return value of functions

[yohji]

Hi,everyone!
I am a beginner of C.When I studied C,I found a problem which isn't
described in many C teaching books.It is like this: I found some of int
or char* functions can be used as void function.For example: we can use
int function printf() like this :
printf("hello,w orld!\n");
How it is like a void function!
Also we can use char* function ltoa()like this:
ltoa(ltemp,buff er,10);
Why??

[Minti]

yohji wrote:

Hi,everyone!
I am a beginner of C.When I studied C,I found a problem which isn't
described in many C teaching books.It is like this: I found some of int or char* functions can be used as void function.For example: we can use int function printf() like this :
printf("hello,w orld!\n");
How it is like a void function!
Also we can use char* function ltoa()like this:
ltoa(ltemp,buff er,10);
Why??

Because, you want to. There are languages in which it is _prohibited_
to discard the return value of a function. C isn't one of them. Also a
lot of programmers, do tend to write the above "functions" as

(void) printf("Hello World!\n");
(void) scanf("%d", &value);

To indicate that _they_ know that they are discarding the value.
BTW, What is the "function" thing doing above?

--
Imanpreet Singh Arora

[Emmanuel Delahaye]

yohji wrote on 09/04/05 :

Hi,everyone!
I am a beginner of C.When I studied C,I found a problem which isn't
described in many C teaching books.It is like this: I found some of int
or char* functions can be used as void function.
Let's rephrase is more accurately...

C functions car return somethinf or no. When a function returns
nothing, its return type is 'void'. On the other cases, there is a
return type ('int', 'char *', or whatever...).

But when you use a non-void function, you don't /have to/ use its
returned value (but in many cases, you should, because the value has
probably some important meaning).

On the contrary, getting a value from a function returning nothing
invokes an undefined behaviour (IOW, a bug)
For example: we can use
int function printf() like this :
printf("hello,w orld!\n");
How it is like a void function!
Also we can use char* function ltoa()like this:
ltoa(ltemp,buff er,10);
Why??

It depends on your paranoid level...

Note that ltoa() is not a standard C function.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"Clearly your code does not meet the original spec."
"You are sentenced to 30 lashes with a wet noodle."
-- Jerry Coffin in a.l.c.c++

[Martin Ambuhl]

yohji wrote:

Hi,everyone!
I am a beginner of C.When I studied C,I found a problem which isn't
described in many C teaching books.It is like this: I found some of int
or char* functions can be used as void function.For example: we can use
int function printf() like this :
printf("hello,w orld!\n");
How it is like a void function!
You can always discard the value of an expression. That is all that is
being done here. You are calling printf for its side-effect, not for
its value. Similarly, the expression 'x++' has both a value and a
side-effect and can be used in the statement
x++;
discarding the value, caring only about the side-effect (incrementing x).
Note that 'a = b' is also an expression yielding a value, but you would
not consider it particularly strange to see
a = b;
discarding the value of the expression and using only the side-effect of
assigning the value of b to a.
Also we can use char* function ltoa()like this:
ltoa(ltemp,buff er,10);

There is no standard function named 'itoa' and any syntax or semantics
are implementation-defined. If a function returns a char *, that could
be any of
a) a pointer to (or into) a buffer specified by the argument list,
b) a pointer to a static buffer
c) a pointer to a dynamically allocated buffer.
d) an error indication (e.g. NULL)
If case (c) holds, then discarding the return value will lead to a
memory leak, and you will feel very silly.

[Keith Thompson]

"Emmanuel Delahaye" <em***@YOURBRAn oos.fr> writes:
[...]

On the contrary, getting a value from a function returning nothing
invokes an undefined behaviour (IOW, a bug)

Hmm. I can't think of a straightforward way to (attempt to) get a
value from a void function. For example, the following won't compile,
so it has no opportunity to invoke UB.

void foo(void) {
}

int main(void)
{
int result = foo();
return result;
}

You could do it by casting a function pointer (which would invoke UB);
was that what you had in mind?

--
Keith Thompson (The_Other_Keit h) ks***@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.

[Emmanuel Delahaye]

Keith Thompson wrote on 09/04/05 :

"Emmanuel Delahaye" <em***@YOURBRAn oos.fr> writes:
[...]

On the contrary, getting a value from a function returning nothing
invokes an undefined behaviour (IOW, a bug)

Hmm. I can't think of a straightforward way to (attempt to) get a
value from a void function. For example, the following won't compile,
so it has no opportunity to invoke UB.

void foo(void) {
}

int main(void)
{
int result = foo();
return result;
}

You could do it by casting a function pointer (which would invoke UB);
was that what you had in mind?

No. I did think it was not a constraint violation and that it may
compile with some warning.

gcc:
main.c: In function `main':
main.c:16: void value not ignored as it ought to be

Borland C 3.1:
Compiling ..\MAIN.C:
Error ..\MAIN.C 16: Value of type void is not allowed

Both are rejected. Fine.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"Clearly your code does not meet the original spec."
"You are sentenced to 30 lashes with a wet noodle."
-- Jerry Coffin in a.l.c.c++

[yohji]

Er,I see._int_ functions in C are special,aren't they?

[yohji]

Er,I see._int_ functions in C are special,aren't they?

[Keith Thompson]

"yohji" <cc***@126.co m> writes:

Er,I see._int_ functions in C are special,aren't they?

Not particularly. What do you mean?

--
Keith Thompson (The_Other_Keit h) ks***@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.

[Richard Bos]

"yohji" <cc***@126.co m> wrote:

Er,I see.
I don't. Post with context, dammit! Google's inadequacy is not an excuse
for you to emulate them.
_int_ functions in C are special,aren't they?

Well... yesno. If a function is declared without a return type, or used
without being declared, then the return type is assumed to be int. This
makes functions that _do_ return int somewhat special, since they're
automatically recognised correctly; but this is the result of this,
historically motivated, rule, not because of some magic in int functions
themselves.
This is all true for C89, but not for C99, BTW.

Richard