C : Call by value or reference

[Army1987]

Re: C : Call by value or reference

On Sun, 15 Jul 2007 17:56:12 +0000, CryptiqueGuy wrote:

On Sun, 15 Jul 2007 13:22:52 +0000, Richard Heathfield wrote:

>>C is a 100% call-by-value language. Function arguments are
>>expressions ,
>>not objects. These expressions are evaluated, and those values are
>>passed to the function.
This statement is not wrong but highly misleading. Esp. it doesn't
cover the distinction between the following:

>>

void foo1 (stuct something sth);
void foo2 (stuct something* ptr);

^^^^^
What happened to 'r'? :-)

>It's all "words". I suspect the OP was looking for pointers - which
>also makes me wonder if he is a troll. Since how can he implement a
>garbage collector and not either (a) know the answer or (b) know how to
>google up the answer.

>
Both your declarations follow pass by value. What is there in it?

Absolutely nothing.
But if foo2 is called as foo2(&someth) and its definition dereferences
ptr, foo2 is able to access a variable local to its caller, which foo1
will never be able to do (unless the struct contains pointers and foo1
dereferences them.

--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)

[Keith Thompson]

Re: C : Call by value or reference

CryptiqueGuy <SRRajesh1989@g mail.comwrites:
[...]

Many poorly written C books call "simulation of pass by reference
using pointers" as pass by reference which confuses a lot of Newbies.
I feel that Call by reference is said to be supported by a language IF
it syntactically and semantically supports that facility. C doesn't
have any syntactical and semantical facility for Call by Reference.
People find it difficult to understand that simple concept and call
"simulation of pass by reference using pointers" as REAL pass by
reference!

Right. C supports pass-by-reference in the same way that it supports
linked lists and many other things -- not as a built-in language
construct, but as something you can construct by programming.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Richard Heathfield]

Re: C : Call by value or reference

Roland Pibinger said:

On Sun, 15 Jul 2007 13:22:52 +0000, Richard Heathfield wrote:

>>C is a 100% call-by-value language. Function arguments are
>>expressions , not objects. These expressions are evaluated, and those
>>values are passed to the function.

>
This statement is not wrong but highly misleading.

Yes it's not wrong, but no it's not misleading either.

Esp. it doesn't
cover the distinction between the following:
>
void foo1 (stuct something sth);
void foo2 (stuct something* ptr);

But he didn't /ask/ about syntax errors.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Jens Thoms Toerring]

Re: C : Call by value or reference

Richard <rgrdev@gmail.c omwrote:

santosh <santosh.k83@gm ail.comwrites:

Nehil wrote:

Does C follow call by value convention or call by reference?
i see that there is nothing like reference in C standard but it is
referenced.
>
still, what should be the answer for the above question?

C only supports call by value. However you can simulate call by
reference using pointers.

and arrays...

http://mces.blogspot.com/2005/08/pas...ence-in-c.html

No, For arrays there's a special rule. Arrays can't be passed
as a whole by value (this would require a copy of the whole
array each time the function is called and thus would be rather
slow) so when an array is found in a context where a value is
expected (e.g. when an array is used as a function argument
but also at the right hand side of an assignment). Instead when
found in such a context ("value context") it is converted auto-
matically to a pointer to the first element of the array, and
this value (i.e. the pointer to the first element of the array)
is what gets passed to the function.

This isn't really passing the array by reference since within
the function you have no information anymore about the 'array-
ness' of the pointer the function received, e.g. you don't
have any information about the size of the array (unless
passed to the function as an additional argument) and can't
distinguish it from a pointer that points to something else
than an array.

The article you cite is a rather useless example for what this
special rule can be (mis-)used - and the accompanying text is
simply false, there is no "automatic memory allocation" (what-
ever that is supposed to be) happening at all, there's just an
one-element array of structures, which is an automatic variable.

Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\______________ ____________ http://toerring.de

[Keith Thompson]

Re: C : Call by value or reference

jt@toerring.de (Jens Thoms Toerring) writes:
[...]

