On Dec 3, 8:50 am, Christina wrote:
> Hi Everybody,
>
> I have declared a static variable in a header file of a library that
> times certain routines in the library. My application links to this
> library. Before terminating, I want to print the value of this
> variable. The library is in archive format. When I try using dlopen on
> this library on Linux, I get the error, "library: invalid ELF header".
>
> Does anybody have any idea, how to extract symbols from an archive
> format?
>
> Or does anybody have any other ideas about how to collect the timing
> of routines in the library in a better way?

I don't really understand what you're trying to do. Can you post an
example?

Christina writes:
>Hi Everybody,
>
>I have declared a static variable in a header file of a library that
>times certain routines in the library. My application links to this
>library. Before terminating, I want to print the value of this
>variable. The library is in archive format. When I try using dlopen on
>this library on Linux, I get the error, "library: invalid ELF header".
>
>Does anybody have any idea, how to extract symbols from an archive
>format?
>
>Or does anybody have any other ideas about how to collect the timing
>of routines in the library in a better way?
>
>I would really appreciate any help and suggestion that anybody could
>offer,
>
>Warm Regards,
>Christina.

If it is a static archive, and is linked with your application, simply
declare the variable 'extern' and access it directly.

scott

I tried declaring it extern. But it is not able to execute. That is
why I tried going for the dlopen/dlsym approach. I think the
components of the archive are dynamic, it is not static. I know that
in AIX, members of a dynamic archive can be obtained using dlopen/
dlsym. But I am not sure that it can be done in Linux.

Regards,
Christina.

On Dec 3, 3:02 pm, sc...@slp53.sl.home (Scott Lurndal) wrote:
> Christina writes:
> >Hi Everybody,
>
> >I have declared a static variable in a header file of a library that
> >times certain routines in the library. My application links to this
> >library. Before terminating, I want to print the value of this
> >variable. The library is in archive format. When I try using dlopen on
> >this library on Linux, I get the error, "library: invalid ELF header".
>
> >Does anybody have any idea, how to extract symbols from an archive
> >format?
>
> >Or does anybody have any other ideas about how to collect the timing
> >of routines in the library in a better way?
>
> >I would really appreciate any help and suggestion that anybody could
> >offer,
>
> >Warm Regards,
> >Christina.
>
> If it is a static archive, and is linked with your application, simply
> declare the variable 'extern' and access it directly.
>
> scott- Hide quoted text -
>
> - Show quoted text -

I know the question is a bit unclear. But it is difficult for me to
write sample code. I'll try to explain it as well as I can.

I have a library code and a header file in that code. A routine in the
code is called repetitively. I want the cumulative time spent by a
function that is called within that routine that is part of another
library. So I inserted timing calls around that routine. At the end of
the program run, I want to see the value of this variable.

Eg:
-------- lib.h file ---------
static double tvar = 0.0; <== newly introduced var

-------- lib.c file ---------
library_routine() {
.....

stime = timing_routine();
third_party_lib_routine();
etime = timing_routine();
tvar += etime - stime;
.....
}
---------- my src code --------
main() {
Access the value of tvar;
}

I was unable to access the value of 'tvar' using extern and the
library is part of an archive. So dlopen on the archive is not
working.

I hope I was able to expain the problem.

Regards,
Christina.

On Dec 3, 2:26 pm, fjbl...@yahoo.com wrote:
> On Dec 3, 8:50 am, Christina wrote:
>
> > Hi Everybody,
>
> > I have declared a static variable in a header file of a library that
> > times certain routines in the library. My application links to this
> > library. Before terminating, I want to print the value of this
> > variable. The library is in archive format. When I try using dlopen on
> > this library on Linux, I get the error, "library: invalid ELF header".
>
> > Does anybody have any idea, how to extract symbols from an archive
> > format?
>
> > Or does anybody have any other ideas about how to collect the timing
> > of routines in the library in a better way?
>
> I don't really understand what you're trying to do. Can you post an
> example?

