GNU Info

(cpp-300.info)Line Control

---

Next: Pragmas Prev: Diagnostics Up: Top

---

Enter node , (file) or (file)node

Line Control
************

   The C preprocessor informs the C compiler of the location in your
source code where each token came from.  Presently, this is just the
file name and line number.  All the tokens resulting from macro
expansion are reported as having appeared on the line of the source
file where the outermost macro was used.  We intend to be more accurate
in the future.

   If you write a program which generates source code, such as the
`bison' parser generator, you may want to adjust the preprocessor's
notion of the current file name and line number by hand.  Parts of the
output from `bison' are generated from scratch, other parts come from a
standard parser file.  The rest are copied verbatim from `bison''s
input.  You would like compiler error messages and symbolic debuggers
to be able to refer to `bison''s input file.

   `bison' or any such program can arrange this by writing `#line'
directives into the output file.  `#line' is a directive that specifies
the original line number and source file name for subsequent input in
the current preprocessor input file.  `#line' has three variants:

`#line LINENUM'
     LINENUM is a non-negative decimal integer constant.  It specifies
     the line number which should be reported for the following line of
     input.  Subsequent lines are counted from LINENUM.

`#line LINENUM FILENAME'
     LINENUM is the same as for the first form, and has the same
     effect.  In addition, FILENAME is a string constant.  The
     following line and all subsequent lines are reported to come from
     the file it specifies, until something else happens to change that.

`#line ANYTHING ELSE'
     ANYTHING ELSE is checked for macro calls, which are expanded.  The
     result should match one of the above two forms.

   `#line' directives alter the results of the `__FILE__' and
`__LINE__' predefined macros from that point on.  Note: Standard
Predefined Macros.  They do not have any effect on `#include''s idea
of the directory containing the current file.