How do I link glibc's implementation of iconv?

Question

The GNU C library provides an implementation of iconv - how do I use it?

Simple program:

#include <iconv.h>

int main( int argc, char **argv ) {
        iconv_t cd = iconv_open( "UTF-8", "ISO-8859-1" );
        iconv_close( cd );
        return 0;
}

Compile and link:

$ gcc -Wall iconv.c -o iconv
/tmp/ccKAfXNg.o: In function `main':
iconv.c:(.text+0x19): undefined reference to `libiconv_open'
iconv.c:(.text+0x29): undefined reference to `libiconv_close'
collect2: ld returned 1 exit status

List the symbols to show they exist!

$ nm -D /lib/libc-2.12.1.so | grep iconv
00017920 T iconv
00017ae0 T iconv_close
00017720 T iconv_open

If I install the GNU libiconv library to /usr/local and link with -liconv it works. How do I link with the glibc implementation of iconv?

EDIT: More information as requested from the comments:

List all iconv.h files in /usr (1 match)

$ find /usr/ | grep "iconv\.h"
/usr/include/iconv.h

Reinstall libc6-dev to ensure the correct header is installed.

$ dpkg -S /usr/include/iconv.h 
libc6-dev: /usr/include/iconv.h
$ apt-get install --reinstall libc6-dev
Reading package lists... Done                                                                                                                                                                                                                  
Building dependency tree                                                                                                                                                                                                                       
Reading state information... Done                                                                                                                                                                                                              
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.                                                                                                                                                                  
Need to get 0B/4,910kB of archives.                                                                                                                                                                                                            
After this operation, 0B of additional disk space will be used.                                                                                                                                                                                
(Reading database ... 143458 files and directories currently installed.)                                                                                                                                                                       
Preparing to replace libc6-dev 2.12.1-0ubuntu10.1 (using .../libc6-dev_2.12.1-0ubuntu10.1_i386.deb) ...                                                                                                                                        
Unpacking replacement libc6-dev ...                                                                                                                                                                                                            
Setting up libc6-dev (2.12.1-0ubuntu10.1) ...

Compile and link again with suggested preprocessor option:

$ gcc -Wall -DLIBICONV_PLUG iconv.c -o iconv
/tmp/ccKAfXNg.o: In function `main':
iconv.c:(.text+0x19): undefined reference to `libiconv_open'
iconv.c:(.text+0x29): undefined reference to `libiconv_close'
collect2: ld returned 1 exit status

Output from gcc -H:

$ gcc -H iconv.c 
. /usr/include/iconv.h
.. /usr/include/features.h
... /usr/include/bits/predefs.h
... /usr/include/sys/cdefs.h
.... /usr/include/bits/wordsize.h
... /usr/include/gnu/stubs.h
.... /usr/include/bits/wordsize.h
.... /usr/include/gnu/stubs-32.h
.. /usr/lib/gcc/i686-linux-gnu/4.4.5/include/stddef.h
Multiple include guards may be useful for:
/usr/include/bits/predefs.h
/usr/include/gnu/stubs-32.h
/usr/include/gnu/stubs.h
/usr/lib/gcc/i686-linux-gnu/4.4.5/include/stddef.h

pastbin copy of /usr/include/iconv.h

Fixed: Reboot fixed the issue. I suspect a cached copy of libiconv was causing the conflicts, even though it was deleted from disk.

Solution

Your program seems fine and compiles fine on my system (Mandriva Linux 2010.1).

I find the libiconv_* references in your compile log worrisome, though. Are you sure that the iconv.h version that gets included comes from glibc and not from a separate libiconv implementation, such as GNU libiconv? It sounds as if it adds a lib prefix to all iconv functions to avoid symbol collisions with the iconv implementation of the C library that came with the system.

Having to explicitly link to libiconv points to a separate iconv implementation too - glibc does not need it.

EDIT:

For the record, I just verified that using the iconv.h header file from libiconv without explicitly linking against it will produce exactly the result that you are seeing - it renames all iconv functions by adding a lib prefix to their names.

Comments

[x-x]

I did install libiconv to /usr/local to test it, but have since removed it with a 'make uninstall' - no traces left that I can see.

[thkala]

@DrTwox: can you verify which iconv.h gets included?

[Logan Capaldo]

@DrTwox you can perform the verification by making use of the -H option to gcc. It will print the path of the files included.

[thkala]

@DrTwox: Does adding -DLIBICONV_PLUG to your compiler options fix it without -liconv? If it does, then you are using the iconv.h version from libiconv.

[x-x]

The computer froze, I reset, it worked. I still don't know what the problem was. Though I uninstalled libiconv from /usr/local, and confirmed it was all deleted, all clues point to the system still seeing the wrong header - stale cache perhaps? I don't understand the deep-down magic of Linux to figure it out myself.