Re: libc/printf bug

"bill pursell" <bill.pursell@xxxxxxxxx> writes:

The following code exhibits unexpected behavior. Either it's
a bug in libc, or the author (me) is blind as a bat. Am I missing
something here, or is something horribly wrong? It appears
that printf is mangling the address unless the second argument
gets cast. I don't see that the cast should have any effect at all
on the output. Any thoughts?

[tmp]$ cat printf_bug.c


int
main()
{

float a=0;

printf("%p\n", &a);

The above is mostly fine, although to be portable outside of Linux,
you might wish to cast &a to (void*).

printf("%x %p\n", a, &a);

This is flat wrong.

The "%x" conversion specifer tells printf() that you've given it an
unsigned int. You've actually given it a double (the float automatically
gets converted to a double when you use it in a variadic function*
like printf(). So, you've accidentally(?) just lied to printf() about
what you're going to pass to it. All kinds of funky stuff could result.

* variadic functions are functions that take a variable number of
arguments.

To use the %x specifier, you should cast to (unsigned int).

printf("%x %p\n", (int)a, &a);

This is better, and pretty likely to work on Linux. For portability
and just all-around correctness, you should cast to (unsigned int), as
I explained earlier.

In general when using printf() and the like, if you're not passing it
a variable that is /exactly/ the type that printf() expects, be sure
and cast it. The exceptions would be chars and shorts, which get
promoted to ints before being passed to printf(); and floats, which
get promoted to doubles.

HTH,
Micah
.

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