What does "undefined reference to ..." mean when using gcc

dangerousdave · 03-10-2005, 08:13 AM

I'm triying to compile a small program I wrote that uses a library I recently installed and I get several "undefined reference to ..." errors for the functions I've used from the library. Can anyone tell me what this error means so I can figure out whats causing it?

Cheers,

jtshaw · 03-10-2005, 08:23 AM

That means your calling a function that has no definition. Either you haven't included the include files that define the functions or you haven't linked the library in with the -l<libraryname> switch on compile.

An example... if you want to compile something with libpthread you would all the -lpthread switch (notice that it is -lpthread and NOT -libpthread). "gcc -Wall -o TestProgram -lpthread TestProgram.c"

dangerousdave · 03-10-2005, 08:45 AM

OK, at the top of my code I have:

Code:

#include <stdio.h>
#include "playerc.h"

I have playerc.h in my /usr/local/include directory. If I change the include line to something invalid I get a different error, so it is definately finding the playerc.h file.

I compile with:
gcc -Wall -o control -lplayerc control.c

I have libplayerc.a in my /usr/local/lib directory, yet I still get undefined reference errors.

Now, if I compile with just:
gcc -Wall -o control control.c

...I get exactly the same error. What does this mean, something not linking right? If I compile with an invalid library I get a different error again, so it is finding the library, just not looking at it for some reason...

Any ideas?

Cheers,

jtshaw · 03-10-2005, 08:47 AM

Moved: This thread is more suitable in Programming and has been moved accordingly to help your thread/question get the exposure it deserves.

itsme86 · 03-10-2005, 09:09 AM

I'm not sure how it works with static libraries, but you might try running ldconfig as root and then trying to compile with the -lplayerc option again. If that doesn't work then gcc -Wall -o control control.c /usr/local/lib/libplayerc.a should definitely work.

bigearsbilly · 03-11-2005, 07:33 AM

it matters what order you do the linking.

It's the loader.
It only sees reference in a left<-right order.

It opens the lib, says: "are there any references I can fulfill?" then closes it. It does NOT go back
for another pass. So as your -o file comes before the lib,
it fails.

try changing the order:

Code:

gcc -Wall     -o control  control.c  -lplayerc

observe:

Code:

billym.primadtpdev>gcc  -o all -lmylib  all.c
Undefined                       first referenced
 symbol                             in file
hello                               /var/tmp/ccgQa4rR1.o
bye                                 /var/tmp/ccgQa4rR1.o
ld: fatal: Symbol referencing errors. No output written to all

Code:

billym.primadtpdev>gcc  -o all  all.c -lmylib
billym.primadtpdev>all
Hello
bye

bigearsbilly · 03-11-2005, 08:22 AM

or try:

Code:

make  control LDLIBS=-lplayerc