Update: I'm working with the GNU-runtime on Linux. The problem does not occur on MacOS with the Apple-runtime.
Update 2: I compiled the GNU-runtime on MacOS and build the example with it. The error does not occur on MacOS with the GNU-runtime. I would say the problem is the glibc (since backtrace
and backtrace_symbols
are glibc extensions).
When printing a backtrace in a GCC compiled Objective-C app using backtrace
and backtrace_symbols
, I don't get any Objective-C symbols. Only the filenames, addresses and C-symbols appear.
I compiled with -g
and linked with -rdynamic
.
My test app:
void _printTrace()
{
void *addr[1024];
int aCount = backtrace(addr, 1024);
char **frameStrings = backtrace_symbols(addr, aCount);
for (int i = 0; i < aCount; i++) {
printf("%s\n", frameStrings[i]);
}
free(frameStrings);
}
@interface TheObject
+ (void)_printTrace;
+ (void)printTrace;
@end
@implementation TheObject
+ (void)_printTrace
{
_printTrace();
}
+ (void)printTrace
{
[self _printTrace];
}
@end
void printTrace()
{
[TheObject printTrace];
}
int main(int argc, char **argv)
{
printTrace();
return 0;
}
and it's output:
./test.bin(_printTrace+0x1f) [0x8048e05]
./test.bin() [0x8048e60]
./test.bin() [0x8048e8b]
./test.bin(printTrace+0x34) [0x8048ec5]
./test.bin(main+0xf) [0x8048eda]
/lib/libc.so.6(__libc_start_main+0xe5) [0xb7643bb5]
./test.bin() [0x8048b51]
Is there a way to let the Objective-C symbols appear in this backtrace?
dladdr()
only reports global and weak symbols. But all Objective-C function symbols are local:
$ readelf -s so_backtrace
Symbol table '.dynsym' contains 29 entries:
…
Symbol table '.symtab' contains 121 entries:
Num: Value Size Type Bind Vis Ndx Name
…
49: 08048a01 13 FUNC LOCAL DEFAULT 14 _c_TheObject___printTrace
50: 08048a0e 47 FUNC LOCAL DEFAULT 14 _c_TheObject__printTrace
…
You can verify that local symbols are never returned by looking at the GNU libc source code yourself. backtrace_symbols()
is defined in sysdeps/generic/elf/backtracesyms.c. It relies on _dl_addr()
, which is defined in elf/dl-addr.c, to provide it with the symbol names. That ultimately calls determine_info()
. If it can, it uses the the GNU hash table, which does not include local symbols by design:
49 /* We look at all symbol table entries referenced by the hash
50 table. */
…
60 /* The hash table never references local symbols so
61 we can omit that test here. */
If the GNU hash table isn't present, it falls back to standard hash table. This includes all the symbols, but the determine_info()
code filters out all but the global symbols and weak symbols:
90 if ((ELFW(ST_BIND) (symtab->st_info) == STB_GLOBAL
91 || ELFW(ST_BIND) (symtab->st_info) == STB_WEAK)
To symbolicate the Objective-C function addresses, you would have to perform the look-up yourself and not filter out the local function symbols. Further, you would have to demangle the Objective-C function symbols to restore _c_TheObject___printTrace
to +[TheObject _printTrace]
.