/sys$common/syshlp/helplib.hlb CC, Language topics, Predefined Macros, LINE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The __LINE__ macro evaluates to a decimal constant specifying the
  number of the line in the source file containing the macro
  reference.

  Example:

       printf("At line %d in file %s", __LINE__, __FILE__);