What exactly does `-rdynamic` do and when exactly is it needed? - c

Question

What exactly does -rdynamic (or --export-dynamic at the linker level) do and how does it relate to symbol visibility as defined by the -fvisibility* flags or visibility pragmas and __attribute__s?
For --export-dynamic, ld(1) mentions:
...
If you use "dlopen" to load a dynamic object which needs to refer back
to the symbols defined by the program, rather than some other dynamic
object, then you will probably need
to use this option when linking the program itself. ...
I'm not sure I completely understand this. Could you please provide an example that doesn't work without -rdynamic but does with it?
Edit:
I actually tried compiling a couple of dummy libraries (single file, multi-file, various -O levels, some inter-function calls, some hidden symbols, some visible), with and without -rdynamic, and so far I've been getting byte-identical outputs (when keeping all other flags constant of course), which is quite puzzling.

Answer 1

Here is a simple example project to illustrate the use of -rdynamic.
bar.c
extern void foo(void);
void bar(void)
{
foo();
}
main.c
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
void foo(void)
{
puts("Hello world");
}
int main(void)
{
void * dlh = dlopen("./libbar.so", RTLD_NOW);
if (!dlh) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
void (*bar)(void) = dlsym(dlh,"bar");
if (!bar) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
bar();
return 0;
}
Makefile
.PHONY: all clean test
LDEXTRAFLAGS ?=
all: prog
bar.o: bar.c
gcc -c -Wall -fpic -o $# $<
libbar.so: bar.o
gcc -shared -o $# $<
main.o: main.c
gcc -c -Wall -o $# $<
prog: main.o | libbar.so
gcc $(LDEXTRAFLAGS) -o $# $< -L. -lbar -ldl
clean:
rm -f *.o *.so prog
test: prog
./$<
Here, bar.c becomes a shared library libbar.so and main.c becomes
a program that dlopens libbar and calls bar() from that library.
bar() calls foo(), which is external in bar.c and defined in main.c.
So, without -rdynamic:
$ make test
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -o prog main.o -L. -lbar -ldl
./prog
./libbar.so: undefined symbol: foo
Makefile:23: recipe for target 'test' failed
And with -rdynamic:
$ make clean
rm -f *.o *.so prog
$ make test LDEXTRAFLAGS=-rdynamic
gcc -c -Wall -o main.o main.c
gcc -c -Wall -fpic -o bar.o bar.c
gcc -shared -o libbar.so bar.o
gcc -rdynamic -o prog main.o -L. -lbar -ldl
./prog
Hello world

Answer 2

-rdynamic exports the symbols of an executable, this mainly addresses scenarios as described in Mike Kinghan's answer, but also it helps e.g. Glibc's backtrace_symbols() symbolizing the backtrace.
Here is a small experiment (test program copied from here)
#include <execinfo.h>
#include <stdio.h>
#include <stdlib.h>
/* Obtain a backtrace and print it to stdout. */
void
print_trace (void)
{
void *array[10];
size_t size;
char **strings;
size_t i;
size = backtrace (array, 10);
strings = backtrace_symbols (array, size);
printf ("Obtained %zd stack frames.\n", size);
for (i = 0; i < size; i++)
printf ("%s\n", strings[i]);
free (strings);
}
/* A dummy function to make the backtrace more interesting. */
void
dummy_function (void)
{
print_trace ();
}
int
main (void)
{
dummy_function ();
return 0;
}
compile the program: gcc main.c and run it, the output:
Obtained 5 stack frames.
./a.out() [0x4006ca]
./a.out() [0x400761]
./a.out() [0x40076d]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f026597f830]
./a.out() [0x4005f9]
Now, compile with -rdynamic, i.e. gcc -rdynamic main.c, and run again:
Obtained 5 stack frames.
./a.out(print_trace+0x28) [0x40094a]
./a.out(dummy_function+0x9) [0x4009e1]
./a.out(main+0x9) [0x4009ed]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f85b23f2830]
./a.out(_start+0x29) [0x400879]
As you can see, we get a proper stack trace now!
Now, if we investigate ELF's symbol table entry (readelf --dyn-syms a.out):
without -rdynamic
Symbol table '.dynsym' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND free#GLIBC_2.2.5 (2)
2: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts#GLIBC_2.2.5 (2)
3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace_symbols#GLIBC_2.2.5 (2)
4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace#GLIBC_2.2.5 (2)
5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail#GLIBC_2.4 (3)
6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf#GLIBC_2.2.5 (2)
7: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main#GLIBC_2.2.5 (2)
8: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
with -rdynamic, we have more symbols, including the executable's:
Symbol table '.dynsym' contains 25 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND free#GLIBC_2.2.5 (2)
2: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterTMCloneTab
3: 0000000000000000 0 FUNC GLOBAL DEFAULT UND puts#GLIBC_2.2.5 (2)
4: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace_symbols#GLIBC_2.2.5 (2)
5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND backtrace#GLIBC_2.2.5 (2)
6: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __stack_chk_fail#GLIBC_2.4 (3)
7: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf#GLIBC_2.2.5 (2)
8: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main#GLIBC_2.2.5 (2)
9: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
10: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMCloneTable
11: 0000000000601060 0 NOTYPE GLOBAL DEFAULT 24 _edata
12: 0000000000601050 0 NOTYPE GLOBAL DEFAULT 24 __data_start
13: 0000000000601068 0 NOTYPE GLOBAL DEFAULT 25 _end
14: 00000000004009d8 12 FUNC GLOBAL DEFAULT 14 dummy_function
15: 0000000000601050 0 NOTYPE WEAK DEFAULT 24 data_start
16: 0000000000400a80 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used
17: 0000000000400a00 101 FUNC GLOBAL DEFAULT 14 __libc_csu_init
18: 0000000000400850 42 FUNC GLOBAL DEFAULT 14 _start
19: 0000000000601060 0 NOTYPE GLOBAL DEFAULT 25 __bss_start
20: 00000000004009e4 16 FUNC GLOBAL DEFAULT 14 main
21: 00000000004007a0 0 FUNC GLOBAL DEFAULT 11 _init
22: 0000000000400a70 2 FUNC GLOBAL DEFAULT 14 __libc_csu_fini
23: 0000000000400a74 0 FUNC GLOBAL DEFAULT 15 _fini
24: 0000000000400922 182 FUNC GLOBAL DEFAULT 14 print_trace
I hope that helps!

Answer 3

I use rdynamic to print out backtraces using the backtrace()/backtrace_symbols() of Glibc.
Without -rdynamic, you cannot get function names.
To know more about the backtrace() read it over here.

Answer 4

From The Linux Programming Interface:
42.1.6
Accessing Symbols in the Main Program
Suppose that we use dlopen() to dynamically load a shared library,
use dlsym() to obtain the address of a function x() from that
library, and then call x(). If x() in turn calls a function y(),
then y() would normally be sought in one of the shared libraries
loaded by the program.
Sometimes, it is desirable instead to have x() invoke an
implementation of y() in the main program. (This is similar to a
callback mechanism.) In order to do this, we must make the
(global-scope) symbols in the main program available to the dynamic
linker, by linking the program using the --export-dynamic linker
option:
$ gcc -Wl,--export-dynamic main.c (plus further options and
arguments)
Equivalently, we can write the following:
$ gcc -export-dynamic main.c
Using either of these options allows a dynamically loaded library to
access global symbols in the main program.
The gcc -rdynamic option and the gcc -Wl,-E option are further
synonyms for -Wl,--export-dynamic.
I guess this only works for dynamically loaded shared library, opened with dlopen(). Correct me if I am wrong.