what are weak external symbols?

[Bahadir Balban <bahadir.balban@gmail.com>]

Hi,

I'm reading the book "Linkers and Loaders" and weak external symbols
are not so clearly explained (for me at least).

In summary it says that weak external symbols is a concept to avoid
linking with unused parts of a library. The printf example is given:
floating-point routines (namely fcvt) are referred as weak references
by printf, such that when printf is used in a routine that didn't use
fcvt, reference to fcvt resolves to zero and no error is produced from
this.

Now the questions:

1) Is my summary above correct?

2) Is this a case such that, these fcvt routines are explicitly
defined in printf as weak references? If so, how do you define it as
such in C source? Or perhaps you use a linker flag when you create the
library?

I thought this question best fits the binutils list, sorry if I'm off-topic.

Many thanks,
Bahadir