Using dlsym() to look for a variable in a statically linked library

Question

We have a program that links in a number of static libraries, which may or may not define a number of symbols depending on compilation options. On OS X, we use dlsym(3) with a NULL handle to obtain the symbol addresses. However, on Linux, dlsym(3) always returns NULL.

Consider a trivial program (sources below) that links in a static library containing a function and a variable and tries to print their addresses. We can check that the program contains the symbols:

$ nm -C test | grep "test\(func\|var\)"
0000000000400715 T testFunc
0000000000601050 B testVar

However, when the program is run, neither can be located:

$ ./test
testVar: (nil)
testFunc: (nil)

Is what we are trying to do possible on Linux, using glibc's implementation of dlsym(3)?

Makefile

(Sorry about the spaces)

LDFLAGS=-L.
LDLIBS=-Wl,--whole-archive -ltest -Wl,--no-whole-archive -ldl

libtest.o: libtest.c libtest.h
    libtest.a: libtest.o
test: test.o libtest.a
clean:
    -rm -f test test.o libtest.o libtest.a

libtest.h

#pragma once
extern void *testVar;
extern int testFunc(int);

libtest.c

#include "libtest.h"
void *testVar;
int testFunc(int x) { return x + 42; }

test.c

#include <stdlib.h>
#include <stdio.h>
#include <dlfcn.h>

int main(int argc, char *argv[]) {
  void *handle = dlopen(NULL, 0);
  void *symbol = dlsym(handle, "testVar");
  printf("testVar: %p\n", symbol);
  symbol = dlsym(handle, "testFunc");
  printf("testFunc: %p\n", symbol);
  return 0;
}

Totally unrelated, but naming a program test in a POSIX environment is a big source of problems, as there is a standard command also named test, often built into modern shells. — Some programmer dude
– Some programmer dude, Commented Dec 17, 2014 at 10:28
True! However, if you have . in your PATH then you deserve what you get. :P — Peter T.B. Brett
– Peter T.B. Brett, Commented Dec 17, 2014 at 10:30

Basile Starynkevitch · Accepted Answer · 2014-12-17 10:35:46Z

4

You should link your program with -rdynamic (or --export-dynamic for ld(1)) so

LDFLAGS += -rdynamic -L.

Then all the symbols are in the dynamic symbol table, the one used by dlsym

BTW, the visibility attribute could be of interest.

edited Dec 17, 2014 at 10:35

answered Dec 17, 2014 at 10:30

Basile Starynkevitch

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1 Comment

legends2k Over a year ago

For those, like me, who didn't know (from the manual): -rdynamic: Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.

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