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From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: extern char *foo vs. extern char foo[]
Summary: Clarification
Message-ID: <16788@haddock.ima.isc.com>
Date: 4 Jun 90 23:37:19 GMT
References: <1990May30.001219.23564@uunet!unhd> <6263@wolfen.cc.uow.oz>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 28

In article <6263@wolfen.cc.uow.oz> pejn@wolfen.UUCP (Paul Nulsen) writes:
>The confusion over the difference between char * and char [] is very common.
>I believe that it is due to the way that C handles the passing of array
>arguments to functions. As rule C passes arguments by value, but it passes
>an array argument by reference.

Actually, function arguments in C are *always* passed by value, but sometimes
the value in question is a pointer, which can give the same result as a call
by reference.  Thus `void f(int *pi); ... f(&i);' can be considered either a
pointer-to-int passed by value, or an int passed by reference.

In the case of an array, strictly speaking, a true call by reference in this
sense would be `void f(int (*pa)[N]); ... f(&a);', where the user explicitly
passes the address of the array (and the caller expects it).  The more common
usage `void f(int *pi); ... f(a);' is not an array passed by reference.  Also,
this effect is not in any way an exception to the usual rules.  In *any*
rvalue context, an array-valued expression `a' decays into the pointer-valued
expression `&a[0]'.

The wart in the language is not that `arrays are passed by reference and
scalars are passed by value', but rather that, in the one special case of
declaring a formal parameter to a function, the language permits the user to
write a pointer declaration using brackets instead of an asterisk.  This
often misleads people into believing that they've declared an array, when in
fact they've declared a pointer.  This is probably the cause of the original
confusion in this thread.

Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint