The length of a standard FITS string keyword is limited to 68 characters because it must fit entirely within a single FITS header keyword record. In some instances it is necessary to encode strings longer than this limit, so FITSIO supports a local convention in which the string value is continued over multiple keywords. This continuation convention uses an ampersand character at the end of each substring to indicate that it is continued on the next keyword, and the continuation keywords all have the name CONTINUE without an equal sign in column 9. The string value may be continued in this way over as many additional CONTINUE keywords as is required. The following lines illustrate this continuation convention which is used in the value of the STRKEY keyword:
LONGSTRN= 'OGIP 1.0' / The OGIP Long String Convention may be used. STRKEY = 'This is a very long string keyword&' / Optional Comment CONTINUE ' value that is continued over 3 keywords in the & ' CONTINUE 'FITS header.' / This is another optional comment.It is recommended that the LONGSTRN keyword, as shown here, always be included in any HDU that uses this longstring convention. A subroutine called FTPLSW has been provided in CFITSIO to write this keyword if it does not already exist.
This long string convention is supported by the following FITSIO subroutines that deal with string-valued keywords:
ftgkys - read a string keyword ftpkls - write (append) a string keyword ftikys - insert a string keyword ftmkys - modify the value of an existing string keyword ftukys - update an existing keyword, or write a new keyword ftdkey - delete a keywordThese routines will transparently read, write, or delete a long string value in the FITS file, so programmers in general do not have to be concerned about the details of the convention that is used to encode the long string in the FITS header. When reading a long string, one must ensure that the character string parameter used in these subroutine calls has been declared long enough to hold the entire string, otherwise the returned string value will be truncated.
Note that the more commonly used FITSIO subroutine to write string valued keywords (FTPKYS) does NOT support this long string convention and only supports strings up to 68 characters in length. This has been done deliberately to prevent programs from inadvertently writing keywords using this non-standard convention without the explicit intent of the programmer or user. The FTPKLS subroutine must be called instead to write long strings. This routine can also be used to write ordinary string values less than 68 characters in length.