On Dec 4, 9:35 am, Christina wrote:
> I know the question is a bit unclear. But it is difficult for me to
> write sample code. I'll try to explain it as well as I can.
>
> I have a library code and a header file in that code. A routine in the
> code is called repetitively. I want the cumulative time spent by a
> function that is called within that routine that is part of another
> library. So I inserted timing calls around that routine. At the end of
> the program run, I want to see the value of this variable.
>
> Eg:
> -------- lib.h file ---------
> static double tvar = 0.0; <== newly introduced var
>
> -------- lib.c file ---------
> library_routine() {
> ....
>
> stime = timing_routine();
> third_party_lib_routine();
> etime = timing_routine();
> tvar += etime - stime;
> ....}
>
> ---------- my src code --------
> main() {
> Access the value of tvar;
>
> }
>
> I was unable to access the value of 'tvar' using extern and the
> library is part of an archive. So dlopen on the archive is not
> working.

Ok, I understand now.

I'm guessing you originally had `double tvar = 0.0;' in lib.h, and it
didn't compile, so you added `static' for no particular reason. Now
it compiles but doesn't work.

Using `static' creates a variable which is visible only to the current
"compilation unit" (normally a single source file, as seen after
preprocessing, and therefore including all header files). So what
this does is to create a *different* variable named `tvar' for every
source file. The `tvar' that the library sets is different from the
one that the main program reads, so the main program never sees the
change.

You really want tvar to be a single global variable. The reason it
didn't compile at first is because the compiler sees a definition,
with initialization, for tvar in every source file that includes
lib.h. It doesn't allow multiple initializations because it can't
check that they are all initializing to the same value, and it would
certainly be an error if they were different.

The correct way to do this is to have lib.h contain only a
*declaration* of the variable:

---lib.h---
extern double tvar;

and then to have a *single* library file contain the *definition*:

---lib.c---
double tvar = 0.0;
/* more code */

Then you can just access tvar in your program like any other global
variable.

Forget about dlopen, it doesn't do what you seem to think it does, and
won't help you here.

I have managed to compile the code and am aware of the connotations of
static. I can change the code to have a single definition.

What I was looking for was whether we can dlopen an archive in Linux?
This is because using extern is not giving me linkage to tvar for some
reason.

Regards,
Christina.

On Dec 4, 2:26 pm, fjbl...@yahoo.com wrote:
> On Dec 4, 9:35 am, Christina wrote:
>
>
>
>
>
> > I know the question is a bit unclear. But it is difficult for me to
> > write sample code. I'll try to explain it as well as I can.
>
> > I have a library code and a header file in that code. A routine in the
> > code is called repetitively. I want the cumulative time spent by a
> > function that is called within that routine that is part of another
> > library. So I inserted timing calls around that routine. At the end of
> > the program run, I want to see the value of this variable.
>
> > Eg:
> > -------- lib.h file ---------
> > static double tvar = 0.0; <== newly introduced var
>
> > -------- lib.c file ---------
> > library_routine() {
> > ....
>
> > stime = timing_routine();
> > third_party_lib_routine();
> > etime = timing_routine();
> > tvar += etime - stime;
> > ....}
>
> > ---------- my src code --------
> > main() {
> > Access the value of tvar;
>
> > }
>
> > I was unable to access the value of 'tvar' using extern and the
> > library is part of an archive. So dlopen on the archive is not
> > working.
>
> Ok, I understand now.
>
> I'm guessing you originally had `double tvar = 0.0;' in lib.h, and it
> didn't compile, so you added `static' for no particular reason. Now
> it compiles but doesn't work.
>
> Using `static' creates a variable which is visible only to the current
> "compilation unit" (normally a single source file, as seen after
> preprocessing, and therefore including all header files). So what
> this does is to create a *different* variable named `tvar' for every
> source file. The `tvar' that the library sets is different from the
> one that the main program reads, so the main program never sees the
> change.
>
> You really want tvar to be a single global variable. The reason it
> didn't compile at first is because the compiler sees a definition,
> with initialization, for tvar in every source file that includes
> lib.h. It doesn't allow multiple initializations because it can't
> check that they are all initializing to the same value, and it would
> certainly be an error if they were different.
>
> The correct way to do this is to have lib.h contain only a
> *declaration* of the variable:
>
> ---lib.h---
> extern double tvar;
>
> and then to have a *single* library file contain the *definition*:
>
> ---lib.c---
> double tvar = 0.0;
> /* more code */
>
> Then you can just access tvar in your program like any other global
> variable.
>
> Forget about dlopen, it doesn't do what you seem to think it does, and
> won't help you here.- Hide quoted text -
>
> - Show quoted text -

Christina writes:
>I have managed to compile the code and am aware of the connotations of
>static. I can change the code to have a single definition.
>
>What I was looking for was whether we can dlopen an archive in Linux?
>This is because using extern is not giving me linkage to tvar for some
>reason.
>

How about providing the error message you are getting presumably from
the linker?

