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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Sun, 04 May 2025 14:09:55 -0700
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Lew Pitcher <lew.pitcher@digitalfreehold.ca> writes:
> Back in the days of K&R, Kernighan and Ritchie published an addendum
> to the "C Reference Manual" titled "Recent Changes to C" (November 1978)
> in which they detailed some differences in the C language post "The
> C Programming Language".
>
> The first difference they noted was that
>   "Structures may be assigned, passed as arguments to functions, and
>    returned by functions."
>
> From what I can see of the ISO C standards, the current C language
> has kept these these features. However, I don't see many C projects
> using them.
>
> I have a project in which these capabilities might come in handy; has
> anyone had experience with assigning to structures, passing them as
> arguments to functions, and/or having a function return a structure?
>
> Would code like
>   struct ab {
>     int a;
>     char *b;
>   } result, function(void);
>
>   if ((result = function()).a == 10) puts(result.b);
>
> be understandable, or even legal?

Struct and union assignment (and argument passing, and returning from
functions) are fundamental features of the C language.  You won't
find a C compiler that doesn't fully and correctly support them,
except perhaps ancient compilers from the 1980s or non-conforming
C-like compilers for tiny embedded systems.

There could be potential performance issues passing large structures,
but I wouldn't worry about that unless (a) the structures are
*very* large or (b) you have unusually tight resource constraints.
Someone in this thread suggested avoiding assignment for structs
bigger than twice the size of a pointer, but I'd set the cutoff
substantially higher than that.

For example, in one particular implementation a C++ std::string is
32 bytes and a std::vector is 24 bytes.  C++ programmers routinely
pass these by value and copy them, and it doesn't cause any serious
problems.  There shouldn't be a problem treating similarly sized
C structs by value.  It Just Works.

Of course if you want to pass a structure to a function and have
the function modify it, you need to pass a pointer.  You'd do the
same for a scalar parameter.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */