Programs without semicolon

[subnet]

I'm not a C expert, but I've seen this topic discussed in the
newsgroup several times. What's the deal with writing such programs?
Why are they considered so interesting?

Thanks

[Peter Nilsson]

Alan Balmer <al******@att.n et> wrote in message news:<mk******* *************** **********@4ax. com>...

On 12 May 2004 10:41:04 -0700, Pl**********@ly cos.co.uk (PlasmaDragon)
wrote:

It compiles fine, but it returns garbage.

Richard

Returns fine for me. Here's a test program:

#include <stdio.h>
#include <stdlib.h>

int main()
{
int x;
x=system("nosem icolon");
printf("%d",x);
printf("\n\n");

system("PAUSE") ;
}

Where "nosemicolo n" is the name of your program that doesn't have a
semicolon. I always get the output 0.

What you're printing is the return value of system, which is
implementation-defined and may or may not reflect the return value of
nosemicolon.

The behaviour is effectively undefined. Once you call system with a
non-null string, you potentially lose the guarantee of conforming
behaviour (assuming system even returns).

--
Peter

[Dan Pop]

In <Pi************ *************** ********@unix41 .andrew.cmu.edu > "Arthur J. O'Dwyer" <aj*@nospam.and rew.cmu.edu> writes:

In C89/C90/C95, falling off the end of 'main' returns
an implementation-defined value to the system.

Nope, the value is *undefined*.

If the main function executes a return that specifies no value,
the termination status returned to the host environment is undefined.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Dan Pop]

In <63************ **************@ posting.google. com> ai***@acay.com. au (Peter Nilsson) writes:

The behaviour is effectively undefined.

The behaviour is mereley defined by another document.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Devarshi Chatterjee]

"Arthur J. O'Dwyer" <aj*@nospam.and rew.cmu.edu> wrote ...

Test programs prove nothing, by definition. Besides,
you're arguing about a stupid quirk that's easily figured
out by consulting the Standards or a relevant FAQ.

In C89/C90/C95, falling off the end of 'main' returns
an implementation-defined value to the system.
In C99, falling off the end of 'main' is exactly equivalent
to returning 0 via 'return 0;' or 'exit(0);'.

A reference to a section in the standards to substantiate
your claim(s) would be appreciated here.

ISO Section 5.1.2.2.3#1 starts as follows:
"If the return type of the main function is a type compatible
with int,..." [snipped for brevity]

Further on, ISO Section 6.8.6.4#1 says:
"A return statement with an expression shall not appear in a
function whose return type is void. A return statement without
an expression shall only appear in a function whose return
type is void."

CMIIW, but my limited intellect says that this implies that
'main' expects a return statement [or exit(...) for the pedants].
--
Devarshi Chatterjee.

[Jeremy Yallop]

Devarshi Chatterjee wrote:

"Arthur J. O'Dwyer" <aj*@nospam.and rew.cmu.edu> wrote ...

In C99, falling off the end of 'main' is exactly equivalent
to returning 0 via 'return 0;' or 'exit(0);'.

A reference to a section in the standards to substantiate
your claim(s) would be appreciated here.

ISO Section 5.1.2.2.3#1 starts as follows:
"If the return type of the main function is a type compatible
with int,..." [snipped for brevity]

... a return from the initial call to the main function is
equivalent to calling the exit function with the value returned by
the main function as its argument; reaching the } that terminates
the main function returns a value of 0.

[Dan Pop]

In <7d************ **************@ posting.google. com> so*******@blueb ottle.com (Devarshi Chatterjee) writes:

"Arthur J. O'Dwyer" <aj*@nospam.and rew.cmu.edu> wrote ...

Test programs prove nothing, by definition. Besides,
you're arguing about a stupid quirk that's easily figured
out by consulting the Standards or a relevant FAQ.

In C89/C90/C95, falling off the end of 'main' returns
an implementation-defined value to the system.
In C99, falling off the end of 'main' is exactly equivalent
to returning 0 via 'return 0;' or 'exit(0);'.
A reference to a section in the standards to substantiate
your claim(s) would be appreciated here.

