Inspecting a Running Janet Process via GDB

It's possible to inspect an already running janet process via gdb, but janet needs to have been compiled with debugging info beforehand.

Flow Summary

Ensure the operating system allows gdb connecting to a running process
Compile janet with debugging info
Create sample janet source file
Run janet with sample file and determine the janet process's PID
Connect to the process using gdb
Perform some fiddling in gdb in preparation
Have fun exploring with gdb

Detailed Steps

From some version of Linux, it may be necessary as root to:
```
# echo 1 > /proc/sys/kernel/yama/ptrace_scope
```
If the previous command was necessary, when done with these experiments, you may (as root again) want to:
```
# echo 0 > /proc/sys/kernel/yama/ptrace_scope
```

Get janet source ready -- for sake of later reference, checkout this commit

$ git clone https://github.com/janet-lang/janet
$ cd janet
$ git checkout a78cbd91dad1066ec82c31cb74b6c4bf2ea44d1b

Compile janet with debugging info -- but first tweak Makefile

$ sed -i "s/^CFLAGS?=-O2/CFLAGS?=-O0 -g3/" Makefile
$ make

Arrange to have a janet file with (os/sleep 60) at the top, e.g.
```
$ echo "(os/sleep 60) (pp {:a 1})" > test.janet
```
Use the debug build of janet to run the test file
```
$ ./build/janet test.janet
```
In another terminal, determine the process id of the running janet process
```
$ pidof janet
```
If there is more than one janet process, use ps to figure out the relevant one.

Start gdb using the determined process id (here it is assumed there is only one running janet process) -- once connected, the janet process may be "paused"

$ gdb -quiet -p `pidof janet`
Attaching to process 216859
Reading symbols from /home/user/.local/bin/janet...
(No debugging symbols found in /home/user/.local/bin/janet)
Reading symbols from /usr/lib/libm.so.6...
(No debugging symbols found in /usr/lib/libm.so.6)
Reading symbols from /usr/lib/libpthread.so.0...
(No debugging symbols found in /usr/lib/libpthread.so.0)
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
Reading symbols from /usr/lib/librt.so.1...
(No debugging symbols found in /usr/lib/librt.so.1)
Reading symbols from /usr/lib/libdl.so.2...
(No debugging symbols found in /usr/lib/libdl.so.2)
Reading symbols from /usr/lib/libc.so.6...
(No debugging symbols found in /usr/lib/libc.so.6)
Reading symbols from /lib64/ld-linux-x86-64.so.2...
(No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
0x00007ff506619762 in read () from /usr/lib/libpthread.so.0

Set an appropriate breakpoint -- here cfun_io_printf is used because pp (which is used in the test file) calls printf which is in turn implemented in terms of cfun_io_printf
```
(gdb) b cfun_io_printf
Breakpoint 1 at 0x5627ea734fe9: file src/core/io.c, line 631.
```

Resume the process via c

(gdb) c
Continuing.

Breakpoint 1, cfun_io_printf (argc=2, argv=0x559c10170568) at src/core/io.c:631
631	    return cfun_io_printf_impl(argc, argv, 1, "out", stdout);

Have fun exploring :)

sogaiu/inspecting-a-running-janet-process-via-gdb.md

Inspecting a Running Janet Process via GDB

Flow Summary

Detailed Steps