Re: libc/printf bug

"Nils O. Selåsdal" <NOS@xxxxxxx> writes:

bill pursell wrote:
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?
There are quite a few errors in this code.
libc is not at fault.

1. Forgot to include <stdio.h>, this is important so
you get a proper prototye for printf, else the compiler
assumes all arguments are int - and who knows what happens
when it itsn't so, and the compiler generates wrong code
accroding to what you intended.

This isn't actually true: it will do exactly the same promotions to
printf()s arguments as /already/ occur when calling printf().

Calling it without a prototype in scope will cause gcc to complain
that the implicit declaration of printf() is incompatible with its
actual prototype, but it apparently still works as expected; however,
as far as the C standard is concerned, its behavior is undefined. This
means that
(a) it could break on other systems besides Linux/glibc systems, and
(b) it may break on future versions of Linux/glibc, as they will not
feel obliged to continue to support this incorrect usage.
.

Relevant Pages

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