Getting hidden symbol error while linking in static libraries but not in shared libraries

Qt Enthusiast

The following code also reproduces the same issue

File : my_lib.h

/* Filename: lib_mylib.h */

int fun();

Filename : my_lib.c

Filename: lib_mylib.c */
#include <stdio.h>
int fun() {
return 2;
}

File 3) my_lib_shared.h
int foo();

File4
#include "my_lib.h"

int foo(void) {
return fun() + 4;
}

and following steps produces same error

gcc -g -fvisibility=hidden -c my_lib.c -o lib_mylib.o
ar rcs libMylib_static_hidden.a lib_mylib.o
gcc -g -fvisibility=hidden -c -fPIC my_lib_shared.c -o my_lib_shared.o
gcc -g -shared -o libMyShare.so my_lib_shared.o
gcc -g driver.c -L . -lMylib_static_hidden -lMyShare

kshegunov

@Qt-Enthusiast
Make the symbol visible (which is equivalent to exporting it on Windows) and you won't have the error any more.

Qt Enthusiast

my questions when I make static libraries as shared , then I do not get this linking error why ? Is there other flag which removes the visibilty. How to check the symbols are hidden in a shared library ?

kshegunov

my questions when I make static libraries as shared , then I do not get this linking error why ?

By default on Linux all (non-static) symbols will be exported (marked visible) from a shared object (dynamic library).

Is there other flag which removes the visibilty.

Not while compiling or linking, but you can set an attribute for individual symbols. For example (ELF ABI, Linux, GCC only):

__attribute__((visibility ("hidden"))) void hiddenSymbol(int arg1, float arg2);

How to check the symbols are hidden in a shared library ?

Try this post.

Qt Enthusiast

Not very clear

I have do

gcc -g -fvisibility=hidden -c my_lib.c -o lib_mylib.o
gcc -g -shared -o libHiddemMyLib.so lib_mylib.o
gcc -g -fvisibility=hidden -c -fPIC my_lib_shared.c -o my_lib_shared.o
gcc -g -shared -o libMyShare.so my_lib_shared.o
gcc -g driver.c -L . -lHiddemMyLib -lMyShare

I do not get this library error why but for static librarries I get this hidden error ? not clear to me yet

kshegunov

@Qt-Enthusiast said in Getting hidden symbol error while linking in static libraries but not in shared libraries:

Not very clear

What's that?

I do not get this library error why but for static librarries I get this hidden error ?

The symbols in the so aren't hidden, so you don't get the error (see first comment of my previous post). The symbols in the archive are marked as hidden, because you've marked them as such, thus you get the errors.

As a word of advice, don't mix up flags for the compiler if you don't know what you're doing. Either go all in and mark everything -fvisibility=hidden or don't and leave the symbols with their default visibility.

Qt Enthusiast

with same files

The following code also reproduces the same issue

File : my_lib.h

/* Filename: lib_mylib.h */
int fun();

Filename : my_lib.c
Filename: lib_mylib.c */
#include <stdio.h>
int fun() {
return 2;
}
File 3) my_lib_shared.h
int foo();

File4
#include "my_lib.h"

int foo(void) {
return fun() + 4;
}

gcc -fvisibility=hidden -fPIC -c my_lib.c -o lib_mylib.o
gcc -fvisibility=hidden -shared -o libMyLibHidden.so lib_mylib.o
gcc -fPIC -c my_lib_shared.c -o my_lib_shared.o
gcc -shared -o libMyShared2.so my_lib_shared.o
gcc -L. -Wall -o test_hidden driver.c -lMyLibHidden -lMyShared2

when I do a

objdump -T libMyLibHidden.so

libMyLibHidden.so: file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
0000000000000448 l d .init 0000000000000000 .init
0000000000000000 w D UND 0000000000000000 gmon_start
0000000000000000 w D UND 0000000000000000 _Jv_RegisterClasses
0000000000000658 g DF .fini 0000000000000000 Base _fini
0000000000000000 w D UND 0000000000000000 _ITM_deregisterTMCloneTable
0000000000000000 w D UND 0000000000000000 _ITM_registerTMCloneTable
0000000000000000 w DF UND 0000000000000126 GLIBC_2.2.5 __cxa_finalize
0000000000200938 g D ABS 0000000000000000 Base __bss_start
0000000000200948 g D ABS 0000000000000000 Base _end
0000000000200938 g D ABS 0000000000000000 Base _edata
0000000000000448 g DF .init 0000000000000000 Base _init

I do not get any reference of fun, fun1 and fun2 and I get error has
[3382] source script_hidden
/tmp/cciyfmzL.o: In function main': driver.c:(.text+0xe): undefined reference to fun'
collect2: error: ld returned 1 exit status

but When I do

gcc -fPIC -c my_lib.c -o lib_mylib.o
gcc -fvisibility=hidden -shared -o libMyLibHidden.so lib_mylib.o
gcc -fPIC -c my_lib_shared.c -o my_lib_shared.o
gcc -shared -o libMyShared2.so my_lib_shared.o
gcc -L. -Wall -o test_hidden driver.c -lMyLibHidden -lMyShared2

I get reference of fun, fun1 but the hidden visibility is supprresed
objdump -T libMyLibHidden.so

libMyLibHidden.so: file format elf64-x86-64

DYNAMIC SYMBOL TABLE:
00000000000004a8 l d .init 0000000000000000 .init
0000000000000650 g DF .text 000000000000000b Base fun
0000000000000000 w D UND 0000000000000000 gmon_start
0000000000000000 w D UND 0000000000000000 _Jv_RegisterClasses
00000000000006b8 g DF .fini 0000000000000000 Base _fini
0000000000000666 g DF .text 000000000000000b Base fun2
0000000000000000 w D UND 0000000000000000 _ITM_deregisterTMCloneTable
0000000000000000 w D UND 0000000000000000 _ITM_registerTMCloneTable
0000000000000000 w DF UND 0000000000000126 GLIBC_2.2.5 __cxa_finalize
0000000000200998 g D ABS 0000000000000000 Base __bss_start
00000000002009a8 g D ABS 0000000000000000 Base _end
0000000000200998 g D ABS 0000000000000000 Base _edata
000000000000065b g DF .text 000000000000000b Base fun1
00000000000004a8 g DF .init 0000000000000000 Base _init

Why ? My question is how to get hidden flag in final shared libraries

kshegunov

You andswered your question as to "why". Apparently gcc applies the hidden attribute on a per object file basis.

My question is how to get hidden flag in final shared libraries

By using -fvisibility=hidden for both compilation and linkage (i.e. everywhere).

Qt Enthusiast

I am still getting undefined reference
for

gcc -fPIC -fvisibility=hidden -c my_lib.c -o lib_mylib.o
gcc -fPIC -fvisibility=hidden -shared -o libMyLibHidden.so lib_mylib.o
gcc -fPIC -c my_lib_shared.c -o my_lib_shared.o
gcc -shared -o libMyShared2.so my_lib_shared.o
gcc -L. -Wall -o test_hidden driver.c -lMyLibHidden -lMyShared2

kshegunov

I am still getting undefined reference

And as far as I can tell you should. You can't reference hidden symbols, that's why they're hidden in the first place.
I really don't know what you want to do exactly, I explained what I knew to the best of my abilities.