You may also need to specify the archive more than once to the linker
depending on how many passes it makes over the .a. Unix 'ld' would
never read a .a twice, so if the reference occurred in a .o/.a which
was read by the linker after your .a, then the reference would be missed.

I don't know if binutils (gnu ld) has similar behavior.

dlopen cannot be used on an archive library on linux or any other
unix except, apparently, aix.

scott

Christina writes:

> I have managed to compile the code and am aware of the connotations of
> static. I can change the code to have a single definition.
>
> What I was looking for was whether we can dlopen an archive in
> Linux?

I can't answer this, but I don't think it matters...

> This is because using extern is not giving me linkage to tvar for some
> reason.

It seems to me you should push to find out why this relatively
ordinary operation (linking to an external variable defined in a
library) is not working as you expect. It is a common usage and
should be sorted out rather than try to find exotic fixes.

As usual, a small example would probably help clarify matters.

--
Ben.

On Dec 5, 10:48 am, Christina wrote:
> I have managed to compile the code and am aware of the connotations of
> static. I can change the code to have a single definition.
>
> What I was looking for was whether we can dlopen an archive in Linux?
> This is because using extern is not giving me linkage to tvar for some
> reason.

No, you can't. dlopen opens a *dynamic* library. Furthermore, using
dlopen would load a new copy, which would be independent of the copy
you linked against. For instance, if you call a library function
foo() which sets a variable `bar', calling dlsym("bar") will return a
pointer to a different variable which isn't the one that was set.

I think you are seriously confused about how linking works. If "using
extern is not giving you linkage to tvar for some reason", I think you
had better figure out why that is. It's almost certainly something
you're doing wrong, because what I described in my previous post is
supposed to work; either that or your compilation environment is badly
messed up. I can't imagine any way that manually loading a library
with dlopen() could help with the sort of thing you mention; it's
completely irrelevant.

fjblurt@yahoo.com writes:
>On Dec 5, 10:48 am, Christina wrote:
>> I have managed to compile the code and am aware of the connotations of
>> static. I can change the code to have a single definition.
>>
>> What I was looking for was whether we can dlopen an archive in Linux?
>> This is because using extern is not giving me linkage to tvar for some
>> reason.
>
>No, you can't. dlopen opens a *dynamic* library.

> Furthermore, using
>dlopen would load a new copy, which would be independent of the copy
>you linked against.

Not always. dlopen(NULL) will allow access to the main program
and all libraries loaded up to that point.

However, you are correct in that dlopen() is pretty useless with an
archive library.

scott

On Dec 6, 4:51 pm, sc...@slp53.sl.home (Scott Lurndal) wrote:
> fjbl...@yahoo.com writes:
> >On Dec 5, 10:48 am, Christina wrote:
> >> I have managed to compile the code and am aware of the connotations of
> >> static. I can change the code to have a single definition.
>
> >> What I was looking for was whether we can dlopen an archive in Linux?
> >> This is because using extern is not giving me linkage to tvar for some
> >> reason.
>
> >No, you can't. dlopen opens a *dynamic* library.
> > Furthermore, using
> >dlopen would load a new copy, which would be independent of the copy
> >you linked against.
>
> Not always. dlopen(NULL) will allow access to the main program
> and all libraries loaded up to that point.
>
> However, you are correct in that dlopen() is pretty useless with an
> archive library.
>
> scott

I have opened libraries before using dlopen(), but I was never aware
that it will open a new copy of the library. I have never had any
problem accessing the values of variables before. But I think the
problem is that I used static keyword and hence there are multiple
copies of the variable in the library. Also, one of the posts mentions
that the linker does not read the archive more than once. At this
point I need to do a whole build again to test this and I got around
the problem by printing the value of the variable in a temporary file
and then seeing the value of the variable after my program runs. I
guess I was trying to do fancy things

Thanks,
Christina.