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Built-in Macros Every C Programmer Should Know

These macros are used like regular identifiers β€” 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.

c

main

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#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.

c

main

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#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 β€” useful for debugging and call tracing.

c

main

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#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 β€” helpful for versioning or debugging builds.

c

main

copy

              
123456

            
#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.

c

main

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123456789101112131415

            
#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.

Task

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 LOG macro 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.

Solution

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Thanks for your feedback!

SectionΒ 4. ChapterΒ 5

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book
Built-in Macros Every C Programmer Should Know

These macros are used like regular identifiers β€” 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.

c

main

copy

              
12345678910

            
#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.

c

main

copy

              
12345678910111213

            
#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 β€” useful for debugging and call tracing.

c

main

copy

              
123456789101112131415

            
#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 β€” helpful for versioning or debugging builds.

c

main

copy

              
123456

            
#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.

c

main

copy

              
123456789101112131415

            
#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.

Task

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 LOG macro 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.

Solution

Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
Everything was clear?

How can we improve it?

Thanks for your feedback!

SectionΒ 4. ChapterΒ 5
Switch to desktopSwitch to desktop for real-world practiceContinue from where you are using one of the options below
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