Built-in Macros Every C Programmer Should Know
These macros are used like regular identifiers — directly in code or within expressions.
They are handled by the preprocessor, before compilation begins.
These require no #define or #include. They’re built into the language.
main.c
#include <stdio.h> void process() { printf("Function %s called from %s:%d at %s %s\n", __func__, __FILE__, __LINE__, __DATE__, __TIME__); } int main() { process(); return 0; }
How to use it?
Logging errors with context
This example defines a LOG_ERROR macro that automatically appends file name and line number to an error message.
main.c
#include <stdio.h> #define LOG_ERROR(msg) \ fprintf(stderr, "ERROR: %s | %s:%d\n", msg, __FILE__, __LINE__) void read_config() { LOG_ERROR("Failed to load config file"); } int main() { read_config(); return 0; }
Function tracing
Automatically prints the function name when it is called — useful for debugging and call tracing.
main.c
#include <stdio.h> void init_system() { printf("Entering function: %s\n", __func__); } void process_data() { printf("Entering function: %s\n", __func__); } int main() { init_system(); process_data(); return 0; }
Embedding build metadata
Displays the date and time when the program was compiled — helpful for versioning or debugging builds.
main.c
#include <stdio.h> int main() { printf("Program compiled on %s at %s\n", __DATE__, __TIME__); return 0; }
Checking compiler standard compliance
The program checks if the compiler conforms to the ISO C standard and prints a corresponding message.
main.c
#include <stdio.h> int main() { #ifdef __STDC__ printf("Standard-compliant compiler detected.\n"); #ifdef __STDC_VERSION__ printf("C standard version: %ld\n", __STDC_VERSION__); #else printf("Likely using ANSI C (C90)\n"); #endif #else printf("Non-standard compiler.\n"); #endif return 0; }
__STDC_VERSION__ is a numeric macro that indicates the version of the C standard, if supported by the compiler.
Swipe to start coding
You're building a utility that logs important events in the code, including location information (file, line, function), and also prints which C standard the program was compiled with. To do this, you'll use predefined preprocessor macros.
- Fill in the blanks in the
LOGmacro so that it prints the message, file name, line number, and function name. - In
main(), check if the compiler is standard-compliant using the appropriate macro. - If the C standard version is available, print it.
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