ISO Section 5.1.2.2.3#1 starts as follows:
"If the return type of the main function is a type compatible
with int,..." [snipped for brevity]

You should have read the text you've snipped, because this is what it
says:

... a return from the initial call to the main function is
equivalent to calling the exit function with the value returned
by the main function as its argument;10) reaching the } that
^^^^^^^^^^^^^^^ ^^^^
terminates the main function returns a value of 0.
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^
Further on, ISO Section 6.8.6.4#1 says:
"A return statement with an expression shall not appear in a
function whose return type is void. A return statement without
an expression shall only appear in a function whose return
type is void."

CMIIW, but my limited intellect says that this implies that
'main' expects a return statement [or exit(...) for the pedants].

Your intellect is too limited: nothing in 6.8.6.4#1 forbids a function
returning something to terminate without a return statement. And
5.1.2.2.3#1 *explicitly* documents what happens when a main defined as
returning int terminates without a return statement. If you're
curious about what happens in other such cases, the answer is provided
by 6.9.1#12:

12 If the } that terminates a function is reached, and the value
of the function call is used by the caller, the behavior is
undefined.

Therefore, the following is a strictly conforming C99 program, with a
well defined termination status. In C89, the termination status would be
undefined.

#include <stdio.h>

int foo(void) { printf("hello world\n"); }

int main() { foo(); }

I would expect a good compiler to emit a warning for each function
defined in this program, but the standard requires no diagnostic.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Ralmin]

"Dan Pop" <Da*****@cern.c h> wrote:

If you're curious about what happens in other such cases, the answer
is provided by 6.9.1#12:

12 If the } that terminates a function is reached, and the
value of the function call is used by the caller, the
behavior is undefined.

Therefore, the following is a strictly conforming C99 program, with
a well defined termination status. In C89, the termination status
would be undefined.
My copy of C89 has equivalent wording in 3.6.6.4#4:

If a return statement without an expression is executed,
and the value of the function call is used by the caller,
the behavior is undefined. Reaching the } that terminates
a function is equivalent to executing a return statement
without an expression.

Following this logic, since the value of the function call to main is used
by its caller, the behaviour is undefined. I don't think there is such a
thing as an "undefined termination status". In this case, the program has
"undefined behaviour".
#include <stdio.h>

int foo(void) { printf("hello world\n"); }

int main() { foo(); }

I would expect a good compiler to emit a warning for each function
defined in this program, but the standard requires no diagnostic.

Agreed.

--
Simon.

[Dan Pop]

In <dc************ *************** ***@news.terane ws.com> "Ralmin" <ne**@ralminNOS PAM.cc> writes:

"Dan Pop" <Da*****@cern.c h> wrote:

If you're curious about what happens in other such cases, the answer
is provided by 6.9.1#12:

12 If the } that terminates a function is reached, and the
value of the function call is used by the caller, the
behavior is undefined.

Therefore, the following is a strictly conforming C99 program, with
a well defined termination status. In C89, the termination status
would be undefined.

My copy of C89 has equivalent wording in 3.6.6.4#4:

If a return statement without an expression is executed,
and the value of the function call is used by the caller,
the behavior is undefined. Reaching the } that terminates
a function is equivalent to executing a return statement
without an expression.

Following this logic, since the value of the function call to main is used
by its caller, the behaviour is undefined. I don't think there is such a
thing as an "undefined termination status". In this case, the program has
"undefined behaviour".

Why didn't you bother checking your copy of C89? You'd have learned that
there is such thing as "undefined termination status" and, therefore, the
program doesn't invoke undefined behaviour:

"Program termination"

A return from the initial call to the main function is equivalent
to calling the exit function with the value returned by the main
function as its argument. If the main function executes a return that
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^
specifies no value, the termination status returned to the host
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^
environment is undefined.
^^^^^^^^^^^^^^^ ^^^^^^^^^^

According to your own quote, falling off the end of main is the equivalent
to executing a return with no value.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de