how to store return values from void in C?

Question

How to store values from void function in main ?

Example

void UsartCallback (uchar* buf, uint len)
{
    return void(*buf);
}

int main()
{
    UsartCallback(&buf,1); // I can get values from void function but how to store this "*buf" value
    // For Example 
    uchar* data=UsartCallback(&buf); //Error: but i want get only "buf" value from void function.
}

*void*, by definition, does *not* return value. You should just call it without assignment: `UsartCallback(&buf)`. The `uchar* data` part is unnecessary — Ian, May 10 '17 at 10:07
What thought process led to the decision to tag a C question [tag:c++] and [tag:c#], pray tell? — Lightness Races in Orbit, May 10 '17 at 10:08
Unclear, and you've changed the language twice, now changed the question a few times in comments too. -1 — Lightness Races in Orbit, May 10 '17 at 10:12
Observing response comments in existing answers, Voting to close as unclear.... — Sourav Ghosh, May 10 '17 at 10:12

Jabberwocky · Answer 1 · 2017-05-10T11:40:38.920

2

You probably want this:

void *UsartCallback (uchar* buf, uint len)
{
  return (void*)buf;
}

int main()
{
  uchar *mybuf = UsartCallback(&buf,1);
}

edited May 10 '17 at 11:40

answered May 10 '17 at 10:10

Jabberwocky

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Oh, right. hah. – Lightness Races in Orbit May 10 '17 at 10:10
ah, that is a void *pointer* – Ian May 10 '17 at 10:12
Maybe, maybe not, we passed pointer to `buf` as argument...... :) – Sourav Ghosh May 10 '17 at 10:12
@user3764118 so your code is different from the code in this answer. Where _exactly_ did you get this error? Maybe you should ask a new question about this specific problem. – Jabberwocky May 10 '17 at 11:42

Sourav Ghosh · Answer 2 · 2017-05-10T10:10:02.003

0

This is invalid code, as it violates the constraint.

Quoting C11, chapter §6.8.6.4

A return statement with an expression shall not appear in a function whose return type is void. [...]

edited May 10 '17 at 10:10

answered May 10 '17 at 10:08

Sourav Ghosh

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can i do this in C++ ? – user3764118 May 10 '17 at 10:09
@user3764118 why? – Sourav Ghosh May 10 '17 at 10:09
@user3764118 Dude are you writing C or is it C++? Would you please _pick one_? – Lightness Races in Orbit May 10 '17 at 10:10
Or is ti C# maybe? – unalignedmemoryaccess May 10 '17 at 10:11
ok, i'm bit confused because i wrote file in C so that way i mention as "C" so is it possible in C++ – user3764118 May 10 '17 at 10:13
@tilz0R why not ruby? python? go? fortan? asm, maybe? :P – Sourav Ghosh May 10 '17 at 10:14
Before you hate, original post had tags for c c++ and c#. @SouravGhosh – unalignedmemoryaccess May 10 '17 at 10:28
@tilz0R OK, i think you got me wrong... 1) i'm the one who edited them out. 2) my question was not "targeted" to you and it's rhetorical...hope it's clear now. :) – Sourav Ghosh May 10 '17 at 10:32

Lightness Races in Orbit · Answer 3 · 2017-05-10T10:12:15.120

You can't. There is no such thing as a "void value". void is the absence of a value. It is true that it looks like you're returning a "thing" here, but you're not.

In C, this is illegal and your program doesn't even compile.
In C++, the code is accepted as an "oddity" of syntax (this is for making templates easier to implement). Your line is effectively the same as return;.

ok is there any way to get "buf" value from void and i want to store that in "main" .. is it possible ?

Okay, completely different question now.

buf is a pointer. So, yes, simply return it. Presumably your callback is required to return void* by some external force?

void* UsartCallback (uchar* buf, uint len)
{
    return buf;
}

I suggest you brush up on pointer syntax; your question (if I now understand it correctly; really hard to tell) betrays a lack of understanding of how to work with pointer types.

ok is there any way to get "buf" value from void and i want to store that in "main" .. is it possible ? — user3764118, May 10 '17 at 10:10

how to store return values from void in C?

3 Answers3