CODEWITHSUNDEEP
c++gdbcore-file
Tito
Tito

Reputation: 2300

GDB reading symbols with "symbol-file" command on a core file

I am trying to analyze segfault on a core file on linux. I am not sure if the following behavior is correct, thus i deliberately caused a segfault using 

#include <signal.h>
int main() {
raise(SIGSEGV); 
}

the binary is build with debug info i.e.

file mainTestFile 
mainTestFile: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/l, for GNU/Linux 3.2.0, with debug_info, not stripped

notice how it does say "with debug_info, not stripped" at the end

when i execute the binary i get a core file generated which is called core-mainTestFile.20474 (In order to generate the core file i hat to set my ulimit to unlimited i.e. ulimit -c unlimited )

if i run only the binary under GDB and do backtrace "bt" then i get the segfault and i get all names of the functions involved printed nicely i.e. notice how the gdb says when starting "reading symbols from ./mainTestFile...done."

gdb ./mainTestFile
GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git

....

reading symbols from ./mainTestFile...done.
(gdb) run
Starting program: /src/exe/mainTestFile 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".


Program received signal SIGSEGV, Segmentation fault.
__GI_raise (sig=<optimized out>) at ../sysdeps/unix/sysv/linux/raise.c:51
51      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=<optimized out>) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x0000000000402dad in main (argc=1, argv=0x7fffffffda38) at /src/exe/main.cpp:53
(gdb)

however if i try to anaylise only the core file with gdb like that 

gdb -c core-mainTestFile.20474

then i get only question marks

the when i execute "bt" then i do not see the names of the methods, instead i get question marks

(gdb) bt
#0  0x00007f34d8842e97 in ?? ()
#1  0x0000000000000000 in ?? ()

they only workaround i found is to supply the binary directly at the command line then it all gets printed nicely. even if i try to tell GDB to use the symbols file and point that to the binary file which does have the symbols i.e.

symbol-file /src/exe/mainTestFile

then GDB says 

Reading symbols from /src/exe/mainTestFile...done

and when i execute bt i see the question marks again? Why is that. Is GDB not able to get the symbols out of the binary?

it only works if supply the binary directly on the command like like:

gdb /src/exe/mainTestFile -c core-mainTestFile.20474

my question is should the GDB be able to read symbols of the binary when directly supplying him the binary over the "symbol-file" command or not. Why is this working when supping him directly the binary over the command line, what is the difference?

Upvotes: 5

Views: 7442

Answers (1)

Employed Russian
Employed Russian

Reputation: 213375

should the GDB be able to read symbols of the binary when directly supplying him the binary over the "symbol-file" command or not.

In theory, using symbol-file and core-file commands in either order in GDB should be equivalent.

But there is a bug: symbol-file followed by core-file works, and the opposite order doesn't.

Since generally the end-user can always rearrange his commands into the order that works, this has never propagated to the top of any GDB developer's queue of things to fix.

Related bug (but not an exact duplicate).

Upvotes: 4

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