdout', which signifies that the input file is to be read from the stdin stream, or written to the stdout stream if a new output file is being created. In the case of reading from stdin, CFITSIO first copies the whole stream into a temporary FITS file (in memory or on disk), and subsequent reading of the FITS file occurs in this copy. When writing to stdout, CFITSIO first constructs the whole file in memory (since random access is required), then flushes it out to the stdout stream when the file is closed. In addition, if the output filename = '-.gz' or 'stdout.gz' then it will be gzip compressed before being written to stdout.

This ability to read and write on the stdin and stdout steams allows FITS files to be piped between tasks in memory rather than having to create temporary intermediate FITS files on disk. For example if task1 creates an output FITS file, and task2 reads an input FITS file, the FITS file may be piped between the 2 tasks by specifying

   task1 - | task2 -

where the vertical bar is the Unix piping symbol. This assumes that the 2 tasks read the name of the FITS file off of the command line.