If you are supposed to return 0 from main, and EXIT_SUCCESS is "implementation defined", and it literally says that it is not required to be 0, then what is the point of it if you can't rely on it being the correct value? Am I missing something?
CodePudding user response：
Returning the integer n from main (if main has an int return value) is equivalent to calling exit(n) -- 5.1.2.2.3.1 in the standard. exit is defined such that both exit(0) and exit(EXIT_SUCCESS) are both implementation-defined forms of the status successful termination -- 7.20.4.3.5 in the standard.
So returning 0 (or having no return and dropping off the end of main) cause a "successful" termination of a program.
I don't think the standard guarantees that return EXIT_SUCCESS and return 0 are exactly equivalent as returns from main, but it's definitely ok to return 0 from main and expect your OS to treat this as a successful termination of your program, no matter what EXIT_SUCCESS is defined as.
Note that websites like cppreference.com are not the best source of information about the C standard, even if they're mostly accurate -- it's much better to go to the actual standard (or at least one of the freely available committee drafts if the official standard is paywalled) and read what it says.
CodePudding user response：
I believe this is a piece of retro-fitted standardisation.
At point of Standardisation (and EXIT_SUCCESS was defined then) it makes sense to permit the common practice (return 0;) but also to have constants and unnecessary to define that the constant is necessarily the common value. Though it usually is.
Having constants is well-aligned to a specific aim of Standardisation which was to make it possible to write portable C programs more easily.
Both 0 and EXIT_SUCCESS indicate successful program execution to the host environment. Though EXIT_SUCCESS is not required to be zero.
So it's wrong to say you can't rely in it being the correct value because it is guaranteed to be one of the correct values for indicating successful execution.
One of the one or more correct values at least one being 0.
I'm not aware of any platform where it isn't 0.
Remember C had diversified onto a number of platforms when it was finally Standardised and by then almost universal practice was that the return value of 0 meant success following the convention that return codes are error-codes and 0 being logical 'false' means 'no error'.
However given the aim of Standardisation was to increase portability it makes sense to provide for implementation defined constants in a critical step of interaction with the host-environment - termination.
I don't know if the constants were introduced during standardisation or included from a contributing implementation.
Remember it's not defined what the Host Environment does with the return value. Yes, common practice is that it's value is returned to the script or process that initiated program execution. But it could be mapped. On a hosting platform that didn't use 0 for success that return value would have to be mapped to different value, quite possibly whatever EXIT_SUCCESS expands to.
It's a bit esoteric because I'm not aware of a platform where EXIT_SUCCESS isn't 0 but the committee was right that you can be have a well defined language by specifying behaviour not concrete value here.
CodePudding user response：
You are supposed to return 0 on UNIXy systems, sure.
Consider targeting an esoteric OS where successful return code is, say, 42, and errors are 222.
That OS's headers will define those constants to 42 and 222, and your program, correctly using EXIT_SUCCESS, won't return zero, a completely weird value to the esoteric OS.
As an additional data point, e.g. FreeBSD defines a variety of other exit codes in sysexits.h.
CodePudding user response：
If you are supposed to return 0 from main, and EXIT_SUCCESS is "implementation defined", and it literally says that it is not required to be 0, then what is the point of it if you can't rely on it being the correct value? Am I missing something?
You are missing the fact that more than one value can indicate a successful exit. A C implementation could define any non-negative value to indicate success, and it could define EXIT_SUCCESS to be 1.