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Wednesday, October 26, 2005
 
C/C++: About __FILE__ & __LINE__ Macros

__FILE__ & __LINE__ macros specified in C & C++ standards are useful to provide some debugging information, during run-time. During pre-processing stage of compilation, __FILE__ macro expands to the name of the file being compiled, in the form of a C string constant; and __LINE__ expands to the line number of the source file that is being compiled, in the form of an integer constant.

Run-time anomalies are not uncommon in production environment. Some developers check for pre-, post-conditions for a block of code with the help of assertions. Depending on the severity of the failure/violation/deviation, the program (application) may either crash or continue to run (of course, at the will of the developer who wrote that piece of code). However to minimize errors/failures during run-time, it is often necessary to log the run-time error messages for a later analysis. So, the developer can introduce some statements with __FILE__ & __LINE__ macros, at appropriate places in the source code to log the exact location of the failure, if an inconsistency was detected during run-time.

Here's an example of poorly written C code, that fails to do proper exception handling. However it logs enough information, to locate the failed statement easily.
% cat -n file-line-directives.c
1 #include <stdio.h>
2 #include <stdlib.h>
3
4 int main(int argc, char **argv)
5 {
6 FILE *fd;
7 char *errinfo = NULL;
8
9 errinfo = (char *) malloc (sizeof (char) * 50);
10
11 fd = fopen (argv[1], "r");
12 if (fd == NULL)
13 {
14 sprintf (errinfo, "Error at line %d of %s", (__LINE__ - 3), __FILE__);
15 perror (errinfo);
16 free (errinfo);
17 }
18 else
19 {
20 printf ("file descriptor = %d", fileno(fd));
21 fclose (fd);
22 }
23
24 return (0);
25 }
Compiling this code with Sun Studio C (or C++) compiler produces the following result, at run-time:
% cc -o directives file-line-directives.c

% ./directives <- needs an argument ie., file name
Error at line 11 of file-line-directives.c: Bad address

% ./directives /tmp/nonexistingfile <- need an existing file name, to open it up for reading
Error at line 11 of file-line-directives.c: No such file or directory

% ls /tmp/*.log
/tmp/activity.log

%./directives /tmp/activity.log
file descriptor = 3
Note:
Since these pre-defined macros are part of the standard(s), they are available with all C and C++ standard compliant compilers.
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Comments:
thanks giri.. very useful info
 
Nice snippet. Appreciate very much.
 
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