Usually, on success, a program returns zero to the OS. For example:

 int main () {
	
	...

	if (!valid_input) {
	printf("Operation failed!");
	return 1;
	}

	return 0;	/* success */
 }

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Likewise, many operating systems and libraries have the convention of returning zero from successful system calls.
* MicrosoftWindows: NOERROR == S_OK == 0, errors are positive.
* MacOs, MacOsx: noErr == 0, minor errors are negative, major errors are positive.
* Unix: many calls that simply return success/failure use 0 for success and -1 for failure.  Also no error if errno==0.
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"One of the main causes of the fall of the Roman Empire was that, lacking zero, they had no way to indicate successful termination of their C programs." -- Robert Firth
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CategoryOperatingSystem