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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Regarding assignment to struct
Date: Fri, 02 May 2025 13:17:39 -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?

C has had structure assignment since before I first learned the
language.  I may not have been aware of it initially, since it's not
mentioned in K&R1, but it's a fundamental feature of the language,
and there are almost certainly no current implementations that
don't fully support it.

I probably don't use structure (or union) assignment very often,
but I wouldn't hesitate to use it if called for, particularly for
small structures that aren't much bigger than a scalar object.

> 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?

I find that particular code is a bit odd.  Defining the type "struct
ab", an object of that type, and a function returning it all in one
declaration is far too terse for my taste, and it's not clear where
it would make sense to *define* the function.  I don't have any
trouble understanding the code, but not because of struct assignment.

Returning structs from functions does raise an issue that your code
doesn't illustrate.  If a struct has a member of array type, and
you apply the indexing operator to the array member of a function
result, you're accessing an array *object* that doesn't really exist
(prior to C11).

For example:

#include <stdio.h>

struct foo { int arr[10]; };

struct foo func(void) {
    struct foo result = { 0 };
    return result;
}

int main(void) {
    printf("%d\n", func().arr[0]);
}

The semantics of the indexing operator require it to refer to an array
*object*, but as of C99 func().arr is an array *value*, not an object
(the expression is not an lvalue).  C11 added the concept of *temporary
lifetime* to deal with this.  It's one of the few cases where you can
have a non-lvalue expression of array type.

See N1570 6.2.4p8.

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