The #line directive changes the contents of _ _LINE_ _ and _ _FILE_ _, which are predefined identifiers. The basic form of the command is
#line number "filename"
where number is any positive integer and the filename is any valid file identifier. The value of number becomes the number of the current source line and filename becomes the name of the source file. The name of the file is optional. #line is primarily used for debugging purposes and special applications.
The _ _LINE_ _ identifier is an integer, and _ _FILE_ _ is a null-terminated string.
For example, the following sets the current line counter to 10 and the filename to “test”:
#line 10 "test"