_Noreturn function specifier in C

The _Noreturn function specifier in C indicates to the compiler that a function will never return control to its caller. Functions marked with _Noreturn either terminate the program (using exit()) or run indefinitely (infinite loops). This helps the compiler optimize code and detect unreachable statements.

Syntax

_Noreturn return_type function_name(parameters)
{
    /* Function body that never returns */
}

Example 1: _Noreturn with exit() Function

When a function marked with _Noreturn calls exit(), it terminates the program without returning control to the caller −

#include <stdio.h>
#include <stdlib.h>

_Noreturn void terminate_program() {
    printf("Function called - terminating program<br>");
    exit(0); 
}

int main() {
    printf("Program started<br>");
    terminate_program(); 
    printf("This line will never execute<br>");
    return 0;
}

Program started
Function called - terminating program

Example 2: _Noreturn with Infinite Loop

A _Noreturn function can also contain an infinite loop that never allows the function to return −

#include <stdio.h>

_Noreturn void infinite_loop() {
    printf("Entering infinite loop<br>");
    int count = 1;
    while (1) {
        printf("Loop iteration: %d<br>", count++);
        if (count > 5) break; /* For demo purposes only */
    }
}

int main() {
    printf("Program started<br>");
    infinite_loop();
    printf("This line will never execute<br>");
    return 0;
}

Program started
Entering infinite loop
Loop iteration: 1
Loop iteration: 2
Loop iteration: 3
Loop iteration: 4
Loop iteration: 5

Example 3: _Noreturn with return Statement (Warning)

Using a return statement in a _Noreturn function violates its contract and generates compiler warnings −

#include <stdio.h>
#include <stdlib.h>

_Noreturn void invalid_function() {
    printf("This function should not return<br>");
    return; /* This causes undefined behavior */
}

int main() {
    printf("Calling invalid _Noreturn function<br>");
    invalid_function();
    printf("Unexpectedly returned<br>");
    return 0;
}

warning: function declared 'noreturn' has a 'return' statement
Calling invalid _Noreturn function
This function should not return
Unexpectedly returned

Key Points

• _Noreturn is a function specifier introduced in C11 standard
• It helps compilers optimize code by marking unreachable statements
• Functions marked _Noreturn must never return under any circumstances
• Common uses include error handlers, program terminators, and infinite event loops

Conclusion

The _Noreturn function specifier is a valuable tool for indicating functions that never return control to their callers. It improves code clarity and enables compiler optimizations while helping detect unreachable code.