| Line Control

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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. filename is interpreted according to the normal rules for a string constant: backslash escapes are interpreted. This is different from `#include`.
  Previous versions of CPP did not interpret escapes in `#line`; we have changed it because the standard requires they be interpreted, and most other compilers do.
#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. See Standard Predefined Macros. They do not have any effect on `#include`'s idea of the directory containing the current file. This is a change from GCC 2.95. Previously, a file reading

    #line 1 "../src/gram.y"
    #include "gram.h"

would search for `gram.h` in `../src`, then the '-I' chain; the directory containing the physical source file would not be searched. In GCC 3.0 and later, the `#include` is not affected by the presence of a `#line` referring to a different directory.
We made this change because the old behavior caused problems when generated source files were transported between machines. For instance, it is common practice to ship generated parsers with a source release, so that people building the distribution do not need to have yacc or Bison installed. These files frequently have `#line` directives referring to the directory tree of the system where the distribution was created. If GCC tries to search for headers in those directories, the build is likely to fail.
The new behavior can cause failures too, if the generated file is not in the same directory as its source and it attempts to include a header which would be visible searching from the directory containing the source file. However, this problem is easily solved with an additional '-I' switch on the command line. The failures caused by the old semantics could sometimes be corrected only by editing the generated files, which is difficult and error-prone.