Predefined Identifier __func__ in C

The __func__ is a predefined identifier in C that provides the name of the current function. It was introduced in C99 standard and is automatically available in every function without any declaration.

Syntax

__func__

The __func__ identifier is implicitly declared as if the following declaration appears at the beginning of each function −

static const char __func__[] = "function-name";

Example 1: Basic Usage

Here's a simple example showing how __func__ returns the current function name −

#include <stdio.h>

void function1(void) {
    printf("Current function: %s<br>", __func__);
}

void function2(void) {
    printf("Current function: %s<br>", __func__);
    function1();
}

int main() {
    printf("Current function: %s<br>", __func__);
    function2();
    return 0;
}

Current function: main
Current function: function2
Current function: function1

Example 2: Debugging with __func__

The __func__ identifier is particularly useful for debugging and logging purposes −

#include <stdio.h>

void calculateSum(int a, int b) {
    printf("Entering function: %s<br>", __func__);
    int sum = a + b;
    printf("Sum calculated in %s: %d<br>", __func__, sum);
}

int main() {
    printf("Starting program in: %s<br>", __func__);
    calculateSum(10, 20);
    printf("Program ending in: %s<br>", __func__);
    return 0;
}

Starting program in: main
Entering function: calculateSum
Sum calculated in calculateSum: 30
Program ending in: main

Example 3: Combined with Other Predefined Identifiers

C provides other similar predefined identifiers that work well with __func__ −

#include <stdio.h>

void displayInfo() {
    printf("Function: %s<br>", __func__);
    printf("File: %s<br>", __FILE__);
    printf("Line: %d<br>", __LINE__);
}

int main() {
    displayInfo();
    return 0;
}

Function: displayInfo
File: main.c
Line: 6

Key Points

• __func__ is automatically available in every function without declaration
• It returns the name of the lexically enclosing function as a string
• Cannot be redefined or assigned to another value
• Available since C99 standard
• Useful for debugging, logging, and error reporting

Conclusion

The __func__ predefined identifier is a valuable tool for debugging and function identification in C programs. It automatically provides the current function name as a string constant, making it ideal for logging and error tracking purposes.