NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/1943

Internet Message Format | 1996-08-05 | 2.9 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: What should be returned?
Date: Wed, 17 Jan 96 22:33:53 GMT
Organization: none
Message-ID: <[e-mail]>
References: <4dj8pv$cjd@eng_ser1.erg.cuhk.hk> <Crawford.821899744@mariner>
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In article <Crawford.821899744@mariner> [e-mail] "CRAWFORD" writes:

>[e-mail] (the CAReLess boy) writes:
>>Hi, all,
>>       It's said that the function main must return an integer value
>>but I just don't know what value should be returned.

The C language defines 3 values which may be passed portably to exit()
and hence returned from an initial invocation of main:

0 - indicates a successful return

EXIT_SUCCESS - indicates a successful return

EXIT_FAILURE - indicates an unsuccessful return

EXIT_SUCCESS and EXIT_FAILURE are defined (along with exit() in stdlib.h.

While 0 and EXIT_SUCCESS mean the same thing, EXIT_SUCCESS doesn't
have to be defined as 0 - there could be more than one value indicating
success. Your implementation can define (or allow you to use) other values
but these values can be used across all ANSI conforming implementations.

>>  Also, if I quit
>>main by exit(), what's the use of the return value?
>>       Any help?

No use at all. If code can never reach the end of a function in C there
is no requirement for the function to end in a return statement. However a
C function must always be defined with an equivalent return type to that
which the caller expects (for example so that the correct call sequence for
that function is generated). A C runtime system is entitled to expect to
call a main function that returns int so, to be portable, you must provide
it with one. Not returning from a function gives you no license to change the
return type of the function, unless you do so in both the caller and callee.
At program startup you have no control over the caller.

>        exit() also causes a value to be returned from main, the POSIX
>prottype for exit() is:
>
>void exit(int status);

POSIX simply adopted the ANSI C prototype for exit().

>        In the UNIX world (and, I believe, DOS), the standard is to
>return zero on success and non-zero on a failure. That's what the
>macros evaluate to, but I'd use the macros anyway, since they're easy
>to understand.

Zero is guaranteed to be transformed by the implementation in whatever way
necessary to generate a successful termination condition. There are
no guarantees for non-zero returns e.g. in VMS 1 is a success indication.
EXIT_FAILURE is the only portable way of generating a failure indication.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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