Purpose

The #line directive allows setting the current line number and name of the file being compiled.

Format

#line integer

or
#line integer "file_name"
or
#line preprocessing_expression

All preprocessor directives begin with the # symbol. It must be the first character on the line or the first character on the line following optional white space.

Spaces or tabs are permitted between the # and line, but not escape characters or other symbols or macros. The preprocessor removes white space and concatenates the # and line together.

If anything follows the #line directive (other than white space) then the program is malformed.

The following are valid uses:

#line 100
# line   100
# /* comments are white space */ line 100

The following are invalid uses:

// #\ is not a valid preprocessor directive
# \t line 100
// #" is not a valid preprocessor directive
# "" line 100

Use

There are three different forms of the #line directive which allow you to change the current line numbering and file name during preprocessing.

First Form – #line integer

When the line number is changed, the line immediately following the #line directive receives the new line number:

#line 123
/* this line is now line number 123 */
/* this line is line number 124 */
/* this line is line number 126 */

Second Form – #line integer file_name

When file_name is specified, the working name of the file being compiled is changed to the specified string. The file_name must be enclosed by double quotes (").

In the following example, the line number remains the same but the name of the file being compiled is changed to new_file_name.abc. Only the name the compiler thinks it is compiling changes (and which it will return using the __FILE__ macro), the actual (physical) file name remains the same:

#line __LINE__ "new_file_name.abc"

Third Form – #line preprocessor_tokens

The final form of the #line directive allows you to specify preprocessor tokens to be expanded. After expansion, the third form must look like one of the first two forms. If it does not, the program is malformed. NOTE: the preprocessor tokens are expanded, but not evaluated.

#define LINE_NUMBER 100
#define FILE_NAME(f) #f

#line LINE_NUMBER FILE_NAME(abc.h)

In the above example, LINE_NUMBER is expanded to 100 and FILE_NAME stringizes the passed parameter.

After compilation, the next line will be line number 100 and the name of the file being compiled will be abc.h.