johnnycrash johnnycrash - 1 year ago 159
Linux Question

dladdr doesnt return the function name

I'm trying to use dladdr. It correctly locates the library, but it does not find the function name. I can call objdump, do a little math, and get the address of the function that I pass dladdr. If objdump can see it, why can't dladdr?

Here is my function:

const char *FuncName(const void *pFunc)
Dl_info DlInfo;
int nRet;

// Lookup the name of the function given the function pointer
if ((nRet = dladdr(pFunc, &DlInfo)) != 0)
return DlInfo.dli_sname;
return NULL;

Here is a gdb transcript showing what I get.

Program received signal SIGINT, Interrupt.
[Switching to Thread 0xf7f4c6c0 (LWP 28365)]
0xffffe410 in __kernel_vsyscall ()
(gdb) p MatchRec8Cmp
$2 = {void (TCmp *, TWork *, TThread *)} 0xf1b62e73 <MatchRec8Cmp>
(gdb) call FuncName(MatchRec8Cmp)
$3 = 0x0
(gdb) call FuncName(0xf1b62e73)
$4 = 0x0
(gdb) b FuncName
Breakpoint 1 at 0xf44bdddb: file threads.c, line 3420.
(gdb) call FuncName(MatchRec8Cmp)

Breakpoint 1, FuncName (pFunc=0xf1b62e73) at threads.c:3420
3420 {
The program being debugged stopped while in a function called from GDB.
When the function (FuncName) is done executing, GDB will silently
stop (instead of continuing to evaluate the expression containing
the function call).
(gdb) s
3426 if ((nRet = dladdr(pFunc, &DlInfo)) != 0)
3427 return DlInfo.dli_sname;
(gdb) p DlInfo
$5 = {dli_fname = 0x8302e08 "/xxx/", dli_fbase = 0xf1a43000, dli_sname = 0x0, dli_saddr = 0x0}
(gdb) p nRet
$6 = 1
(gdb) p MatchRec8Cmp - 0xf1a43000
$7 = (void (*)(TCmp *, TWork *, TThread *)) 0x11fe73
(gdb) q
The program is running. Exit anyway? (y or n) y

Here is what I get from objdmp

$ objdump --syms /xxx/ | grep MatchRec8Cmp
0011fe73 l F .text 00000a98 MatchRec8Cmp

Sure enough, 0011fe73 = MatchRec8Cmp - 0xf1a43000. Anyone know why dladdr can't return dli_sname = "MatchRec8Cmp" ???

I'm running Red Hat Enterprise Linux Server release 5.4 (Tikanga). I have seen this work before. Maybe it's my compile switches:

CFLAGS = -m32 -march=i686 -msse3 -ggdb3 -pipe -fno-common -fomit-frame-pointer \
-Ispio -fms-extensions -Wmissing-declarations -Wstrict-prototypes -Wunused -Wall \
-Wno-multichar -Wdisabled-optimization -Wmissing-prototypes -Wnested-externs \
-Wpointer-arith -Wextra -Wno-sign-compare -Wno-sequence-point \
-I../../../include -I/usr/local/include -fPIC \

I have tried it with -g instead of -ggdb3 although I don't think debugging symbols have anything to do with elf.


Answer Source

If objdump can see it, why can't dladdr

dladdr can only see functions exported in the dynamic symbol table. Most likely

 nm -D /xxx/ | grep MatchRec8Cmp

shows nothing. Indeed your objdump shows that the symbol is local, which proves that this is the cause.

The symbol is local either because it has a hidden visibility, is static, or because you hide it in some other way (e.g. with a linker script).


Those marked with the 'U' work with dladdr. They get "exported" automatically somehow.

They work because they are exported from some other shared library. The U stands for unresolved, i.e. defined elsewhere.