Doubt

[Anarki]

int main(a,b,c,d,e, f,g)
{
printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));
return 0;
}

The above program perfectly compiles in cygwin using gcc

and my questions are

1.How does the compiler know what are the types of variables a, b, c,
d, e, f and g?? Where is its declaration?
2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

[Ian Collins]

Re: Doubt

Anarki wrote:

int main(a,b,c,d,e, f,g)
{
printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));
return 0;
}
>
The above program perfectly compiles in cygwin using gcc
>
and my questions are
>
1.How does the compiler know what are the types of variables a, b, c,
d, e, f and g?? Where is its declaration?

It's assuming they are int.

2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

It does.

gcc /tmp/x.c
/tmp/x.c: In function 'main':
/tmp/x.c:3: warning: incompatible implicit declaration of built-in
function 'printf'

--
Ian Collins.

[Richard Heathfield]

Re: Doubt

Anarki said:

int main(a,b,c,d,e, f,g)
{
printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));
return 0;
}
>
The above program perfectly compiles in cygwin using gcc
>
and my questions are
>
1.How does the compiler know what are the types of variables a, b, c,
d, e, f and g?? Where is its declaration?

In C90 (which is what you're using), the code is perfectly legal (albeit in
an obsolete style), and the compiler knows that a, b, etc are ints because
that's the default type where a type is not specified within a
declaration.

In C99, which you are *not* using but which might become popular any
century now, the code is illegal, and a diagnostic message is required.

2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

In C90, no diagnostic message is required when a function is called without
a prototype being in scope (or at least, one might be, but it will be for
a reason other than the mere absence of the prototype) - the compiler will
assume that the args are fine and that the function returns int. (So if,
for example, you don't bother to include <stdlib.hwhen calling malloc,
the compiler will assume it returns int, but you're assigning that value
to a pointer object, and that's why you get a diagnostic message along the
lines of "cannot convert integer to pointer without a cast" - the right
solution is *not* to add a cast, but to add a prototype!)

If the function really does return int and if you get the parameters right,
you can get away with this most of the time. In the case of variadic
functions like printf, however, the behaviour is undefined unless you
provide a prototype.

In C99, you... well, you don't necessarily need a function prototype before
calling a function (although it really really really is a good idea), but
you do at least need a function declaration before the call. Otherwise, a
diagnostic message is required.

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

[Anarki]

Re: Doubt

On Jul 31, 3:44 pm, Ian Collins <ian-n...@hotmail.co mwrote:

Anarki wrote:

int main(a,b,c,d,e, f,g)
{
   printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));
   return 0;
}

>

The above program perfectly compiles in cygwin using gcc

>

and my questions are

>

1.How does the compiler know what are the types of variables a, b, c,
d, e, f and g?? Where is its declaration?

>
It's assuming they are int.
>

2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

>
It does.
>
gcc /tmp/x.c
/tmp/x.c: In function 'main':
/tmp/x.c:3: warning: incompatible implicit declaration of built-in
function 'printf'
>
--
Ian Collins.

y no warning for me! ok if at all it complains how does this program
link with the printf? Does it look in default include files/libs ??

By the way I use Cygwin(windows Xp) is there a way to redirect the
warning/errors to a text file. If so please tell me how

[santosh]

Re: Doubt

Anarki wrote:

int main(a,b,c,d,e, f,g)

You are using the so-called "old style" function definition without a
declaration list specifying the types of the parameters. In such a case
they default to type int.

Further note that unless your implementation specifically defines this
form of main, (i.e., int main(int, int, int, int, int, int, int)), you
are invoking undefined behaviour.

{
printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));

The sizeof operator yields a value of type size_t. The 'd' type
specifier is for int values. Use 'z' if available, or 'lu' and cast to
unsigned long.

return 0;
}
>
The above program perfectly compiles in cygwin using gcc
>
and my questions are
>
1.How does the compiler know what are the types of variables a, b, c,
d, e, f and g?? Where is its declaration?

As above.

2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

If you had invoked your compiler in conforming mode then it would have.
For gcc use the -ansi/-std=c99 and -pedantic flags for C90 or C99
conformance. C99 conformance is incomplete.

[santosh]

Re: Doubt

Anarki wrote:

On Jul 31, 3:44 pm, Ian Collins <ian-n...@hotmail.co mwrote:

>Anarki wrote:

int main(a,b,c,d,e, f,g)
{
printf("Size of a = %d\nSize of b =%d",sizeof(a), sizeof(b));
return 0;
}

>>

The above program perfectly compiles in cygwin using gcc

>>

and my questions are

>>

1.How does the compiler know what are the types of variables a, b,
c, d, e, f and g?? Where is its declaration?

>>
>It's assuming they are int.
>>

2.I haven't included stdio.h yet the printf compiles.The compiler
doesn't complain about the requirement of function prototype, why?

>>
>It does.
>>
>gcc /tmp/x.c
>/tmp/x.c: In function 'main':
>/tmp/x.c:3: warning: incompatible implicit declaration of built-in
>function 'printf'
>>
>--
>Ian Collins.

Please snip signatures unless you are commenting on them.

y no warning for me! ok if at all it complains how does this program
link with the printf? Does it look in default include files/libs ??

A compiler is allowed to do whatever it wants after emitting the
required diagnostics, including making guesses about what you meant and
trying to generate the best possible code.

By the way I use Cygwin(windows Xp) is there a way to redirect the
warning/errors to a text file. If so please tell me how

Use:

gcc -whatever_flags_ you_want file_list_etc 2errors.txt

Now all diagnostic messages will be redirected to a file named
errors.txt.

Using 2>instead of 2will append the output of the command to the
existing contents of errors.txt.

For more help ask in a Unix group like comp.unix.progr ammer or on the
mailing lists for Cygwin.

[Richard Heathfield]

Re: Doubt

santosh said:

Anarki wrote:
>

<snip>

>

>2.I haven't included stdio.h yet the printf compiles.The compiler
>doesn't complain about the requirement of function prototype, why?

>
If you had invoked your compiler in conforming mode then it would have.

Chapter and verse for C90, please. (I accept that your claim is correct for
C99.)

<snip>

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

[santosh]

Re: Doubt

Richard Heathfield wrote:

santosh said:
>

>Anarki wrote:
>>

<snip>

>>

>>2.I haven't included stdio.h yet the printf compiles.The compiler
>>doesn't complain about the requirement of function prototype, why?

>>
>If you had invoked your compiler in conforming mode then it would
>have.

>
Chapter and verse for C90, please. (I accept that your claim is
correct for C99.)
>
<snip>

Um, does 0.0 p0 qualify? :-)

You're right. The rules of C90 do not require a diagnostic. This is what
comes of relying more on gcc than on the Standard document itself.