lldb debugger cannot print structure contents

Question

I've compiled a small program that uses asl_log, and when running in lldb, it failed to print the contents of a global variable from type 'aslclient', although i compiled in debug mode ('-g' flag).

perhaps you can tell me if this is related to the following bug, and how to workaround this problem

[lldb-dev] [Bug 16191] New: LLDB fails to evaluate expressions that 
dereference a struct when inferior is built with recent Clang

the input from debugger :

(lldb) print log_asl_client
(aslclient) $5 = 0x0000000100200000
(lldb) print *log_asl_client
(lldb) print *log_asl_client
error: incomplete type '__asl_object_s' where a complete type is required
note: forward declaration of '__asl_object_s'
error: 1 errors parsing expression

my compilation command :

clang -g -c -Wall -DDEBUG=1 example.c -o example.o
clang  example.o -o example

the code :

aslclient log_asl_client;
...
int main(int argc, char * const *argv) {
...
log_asl_client = asl_open(identity, facility, client_opts);
... 
--> at this point i initiate the print command in debug mode.

the version i use :

clang --version
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.3.0
Thread model: posix

thanks,

Answer 1

The debug information has a record for the forward reference to __asl_object_s , but not for the full type. This isn't entirely surprising in this particular case, since the only appearance of __asl_object_s in the public header files on OS X is:

typedef struct __asl_object_s *asl_object_t;

so this is an opaque reference to the struct, and there isn't a real definition anywhere. Presumably __asl_object_s is a placeholder and the pointer gets cast to whatever it really is when it is used.

Anyway, the debugger isn't refusing to show you something, there's actually nothing there to see...