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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


              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.

main.c


              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 — useful for debugging and call tracing.

main.c


              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 — helpful for versioning or debugging builds.

main.c


              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.

main.c


              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.

Opgave

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.

Løsning

Skift til skrivebord for at øve i den virkelige verdenFortsæt der, hvor du er, med en af nedenstående muligheder

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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


              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.

main.c


              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 — useful for debugging and call tracing.

main.c


              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 — helpful for versioning or debugging builds.

main.c


              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.

main.c


              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.

Opgave

Swipe to start coding

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.

Løsning

Skift til skrivebord for at øve i den virkelige verdenFortsæt der, hvor du er, med en af nedenstående muligheder

Var alt klart?

Tak for dine kommentarer!

Stryg for at vise menuen

Built-in Macros Every C Programmer Should Know

How to use it?

Logging errors with context

Function tracing

Embedding build metadata

Checking compiler standard compliance

Løsning

Awesome!

Built-in Macros Every C Programmer Should Know

How to use it?

Logging errors with context

Function tracing

Embedding build metadata

Checking compiler standard compliance

Løsning

Awesome!