Why does backtrace not contain Objective-C symbols

问题:

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 backtraceand 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?

回答1:

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].

回答2:

GNUstep's NSException implementation doesn't use backtrace, instead it uses libbfd (binary file descriptor). I think the function that actually does the work is called static void find_address, which you can view here. Using this trivial example, I get the results that follow.

#include <Foundation/Foundation.h>

@interface Test : NSObject {}
+ (void) test;
@end

@implementation Test
+ (void) test
{
    Class GSStackTrace = objc_getClass("GSStackTrace");

    id stack = [GSStackTrace currentStack];

    for (int i = 0; i < [stack frameCount]; i++)
    {
        NSLog (@"%@", [[stack frameAt:i] function]);
    }
}
@end

int main(int argc, char **argv)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    [Test test];

    [pool release];

    return 0;
}

Output (when compiled with debug symbols):

2010-10-18 14:14:46.188 a.out[29091] +[GSStackTrace currentStack]
2010-10-18 14:14:46.190 a.out[29091] +[Test test]
2010-10-18 14:14:46.190 a.out[29091] main
2010-10-18 14:14:46.190 a.out[29091] __libc_start_main

You may be able to pick apart GSStackTrace. It is a “private” class (that's why I need to use objc_getClass, you'll also get lots of unrecognised selector warnings), but it seems to contain all the code necessary to read Objective-C class names.

Tested on Ubuntu 9.04 with GNUstep configured with --enable-debug (so that GSFunctionInfo is included in the build).

回答3:

I expect you'll need to ask the ObjC run time about the addresses to get symbol information. The addresses returned from backtrace() could probably be passed to something like object_getClass() to get the class, for example. I haven't tried any of this but it's where I'd look next in this case.