C Predefined Macros

Predefined macros are ready-made names C fills in for you, like the current file, line, date, and time. They help with messages and debugging.

In this page:

The FILE Macro
The LINE Macro
The DATE Macro
The TIME Macro
The STDC Macro

Syntax

__FILE__  // current source file name
__LINE__  // current line number
__DATE__  // compile date
__TIME__  // compile time

The FILE Macro

This is invaluable for debugging and logging -- a macro like #define LOG(msg) printf("[%s] %s\n", __FILE__, msg) automatically tags every log message with the exact source file it came from, without you having to type the filename manually.

Example: The FILE Macro

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%s", __FILE__);
	// Return 0 to signal that the program finished successfully
	return 0;
}

The LINE Macro

Combined with __FILE__, this pair is the foundation of custom assertion and error-reporting macros that tell you exactly where in the source code something went wrong, down to the specific line.

Example: The LINE Macro

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%d", __LINE__);
	// Return 0 to signal that the program finished successfully
	return 0;
}

The DATE Macro

This is useful for embedding build metadata directly into compiled binaries, such as printing a program's build date in its version or about screen without needing external build scripts.

Example: The DATE Macro

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%s", __DATE__);
	// Return 0 to signal that the program finished successfully
	return 0;
}

The TIME Macro

Paired with __DATE__, this gives you a complete build timestamp, which is commonly combined into a single version string printed when a program starts up in debug builds.

Example: The TIME Macro

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("%s %s", __DATE__, __TIME__);
	// Return 0 to signal that the program finished successfully
	return 0;
}

The STDC Macro

Checking #if __STDC__ == 1 lets code adapt its behavior or feature usage depending on whether it's being compiled by a strictly standards-conformant compiler versus one with vendor-specific extensions enabled.

Example: The STDC Macro

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	#if __STDC__ == 1
	// Print formatted text to the screen
	printf("Standards-conformant compiler");
	#endif
	// Return 0 to signal that the program finished successfully
	return 0;
}

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