使用gdb遇到问号调试Go代码

Question

I read golang official gdb document here https://golang.org/doc/gdb

And I'm curious about what causes question mark when inspecting the stack.

(gdb) bt  # backtrace
#0  regexp.TestFind (t=0xf8404a89c0) at /home/user/go/src/regexp/find_test.go:148
#1  0x000000000042f60b in testing.tRunner (t=0xf8404a89c0, test=0x573720) at /home/user/go/src/testing/testing.go:156
#2  0x000000000040df64 in runtime.initdone () at /home/user/go/src/runtime/proc.c:242
#3  0x000000f8404a89c0 in ?? ()
#4  0x0000000000573720 in ?? ()
#5  0x0000000000000000 in ?? ()

So, Is the gdb optimizations caused the question mark? Or what on earth result in such question mark?

One more question, How can I retrieve the message hiden behind question mark?

Answer 1

The code, probably, has stripped debugging symbols:

gcc -s

You can see that behavior with this simple sample

#include <stdio.h>

void riskyCode() {
  char *error_str = "This code will fail with SIGSEGV";
  *error_str = 'g';
}

int c() {
  riskyCode();
  return 1;
}

int b() {
  return c();
}

int a() {
  return b();
}

int main() {
  return a();
}

Compile it like this

gcc -s -O0 -o main main.c

And run code in gdb

gdb main
> run

Then, you will see ?? in stack locations

bt
#0  0x000000000040056a in ?? ()
#1  0x000000000040057d in ?? ()
#2  0x0000000000400592 in ?? ()
#3  0x00000000004005a2 in ?? ()
#4  0x00000000004005b2 in ?? ()
#5  0x00002aaaaacebc36 in __libc_start_main () from /lib64/libc.so.6
#6  0x0000000000400429 in ?? ()
#7  0x00007fffffff93f8 in ?? ()
#8  0x000000000000001c in ?? ()
#9  0x0000000000000001 in ?? ()
#10 0x00007fffffff9ca2 in ?? ()
#11 0x0000000000000000 in ?? ()

Try to use very same library, but compiled with debugging symbols?