| __LINE__

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This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it's a pretty strange macro, since its "definition" changes with each new line of source code.
`__FILE__` and `__LINE__` are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example,

    fprintf (stderr, "Internal error: "
                     "negative string length "
                     "%d at %s, line %d.",
             length, __FILE__, __LINE__);

An `#include` directive changes the expansions of `__FILE__` and `__LINE__` to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the `#include` directive, the expansions of `__FILE__` and `__LINE__` revert to the values they had before the `#include` (but `__LINE__` is then incremented by one as processing moves to the line after the `#include`).
A `#line` directive changes `__LINE__`, and may change `__FILE__` as well. See Line Control.
C99 introduces `__func__`, and GCC has provided `__FUNCTION__` for a long time. Both of these are strings containing the name of the current function (there are slight semantic differences; see Function Names as Strings). Neither of them is a macro; the preprocessor does not know the name of the current function. They tend to be useful in conjunction with `__FILE__` and `__LINE__`, though.