During translation, two macros can be used to get information about the translation into the translated program. Their expansions change during translation. __FILE__ is a macro expanding to a string literal identifying the source file in which it appears. __LINE__ is a macro expanding to an integer expression giving the line number where it is expanded.

Since C99, __func__ is an object internal to the function in which it appears, giving the name of that function as a null-terminated string in an array of char. It is as if every function definition begins with a hidden static string:

void my_func(void)
{
  static const char __func__[] = "my_func";

  printf("Inside %s now\n", __func__);
}

Altogether, __FILE__, __LINE__ and __func__ can be used to build macros that report on the flow of control through an executing program:

// In foo.c
#include <stdio.h>

#define TRACE(msg) \
  fprintf(stderr, "TRACE: %s %s:%d %s\n", \
          __func__, __FILE__, __LINE__, msg)

int main(void)
{
  TRACE("here");
  return 0;
}

TRACE: main foo.c:10 here

control-line

# line pp-tokens new-line

The preprocessing directive #line changes the line number and file name reported in diagnostic messages, including the expansions of __LINE__ and __FILE__. This sets only the line number:

#line digit-sequence new-line

This sets the line number and file name:

#line digit-sequence "s-char-sequenceopt" new-line

Anything else of the following form has its tokens scanned for macro invocations, which are expanded, resulting in one of the two forms above:

#line pp-tokens new-line

#line is most useful in generated source. For example, an IDL might be processed by a tool to yield equivalent C code, and compile-time errors arising in that code might be the result of errors in the IDL. The tool that translates IDL into C can insert #line directives to make the errors refer to lines in the IDL instead of C.