Re: can't understand linking behaviour

["Pedro de Medeiros" <pedrovmm+lists@gmail.com>]

On 6/15/07, Shriramana Sharma <samjnaa@gmail.com> wrote:
> Glynn Clements wrote:
> > Libraries can have undefined references. Sometimes this is necessary,
> > e.g. if the library depends upon a function which is supposed to be
> > provided by the executable.
>
> To my mind, in the real world, if a library must access a function that
> the executable provides, it will only be through a function pointer --
> the library may provide, for e.g. a binary search facility, and the
> program uses this to carry out a binary search on a function in its body.


Actually, that's what you do when you allow the function called by the
library to have any name: you call it by a pointer. It is more
flexible, but not necessarily the only way around.


> What real world application would make a library A to call a function
> foo() when foo() is defined in the body of an executable X that itself
> depends on A? If foo() were in a dependency of A, say library B, then it
> can be understood, but to expect the function to come from X is,
> although *theoretically* possible, how practically necessary?


I suppose it is just because it is more simple and flexible to
implement it this way. Because for library A (and for the compiler and
linker) it doesn't matter if foo() comes from executable X or library
B. At compile time, those symbols are simply inserted in the table of
defined symbols, so it doesn't matter where they come from (and C
doesn't even have namespaces or something like that anyway; symbols
are just global or local).


> >> This seems highly counter-intuitive. Why would the linker require *only*
> >> at *executable* build-time the libraries,
> >
> > Why would the linker require all external symbols to be resolved at
> > the time the library is built?
>
> If you prove that there is a valid real-life situation where the library
> needs a symbol from an application, then the linker would not require
> all external symbols to be resolved.


Here is an example of library using a symbol from the application: the
external environ variable referenced in unistd.h is defined by the
application. The exec* family of functions (excluding execle() I
guess) use it internally only.


Cheers,
Pedro.

-- 
Pedro de Medeiros - Ciência da Computação - Universidade de Brasília
Home Page: http://www.nonseq.net - Linux User No.: 234250
-
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html