What Does Return 0 Do in C?

Return 0 is an iconic C programming statement, especially as the final line closing main(). But why is a zero return value significant? This comprehensive guide dives deep on the meaning and usage of return 0 in C.

Main Function Should Return an Integer Value

The C language treats main() as any other function that needs to return a value. By convention, it is defined to return an integer:

int main() 

Or more commonly:

int main(int argc, char *argv[]) 

This declares main() prototype indicating that an integer return type is expected.

So at the conclusion of program execution, main() is required to return some integer value. This return value is then interpreted by the calling environment (like shell or operating system) to determine whether the program ran successfully or encountered errors.

Return 0 Indicates Successful Execution

By convention, a return value of 0 from main() has come to indicate successful execution with no unexpected errors.

So C programs usually finish main() function with:

int main() {

  // program code

  return 0;
}

The 0 return value informs the shell or operating system that the program completed its intended objective without any issues. This enables seamlessly integrating C programs into UNIX pipelines or using them in scripts.

Evolution of Return 0 Convention

The practice of using return 0 to denote success dates back to early versions of the C language and UNIX operating system in the 1970s. Over time, this convention was standardized across compilers and environments, making return 0 a ubiquitous indicator of correctness in C programs.

Error Indicator Values

While return 0 became established for success, various non-zero return values evolved to indicate possible error conditions:

• 1 – Generic or unknown error
• 2 – Misuse of shell builtins
• 126 – Command invoked failed to execute
• 127 – "Command not found"
• 128+ – Fatal error signal

So return values other than 0 inform that the program terminated due to some exceptional condition. Interpreting the meaning of different non-zero codes is environment dependant.

Benefits of Standardized Return Codes

The convention of return 0 for success brings significant benefits:

• Portability across any Unix-like environment
• Scripting language integration
• Facilitates reuse via pipelines and redirection
• Enables error-handling in automation workflows

Following standardized return codes is key for interoperability.

Void Main() Function

In C, main() can also be defined with a void return type:

void main() {

  // program code 

}

Here, main() does not return any value. So there is no need for return 0 statement in this case.

However void main() is considered non-standard and compilers may issue warnings for this usage. Sticking to int main() is recommended.

Return 0 Usage in User Code

Besides main(), return 0 can similarly be used in user-defined C functions to indicate successful completion:

int validate_input(char *input) {
   // validation code

   return 0; // success
}

By convention, 0 return value denotes logical false in C. So return 0; would signify false from a boolean perspective.

Omitting Return 0 Statement

The return 0 statement is not mandatory in main(). If no return statement is provided, compilers will automatically insert return 0;

So the following is acceptable:

int main() {
  // program code
}

But leaving out return 0 is not considered good coding practice, especially for larger programs. Explicitly adding return 0; makes the developer‘s intent clear.

Return Values in Other Languages

The convention of return 0 indicating success is much more prevalent in system programming languages like C/C++.

In contrast, Python relies on exception handling for signaling and responding to unexpected errors. Main python programs generally do not return integer codes.

JavaScript functions simply use return; to exit out of the function without returning any values.

So the return 0 idiom is quite unique to C-family languages.

C Standard Perspective on Return 0

The return 0 behavior is formally defined in the ISO C standard documents.

ISO/IEC 9899:201x

Section 5.1.2.2.3 outlines the semantics of returning a value from main():

"reaching the } that terminates the main() function returns a value of 0."

So by language specification, main() will implicitly return 0 even if no return statement is present.

C Standards Evolution

Early versions of the K&R C language specification also established return 0 convention for main():

"Program execution begins by executing the main function. The integer returned… is used as the program‘s exit code."

K&R C (Pre-standard)

So return 0 for success is a fundamental part of C language design.

Compiler Handling of Return 0

It is interesting to analyze what C compilers actually do when return 0 is omitted.

Consider this simple main program:

int main() {
   printf("Hello World"); 
}

Compiled using GCC 10.3 with optimizations disabled (-O0), the generated assembly language output shows:

main:
    ...
    mov eax,0  
    ret

Here the compiler automatically inserts mov eax,0 before the ret instruction to populate the return register eax with 0.

So the compiler mandates a return, adhering to the C standard requirement of int return type for main(). Return 0 gets added even if developer does not explicitly write return statement.

Return 0 Optimization

With optimization enabled, the compiler can even skip the extra mov instruction:

main:
   ...
   xor eax, eax
   ret

Here eax register is set to 0 simply using the XOR bitwise operation.

Integration with Linux Ecosystem

Return codes are deeply integrated into the UNIX philosophy that underpins Linux and GCC toolchain.

Pipelines

In pipe model, the return code from preceding command determines behavior of next command.

gcc -o app main.c; ./app | grep "Success"

Here grep only executes if gcc compilation succeeded (return 0).

Makefiles

In Make build automation, subsequent build rules depend on the exit code of previous statements.

So return 0 allows seamlessly incorporating C compilation into Linux infrastructure.

Conclusion

Return 0 has become ubiquitous C coding style, especially to indicate successful termination of main(). By convention a 0 return value informs calling environment that program completed its intended purpose without errors.

Understanding return codes like 0 is key for C developers working in Linux ecosystem. Following standardized return conventions improves reliability and interoperability.