用来重新设定源文件的名字和行号，指向另外的一个地方，如果出现编译错误，编译器的输出信息将指向设置后的文件和偏移， LEX & YACC，Ragel，很多这种宏。
MSDN的解释
The #line directive tells the preprocessor to change the compiler’s internally stored line number and filename to a given line number and filename. The compiler uses the line number and filename to refer to errors that it finds during compilation. The line number usually refers to the current input line, and the filename refers to the current input file. The line number is incremented after each line is processed.
Syntax
#line
digit-sequence "filename"opt
The digit-sequence value can be any integer constant. Macro replacement can be performed on the preprocessing tokens, but the result must evaluate to the correct syntax. The filename can be any combination of characters and must be enclosed in double quotation marks (" "). If filename is omitted, the previous filename remains unchanged.
You can alter the source line number and filename by writing a #line directive. The translator uses the line number and filename to determine the values of the predefined macros __FILE__ and __LINE__. You can use these macros to insert self-descriptive error messages into the program text. For more information on these predefined macros, see Predefined Macros.
The __FILE__ macro expands to a string whose contents are the filename, surrounded by double quotation marks (" ").
If you change the line number and filename, the compiler ignores the previous values and continues processing with the new values. The #line directive is typically used by program generators to cause error messages to refer to the original source file instead of to the generated program.
The following examples illustrate #line and the __LINE__ and __FILE__ macros.
In this statement, the internally stored line number is set to 151 and the filename is changed to copy.c.
#line 151 "copy.c"
In this example, the macro ASSERT uses the predefined macros __LINE__ and __FILE__ to print an error message about the source file if a given “assertion” is not true.
#define ASSERT(cond)
if( !(cond) )\
{printf( "assertion error line %d, file(%s)\n", \
__LINE__, __FILE__ );}