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From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: MSC-all C's
Message-ID: <6676@brl-smoke.ARPA>
Date: Sat, 14-Nov-87 15:59:35 EST
Article-I.D.: brl-smok.6676
Posted: Sat Nov 14 15:59:35 1987
Date-Received: Sun, 15-Nov-87 19:07:54 EST
References: <3094@rosevax.Rosemount.COM>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 33

In article <3094@rosevax.Rosemount.COM> richw@rosevax.Rosemount.COM (Rich Wagenknecht) writes:
>  1) What happens in C when a function returns a value which the
>     calling line does not assign to a variable? Where does this
>     value end up? (example: strcpy(str1,str2); ) Could someone
>     explain the process that is gone through when a function is
>     called?

Just as with the value of "x = 1", the value of the expression
is used only if the programmer chooses to use it.  It is misleading
to ask where the value ends up; it is up to the compiler to do the
right thing (which amounts to not having the value "end up" anywhere
that it was not explicitly placed by the programmer).

The process that occurs when a function-call expression is evaluated
at run time is:
	1. The arguments are evaluated (in unspecified order).
	2. The formal parameter variables in the function definition
	receive the corresponding values of the arguments.
	3. The body of the function definition is evaluated until an
	explicit "return" expression is evaluated or the end of the
	body is reached (which constitutes an implicit "return;").
	4. If the function returns a value and its evaluation ended
	with a "return" that had a value as its operand, that value
	replaces the function invocation in the calling context, and
	that higher-level evaluation process resumes using that value.
	(If the value is not explicitly used by the programmer, then
	evaluation simply proceeds to the next expression.)
This is really no different from most programming languages that
support functions, although C is unusual in that all arguments are
passed by value (array names are apparent exceptions; by the funny C
rules for array names in various contexts, they are converted to
pointers to the first members of the arrays when passed as arguments,
which is similar to "call by reference" found in some languages).