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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.c
Subject: Re: That depends... (Was: Regarding assignment to struct)
Date: Sat, 3 May 2025 04:31:14 -0000 (UTC)
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Message-ID: <20250502211854.795@kylheku.com>
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On 2025-05-03, Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
> If it is legal, then why isn't it used more often? Is it a readability/
> maintainability issue, or is it something else?

It's not used more often because

- programmers are irrationallyafraid that there will be lots of
  overhead when the structure gets large. 

  (In fact, passing large structs by value is implemented
  using a hidden pointer; a copy is not made unless the calee
  modifies the parameter. Returning likewise: caller passes
  a pointer to a location where to place the structure.
  It is not as bad as you might think.)

  Some of the fear is rational: you usually have a very good
  idea how pointer passing is implemented, but not necessarily how
  struct passing is.

- some structures cannot be copied; duplicating objects can
  be nontrivial when they manage resources or are sensitive
  to their own address (like have pointers to parts of themselves).
  E.g.  you can't just have a FILE parameter and pass (*stdout) to it.
  Under object-based/oriented programming in C, we usually pass
  around pointers to objects. Treating OOP objects by value requires
  techniques found in C++: you need handlers to be invoked when
  objects are copied ("copy constructors").

- ABI concerns might prevent you from writing a public API that has
  functions that take and/or return structs by value for fear of
  causing complications to FFI users, or incompatibilities when
  differnet compiler are used.

  (We have such APIs in ISO C, though: ldiv and friends.)

  (In POSIX, I seem to recall, some platforms have made pthread_t a
  struct. Glibc on Linux might have had it that way? I think it's an
  unsigned int or long now.  Anyway, in those implementations,
  pthread_equal takes two structs by value. And, aha, that's why there
  is pthread_equal; because pthread_t types might not be scalar values
  comparable with ==: i.e. pointers or integers.)

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
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