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--- gdb [2016/10/16 20:19] – created dblume
+++ gdb [2024/10/22 19:07] (current) – [Attaching to a remote target] dblume
@@ Line 1: / Line 1: @@
 ====== gdb ======
+===== Example code with a deadlock =====
+Build your target with debug symbols. For example, with [[http://git.dlma.com/testcode.git/|the testcode project]], make with the target "debug"
+  make debug
+If the deadlock was compiled in, then run it like so:
+  product/testcode &
+Then you can run gdb and attach to the process in one of the following ways.
+  $ gdb -p <pid-of-testcode>
+  $ gdb product/testcode <pid-of-testcode>
+  $ gdb
+  (gdb) attach <pid-of-testcode>
+===== Tips =====
 Show all the backtraces:
@@ Line 5: / Line 27: @@
   (gdb) set pagination off
   (gdb) thread apply all bt
+Other useful tips:
+  (gdb) t a a f       # thread apply all frame
+  (gdb) t a a bt 3    # thread apply all backtrace bottom three frames
+  (gdb) t a a bt -3   # thread apply all backtrace top three frames
+===== Detecting a Deadlock =====
 Get high level info on the threads:
@@ Line 47: / Line 77: @@
 Note the <nowiki>__owner</nowiki> of the mutex that thread 2 is waiting on. It's 24793. That's thread 3. There's your deadlock.
+==== Attaching to a remote target ====
+  - Deploy gdb server with the remote target. Launch remote target with gdb server.
+  - Untar remote libraries to a local dir. Eg., 487.72E04128A-2371582-rootfs.tar.gz in my ~/Downloads directory.
+    $ /usr/local/arm/bin/arm-linux-gdb builds/myapp.sym
+    GNU gdb (GDB) 7.5.1
+    This GDB was configured as "--host=i686-build_pc-linux-gnu --target=arm-brcm-linux-gnueabi".
+    Reading symbols from builds/myapp.sym...done.
+    (gdb) set sysroot ~/Downloads/rootfs/firmware.obj/root/
+    (gdb) set solib-search-path builds/myapp.dir/
+    (gdb) target remote 10.15.24.54:5555
+    Remote debugging using 10.15.24.54:5555
+    ...
+    (gdb) c
+===== Processing a backtrace from code =====
+When using a define like this...
+<code cpp>
+#define PRINT_BACKTRACE { void *stack[32];                                   \
+    const int nptrs = ::backtrace(stack, sizeof(stack) / sizeof(stack[0]));  \
+    if (nptrs) ::backtrace_symbols_fd(stack, nptrs, STDERR_FILENO);          \
+}
+</code>
+You might get output like this:
+    /lib/libUILib.so(_ZN12FakeSymbolDoesNotMatterEb+0x288)[0xb3831a78]
+    Application[0x4c51d0]
+You can get source code lines if you can calculate the offsets from each module. You can get those module base addresses from /proc/<pid>/maps
+    target:$ egrep " r-xp .*/(Application|libUILib.so)" /proc/$(ps -a | awk '/Application/{print $1}')/maps
+    00010000-00dfb000 r-xp 00000000 00:0d 135649148  /bin/Application
+    b3578000-b3f45000 r-xp 00000000 00:0d 68148928   /lib/libUILib.so
+So, for example, the first frame is at: libUILib.so + (0xb3831a78 - 0xb3578000). You can use addr2line to get that:
+    $ addr2line -e path/to/libUILib.so 0x2b9a78
+    path/to/MySource.cpp:5712