From:	DavMac AT iname DOT com (Davin McCall)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: difference between libraries and headers
Date:	Thu, 18 Nov 1999 02:08:13 GMT
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On Wed, 17 Nov 1999 18:38:10 -0600, "Morpheus" <hall AT cs DOT unt DOT edu>
wrote:

>I know the difference between a library file and a header file, but can
>someone tell me the particulars involved -- the differences between
>compiling your program with included headers and including a library on the
>prompt? I know the header files on the include path don't contain a full
>implementation of their functions/classes, so where is this implementation?

In a library file.

>If it's in the library files, how does the compiler know which library to
>get without specifying it at the prompt?

It generally doesn't. In the case of the "standard headers" (eg
stdio.h, stdlib.h, string.h), the implementations are contained in the
standard library, which is automatically linked in (you don't need to
specify it on the command line). For other headers, you generally also
need to specify the library file.

> If compiling with libraries and
>headers are independent things, then what is the advantage of compiling with
>library files?

Hopefully you understand by now, but to further clarify: Including
headers and linking libraries are not completely independent: They
supplement each other.

In particular, the header file tells the compiler what functions (and
variables and types etc) are available in the library, so that it
knows how many arguments each of the functions takes and can generate
appropriate errors if the wrong number is used, etc.

The library on the other hand contains the implementation, which is
pre-compiled code. The linker uses the library file by extracting the
appropriate code and including it in the program (incidentally, the
compiler calls the linker automatically, and passes the appropriate
library files as specified by the user. It also adds the standard
library at this point).

Davin.

__________________________________________________________
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