C preprocessor defined macros

This page shows how to use some of the C preprocessor defined variables.

The C preprocessor has many pre-defined macros which are values (acting as variables but not variables) defined at compile time (not run time). Some are shown here.

The preprocessor "pre processes" the source code before the compiler sees it.

We want to look at the preprocessor step in more detail.

The __DATE__ and __TIME__ are the compile date and time, not the date and time when the program is run.

__DATE__ __TIME__

These "v" can be used, for example, as part of the version of the software. They are not really "variables" but are dynamic textual replacements done at compile time by the preprocessor.

The __FILE__ and __LINE__ can be useful for debugging purposes (see below).

__FILE__ __LINE__

Here is the C code.

#include <stdio.h>

int main() {
	printf("The compile date is %s.\n",__DATE__);
	printf("The compile time was %s.\n",__TIME__);
	printf("\n");
	printf("The source file is \"%s\".\n",__FILE__);
	printf("The source line is %d.\n",__LINE__);
	return 0;
	}

Here is the output of the C code.

The compile date is Apr 30 2020.
The compile time was 06:39:20.

The source file is "D:\W\PROGRAM1.C\debug24-01.c".
The source line is 9.

A better debug output requires some system help in post-processing the output file or another generated file.

Here a beginning program is presented and then a preprocessor macro called printd added for debugging output support.

Here is the starting example program.

Here is the C code.

#include <stdio.h>

int main() {
	printf("One\n");
	printf("Two\n");
	printf("Three\n");
	return 0;
	}

Here is the output of the C code.

One
Two
Three

Here is the modified program example that uses a printd macro for text replacement and added debugging output.

Here is the C code.

#include <stdio.h>

#define printd(...) printf(__VA_ARGS__); printf(":MARK f=[%s],r=%d\n",__FILE__,__LINE__)

int main() {
	printd("One\n");
	printd("Two\n");
	printd("Three\n");
	return 0;
	}

Here is the output of the C code.

One
:MARK f=[D:\W\PROGRAM1.C\debug24-03.c],r=7
Two
:MARK f=[D:\W\PROGRAM1.C\debug24-03.c],r=8
Three
:MARK f=[D:\W\PROGRAM1.C\debug24-03.c],r=9

Note the following:

The __FILE__ is necessary for complex projects consisting of many files.
A convention such as always ending the printd string with a newline is needed (where printd is used).
Each printd should print one (and preferably only one) line, though there are ways around this.
A post-processing step is needed to extract the marks from the output and generate a suitable output.
Some suitable output, such as web-based HTML with JavaScript actions for click on the line is needed.
The editor can, for example, color-code such debugging statements so that they are easily seen.

The -E option to gcc allows one to see the intermediate code.

gcc -E mycode.c

This code has a lot of lines in it. There are 1478 lines for the above program (of 11 lines). Many lines are blank and many are comments with debugging line information in them. Remember, the include has many other includes and all of them are represented. For the above program, here are the last few lines.

# 6 "D:\\W\\PROGRAM1.C\\debug24-03.c" int main() { printf("One\n"); printf(":MARK f=[%s],r=%d\n","D:\\W\\PROGRAM1.C\\debug24-03.c",7); printf("Two\n"); printf(":MARK f=[%s],r=%d\n","D:\\W\\PROGRAM1.C\\debug24-03.c",8); printf("Three\n"); printf(":MARK f=[%s],r=%d\n","D:\\W\\PROGRAM1.C\\debug24-03.c",9); return 0; }

Note: The first line starting with a hash sign "#" is used for debugging information, much as the ":MARK" is used in the example program.

The C preprocessor can be used to determine the compiler platform on which the code is being compiled. Certain compiler platforms may have specific limitations, etc.

Note: One can also define symbols for different versions of software or the platform for which the code being compiled to be run (as opposed to the platform being used to compile the code to run). Here is the C code.

#include <stdio.h>

int main() {
	printf("Compiled on: ");
	#ifdef __APPLE__
	printf("APPLE\n");
	#endif
	#ifdef __MINGW32__
	printf("MINGW32\n");
	#endif
	#ifdef __LINUX__
	printf("LINUX\n");
	#endif
	#ifdef __CYGWIN__
	printf("CYGWIN\n");
	#endif
	return 0;
	}

Here is the output of the C code.

Compiled on: CYGWIN

This code was compiled on a CYGWIN platform (at the time these notes were created).