No, For arrays there's a special rule. Arrays can't be passed
as a whole by value (this would require a copy of the whole
array each time the function is called and thus would be rather
slow) so when an array is found in a context where a value is
expected (e.g. when an array is used as a function argument
but also at the right hand side of an assignment). Instead when
found in such a context ("value context") it is converted auto-
matically to a pointer to the first element of the array, and
this value (i.e. the pointer to the first element of the array)
is what gets passed to the function.

[...]

Just a minor quibble: the idea that arrays are not passed as arguments
because they're too big is a shaky rationale. Structures (and unions)
can be passed as arguments (by value, of course), and structures can
be arbitrarily large. They can even contain arrays.

Arrays are "second-class" types because the language defines them that
way; it's a consequence of the way most operations on arrays are
defined in terms of pointer operations. (But arrays are not pointers,
and pointers are not arrays; see section 6 of the comp.lang.c FAQ.)

In the dim past, before the first C standard, structures were also
second-class types; you couldn't assign structure values, and you
couldn't pass them as function arguments. Operations on structures
were done with pointers. (This explains the odd way some of the
functions in the C standard library are declared; they were first
implemented before the rules changed.) It was found that the language
could be changed to allow structure assignment and argument passing
without breaking anything. The same is not true for arrays. Any
change allowing arrays to be treated as first-class types would break
existing code, so we're stuck with the historical baggage.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Re: C : Call by value or reference

Keith Thompson wrote:

[...] It was found that the language
could be changed to allow structure assignment and argument passing
without breaking anything. The same is not true for arrays. Any
change allowing arrays to be treated as first-class types would break
existing code, so we're stuck with the historical baggage.

Ritchie's "The Development of the C Language" (GIYF)
gives a pretty clear explanation of his motivation to
invent the "baggage."

(As for myself, I think highly of baggage. When the
airline loses it, I am not pleased.)

--
Eric Sosman
esosman@ieee-dot-org.invalid

[Chris Dollin]

Re: C : Call by value or reference

Army1987 wrote:

On Sun, 15 Jul 2007 17:56:12 +0000, CryptiqueGuy wrote:

On Sun, 15 Jul 2007 13:22:52 +0000, Richard Heathfield wrote:
>>>C is a 100% call-by-value language. Function arguments are
>>>expression s,
>>>not objects. These expressions are evaluated, and those values are
>>>passed to the function.
>This statement is not wrong but highly misleading. Esp. it doesn't
>cover the distinction between the following:
>>>
>void foo1 (stuct something sth);
>void foo2 (stuct something* ptr);

^^^^^
What happened to 'r'? :-)

It's stuct in his keyboad.

--
Chis "puns ' us" Dollin

Hewlett-Packard Limited Cain Road, Bracknell, registered no:
registered office: Berks RG12 1HN 690597 England

[Chris Dollin]

Re: C : Call by value or reference

Richard wrote:

Chris Dollin <eh@electriched gehog.netwrites :
>

>Roland Pibinger wrote:
>>

>>On Sun, 15 Jul 2007 13:22:52 +0000, Richard Heathfield wrote:
>>>>C is a 100% call-by-value language. Function arguments are expressions,
>>>>not objects. These expressions are evaluated, and those values are
>>>>passed to the function.
>>>
>>This statement is not wrong but highly misleading. Esp. it doesn't
>>cover the distinction between the following:
>>>
>>void foo1 (stuct something sth);
>>void foo2 (stuct something* ptr);

>>
>Two functions, distinguished by argument types only. So what?
>Structs are passed by value, and pointers are passed by value.
>What distinction are you referring to?

>
Because 99% of the time that C nOObs ask this question they are
enquiring how to pass a pointer to a large object as opposed to the
entire variable.

Oh, I /see/. Hmm. I'll try and bear that in mind the next time I
respond to that question -- the usual "what are you actually trying
to do?" should get mixed in.

--
Chris "impressed by the precision of '99%'" Dollin

Hewlett-Packard Limited Cain Road, Bracknell, registered no:
registered office: Berks RG12 1HN 690597 England