Re: [xml] The order of arguments when compiling

[Csaba Raduly <rcsaba gmail com>]

Hi spam,

On Thu, Feb 16, 2012 at 4:01 PM,   wrote:
> Hello,
>
> On a first computer, when I compile a C program using the libxml2 library I do :
> $ gcc `xml2-config --cflags` `xml2-config --libs` main.c
> or
> $ gcc main.c `xml2-config --cflags` `xml2-config --libs`
> And it works (but the first method is better because it respects the order of arguments in the gcc manual).
>
> On a second computer, if I compile like this, there is an error :
> $ gcc `xml2-config --cflags` `xml2-config --libs` main.c
> /tmp/cc7uNwed.o: In function `parseDoc':
> main.c:(.text+0xd): undefined reference to `xmlParseFile'
> main.c:(.text+0x51): undefined reference to `xmlCleanupParser'
> main.c:(.text+0x63): undefined reference to `xmlFreeDoc'
> main.c:(.text+0x68): undefined reference to `xmlCleanupParser'
> collect2: ld returned 1 exit status
> But if I compile like this, there is no error :
> $ gcc main.c `xml2-config --cflags` `xml2-config --libs`
>
> I thought this was a gcc problem but someone on the gcc mailing-list tell me that probably I have compiled 
> libxml2 on the second computer "statically". And I should compile it "dynamically". It's true that on the 
> second computer, I have compiled myself the library (configure, make, make install). On the first computer, 
> this is my distro package.
>
> How should I compile libxml2 on the 2nd computer to have the same behavior on the two computers?

You shouldn't; just use the last compilation command (with
`xml2-config --libs` at the end).
Most Unix linkers are one-pass; because of this, an object which needs
a symbol must appear before the object (or library) which supplies
that symbol.

See for example http://webpages.charter.net/ppluzhnikov/linker.html

GNU make's built-in rule for linking boils down to:

%: %.c
#  commands to execute (built-in):
         $(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)      $^
    $(LOADLIBES) $(LDLIBS) -o $@

%: %.o
        $(CC) $(LDFLAGS) $(TARGET_ARCH)      $^      $(LOADLIBES)
$(LDLIBS) -o $@

As you can see, the list of source or object files ($^) is always
_before_ the list of libraries (in $(LDLIBS) or $(LOADLIBES) ).

Hope this helps,
Csaba
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