C macros __LINE__, __FILE__ and __func__

puss_in_boots

It is possible for a C program to print the currently executing line of source code, the file of the source code, and the name of the current function. The currently executing line is available in a preprocessor variable called __LINE__:

#include <stdio.h>

int main ()
{
    printf ("This is line %d.\n", __LINE__);
    return 0;
}

(download)

This prints out
This is line 5.

To get the name of the file, use the preprocessor variable __FILE__:

#include <stdio.h>

int main ()
{
    printf ("This is line %d of file \"%s\".\n",
            __LINE__, __FILE__);
    return 0;
}

(download)

This prints out
This is line 6 of file "/usr/home/ben/lemoda/c/line-file-func/file.c".

C99 (the most recent version of the C language, at the time of writing) introduced another macro called __func__ which also names the function in use:

#include <stdio.h>

#define BLURT printf ("This is line %d of file %s (function %s)\n",\
                      __LINE__, __FILE__, __func__)

void silly_function ()
{
    BLURT;
}

int main ()
{
    BLURT;
    silly_function ();
    return 0;
}

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This prints out
This is line 13 of file /usr/home/ben/lemoda/c/line-file-func/func.c (function main)
This is line 8 of file /usr/home/ben/lemoda/c/line-file-func/func.c (function silly_function)

You can change the values used for __FILE__ and __LINE__ using a line directive, which is a preprocessor command of the form

#line 20 "some-file"

which tells the C compiler that the next line after the line directive is line 20 of some-file.

#include <stdio.h>

#define BLURT printf ("This is line %d of file \"%s\" (function <%s>)\n",\
                      __LINE__, __FILE__, __func__)

#line 99 "grody-to-the-max"
void silly_function ()
{
    BLURT;
}

#line 999 "bassetts-liquorice-allsorts"
int main ()
{
    BLURT;
    silly_function ();
    return 0;
}

(download)

This prints out
This is line 1001 of file "bassetts-liquorice-allsorts" (function <main>)
This is line 101 of file "grody-to-the-max" (function <silly_function>)

This line directive is used by facilities like "lex" and "yacc" which output C code so that error messages are related to the input file rather than the output C file.

GCC (the "Gnu Compiler Collection") also defines __FUNCTION__ and __PRETTY_FUNCTION__. These are both the same as __func__ for C. For C++, __PRETTY_FUNCTION__ is something different.

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