The #line directive

A preprocessor line control directive supplies line numbers for compiler messages. It causes the compiler to view the line number of the next source line as the specified number.


#line directive syntax

>>-#--line------------------------------------------------------>

     .-------.                                            
     V       |                                            
>--+---+---+-+--decimal_constant--+-----------------+-+--------><
   |   '-0-'                      '-"--file_name--"-' |   
   '-characters---------------------------------------'

In order for the compiler to produce meaningful references to line numbers in preprocessed source, the preprocessor inserts #line directives where necessary (for example, at the beginning and after the end of included text).

A file name specification enclosed in double quotation marks can follow the line number. If you specify a file name, the compiler views the next line as part of the specified file. If you do not specify a file name, the compiler views the next line as part of the current source file.

In all C and C++ implementations, the token sequence on a #line directive is subject to macro replacement. After macro replacement, the resulting character sequence must consist of a decimal constant, optionally followed by a file name enclosed in double quotation marks.

You can use #line control directives to make the compiler provide more meaningful error messages. The following example program uses #line control directives to give each function an easily recognizable line number:

/**
 ** This example illustrates #line directives.
 **/

#include <stdio.h>
#define LINE200 200

int main(void)
{
   func_1();
   func_2();
}

#line 100
func_1()
{
   printf("Func_1 - the current line number is %d\n",__LINE__);
}

#line LINE200
func_2()
{
   printf("Func_2 - the current line number is %d\n",__LINE__);
}

This program produces the following output:

Func_1 - the current line number is 102
Func_2 - the current line number is 202

C++11 In C++11, the increased limit for #line directive from the C99 preprocessor are adopted to provide a common preprocessor interface for C and C++ compilers. The upper limit of #line <integer> preprocessor directives has been increased from 32,767 to 2,147,483,647 for the C++ preprocessor in conformance with the C99 preprocessor. For more information, see C99 preprocessor features adopted in C++11 (C++11).