Re: fork() Problem?

Helge Hafting (helge.hafting@c2i.net)
Thu, 06 May 1999 00:20:35 +0200

> On Wed, 5 May 1999, Steve Dodd wrote:
>
> > On Wed, May 05, 1999 at 09:52:23AM -0400, Richard B. Johnson wrote:
> >
> > > > if (pid = fork())
> > > ^^^^________ logical test of an assignment? This will always
> > > be true!
> >
> > Not if the code is written in C, which it seems to be.
> >
> > It'll be true if the result of fork() is true, or false otherwise. pid will
> > be assigned the result of fork() in either case. As fork() returns 0 (false)
> > in the child, and the PID of the child in the parent (true), what's the
> > problem?[1]
> >
> > S.
> >
> > [1] Assuming fork() doesn't fail of course, which is reasonable in test code.
> >
>
> The problem is that if anybody writes code like this, they should be
> terminated with extreme prejudice. This is using a gcc compiler bug as
> a feature.
Nope. The C language is defined like that, it is *not* a
gcc oddity! This works with any C compiler.
You may dislike it but you can't change the standard.

> #include <stdio.h>
>
>
> int foo()
> {
> return 0;
> }
>
> int main()
> {
> int i;
> if(i=foo())
> (void)puts("If this is not seen, it is a compiler BUG!");
>
> return 0;
> }
Wrong! if (i=foo()) {} is equivalent to i=foo(); if (i) {}
"=" is just an assignment, and returns the value assigned. There is no
similiarity to the comparison operator "==" other than the fact
that both are written using equal signs.
That's how C works - live with it!

>
>
> LCLint 2.2a --- 04 Sep 96
>
> xxx.c: (in function main)
> xxx.c:13,5: Test expression for if is assignment expression: i = foo()
> The condition test is an assignment expression. Probably, you mean to use ==
> instead of =. If an assignment is intended, add an extra parentheses nesting
> (e.g., if ((a = b)) ...) to suppress this message. (-predassign will suppress
> message)
> xxx.c:13,5: Test expression for if not bool, type int: i = foo()
> Test expression type is not boolean or int. (-predboolint will suppress
> message)
>
> Finished LCLint checking --- 2 code errors found
>
>
> gcc 'fixes' obvious coding errors like this. If a compiler fixes
> these errors, it should fix the source while it's at it so I don't
> have to see them again.

No, gcc doesn't "fix" anything. You get a warning because writing "="
in a if-test is a common typo among C beginners and people who
program other languages such as pascal. (Pascal use "=" for comparison)

It is just a warning that something might be wrong. I always turn it
off as it annoys me. A good C programmer shouldn't have trouble
with it and it is perfectly readable for those fluent in C.

Some people thinks if (a=b) is bad practice, some like it. It
is definitely not a compiler bug though.

Helge Hafting

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