2.4. Program Termination: exit and return

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The operating system (OS) is responsible for running an application program by spawning a new processes. Every process utilizes some computer resources that the OS must reclaim when the process finishes. Programs signal their termination to the OS in two ways: the return operator or the exit function. As part of its termination, the program sends or returns a single integer value, called the exit status or exit code, to the OS.

The return Operator

The introduction to the Hello, World! program in the previous chapter stated that "Every C++ program contains exactly one function named main." That main function, and those presented in the examples throughout the text, end with the statement return 0;. This statement doesn't seem to contribute anything to writing a message to the console or any problem solution, but its ubiquity suggests that we must understand it. Trying to describe the return operator's behavior without using the word "return" in the description is a bit awkward, but we might say that it reverts program control from a function back to its starting location. Or, simply, it returns control to the calling point.

The return operator provides programs with two services. First, it ends a function by returning control to where it was called or started. Second, it implements a mechanism that sends a value - the result of the function's operation - to the calling location. We'll learn more about functions in Chapter 6.

int main()
{
	   .
	   .
	   .
	return 0;
}
 
void main()
{
	   .
	   .
	   .
	          
}
(a)(b)
Program termination with return. The operating system starts a program by calling or running function main, and it is to the operating system that the return operator returns control. However, the return operator only terminates the program when called in main. Calling it anywhere else in the program causes a different effect.
  1. The current ANSI C++ standard.
  2. Before the ANSI standard changed in the 1990s, it allowed two ways of terminating a program from main. This version is now deprecated, but many compilers still accept it without causing either a warning or an error.

Exit Status

The value following the return operator, "0" in this example and those appearing throughout the text, is the exit status. A zero exit status indicates that the program is terminating or ending without error; a non-zero exit status indicates that the program is terminating with an error. However, there is no correspondence between the exit value and a particular error - individual programs assign specific exit values to particular errors. (Confusingly, Windows specifies standard exit values for batch programs, but not applications.) The program returns the exit status to the operating system, where shell scripts or batch files can use it.

C++ Program Windows Batch File bash Shell Script 
#include <iostream>
using namespace std;

int main()
{
    int status;
    cout << "Enter an exit status: ";
    cin >> status;

    return status;
}
 
myprogram

if %ERRORLEVEL% NEQ 0 echo Error
 
 
 
 
 
 
 
 
 
#!/bin/bash

./myprogram

if [ $? -ne 0 ]
then
        echo Error
fi
 
 
 
Exit status: batch files and shell scripts. Most general-purpose operating systems provide three user interfaces: graphical, command line, and batch, often implementing the latter via scripting languages. Batch files and scripts are relatively simple programs that can run other programs written in various languages, such as C++. An exit status (aka exit code) doesn't affect the returning program. However, when a script runs a program, the operating system harvests the exit status and passes it to the script. Scripts can use the exit status as a control structure condition.
  1. A simple program demonstrating batch files and shell scripts.
  2. %ERRORLEVEL% is a Windows environment variable saving the exit status of the last terminated program. NEQ is the not equal operator.
  3. The Bourne again shell (bash) variable $? saves the exit status of the last terminated program. -ne is the bash not equal operator.

The exit Function

The return operator only ends a program and returns an exit status when the program runs it from main. Later, we will write larger programs with more than one function, and sometimes, it will be desirable to terminate a program from within one of these other functions. A return statement in a function other than main returns control to the calling function but does not terminate the program. Programmers may call the exit function anywhere in a program, and when called, it terminates the program.

void other_function()
{
	   .
	   .
	   .
	if (condition1)
		exit(2);		// error
	else if (condition2)
		exit(0);		// non-error
	   .
	   .
	   .
}
Terminating a program with the exit function. The exit function requires a single integer argument - the exit status. As illustrated here, programmers typically call the exit function from within an if-statement or switch, but they may call it wherever they need to terminate a program.