Make gdb search for symbol files again - debugging

Question

My distribution (Debian) ships debug files in separate packages. So what happens often is that I run a program in gdb until it crashes, in order to obtain a usable backtrace for a bug report. But bt is rather useless, missing the symbol information – because I did not install the corresponding -dbg package.
If I install the package now, is there a way to make gdb search for the symbol files again, without losing my current backtrace?

Answer 1

There is a trick you can use to make gdb try to read symbol files again:
(gdb) nosharedlibrary
(gdb) sharedlibrary
The first command tells it to forget all the symbol information it has, and the second command tells it to re-read it.

Answer 2

I am going to suggest an alternative approach with gdb gcore command, possibly it is suitable for you.
This is gcore description:
(gdb) help gcore
Save a core file with the current state of the debugged process.
Argument is optional filename. Default filename is 'core.<process_id>'
So I have a program that causes a crash:
include <iostream>
int f()
{
time_t curr_ts = time(0);
std::cout << "Before crash " << curr_ts << std::endl;
int * ptr = 0;
*ptr = *ptr +1 ;
std::cout << "After crash " << curr_ts << std::endl;
return *ptr;
}
int main()
{
std::cout << "Before f() " << std::endl;
f();
std::cout << "After f() " << std::endl;
return 0;
}
I compiled it with debug info. However I put the executable with debug info in an archive and for tests use a stripped version.
So it crashes under gdb:
$ gdb ./a.out
Reading symbols from ./a.out...(no debugging symbols found)...done.
(gdb) r
Starting program: /home/crash/a.out
Before f()
Before crash 1435322344
Program received signal SIGSEGV, Segmentation fault.
0x000000000040097d in ?? ()
(gdb) bt
#0 0x000000000040097d in ?? ()
#1 0x00000000004009e0 in ?? ()
#2 0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
#3 0x00000000004007f9 in ?? ()
#4 0x00007fffffffde58 in ?? ()
#5 0x000000000000001c in ?? ()
#6 0x0000000000000001 in ?? ()
#7 0x00007fffffffe1a9 in ?? ()
#8 0x0000000000000000 in ?? ()
(gdb) gcore crash2.core
Saved corefile crash2.core
I simply generate core file with gcore and leave gdb. Then I get from the archive the version with debug symbols and I can see all symbols:
$ gdb ./a.out ./crash2.core
Reading symbols from ./a.out...done.
warning: exec file is newer than core file.
[New LWP 15215]
Core was generated by `/home/crash/a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x000000000040097d in f () at main.cpp:8
8 *ptr = *ptr +1 ;
(gdb) bt
#0 0x000000000040097d in f () at main.cpp:8
#1 0x00000000004009e0 in main () at main.cpp:17
(gdb) info locals
curr_ts = 1435322344
ptr = 0x0
Update
if you set backtrace past-main on you will see at least this __libc_start_main. What is above __libc_start_main is not printed if you analyze only core file (possibly even not saved there) saved wit gcore:
$ gdb ./a.out crash2.core
Reading symbols from ./a.out...done.
warning: exec file is newer than core file.
[New LWP 15215]
Core was generated by `/home/crash/a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x000000000040097d in f () at main.cpp:8
8 *ptr = *ptr +1 ;
(gdb) set backtrace past-main on
(gdb) bt
#0 0x000000000040097d in f () at main.cpp:8
#1 0x00000000004009e0 in main () at main.cpp:17
#2 0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
Backtrace stopped: Cannot access memory at address 0x4007d0
(gdb)
But if I reproduce the crash under gdb with my test program (with debug info in it) I can see all (see set backtrace past-main on && set backtrace past-entry on):
$ gdb ./a.out
Reading symbols from ./a.out...done.
(gdb) r
Starting program: /home/crash/a.out
Before f()
Before crash 1435328858
Program received signal SIGSEGV, Segmentation fault.
0x000000000040097d in f () at main.cpp:8
8 *ptr = *ptr +1 ;
(gdb) bt
#0 0x000000000040097d in f () at main.cpp:8
#1 0x00000000004009e0 in main () at main.cpp:17
(gdb) set backtrace past-main on
(gdb) set backtrace past-entry on
(gdb) bt
#0 0x000000000040097d in f () at main.cpp:8
#1 0x00000000004009e0 in main () at main.cpp:17
#2 0x000000314981ed1d in __libc_start_main () from /lib64/libc.so.6
#3 0x00000000004007f9 in _start ()
#4 0x00007fffffffde58 in ?? ()
#5 0x000000000000001c in ?? ()
#6 0x0000000000000001 in ?? ()
#7 0x00007fffffffe1a9 in ?? ()
#8 0x0000000000000000 in ?? ()
(gdb)