Using GDB with the Julia compiler

Context

When starting to dive in the Julia internals, you will encounter C and C++ files at some point. Debugging these can be a pain... Without the right tools. And GDB is definitely one of them. It can be scary to use though... Especially when there is Julia running on top!

Things aren't that complicated though. See for yourself!

(Note that these are instructions for Linux. They should be somewhat reproducible with Windows/MacOS, but I can't guarantee it.)

Connecting GDB and Julia

Attaching GDB to a running Julia REPL

• In your Julia REPL, type getpid() to get Julia's current process id.

• In another terminal, type sudo gdb --pid JULIA_PID, replacing JULIA_PID with the process id you just got. sudo is required to attach gdb to the Julia process like this.

• Setup breakpoints: you can type break function_name to break when the function is called, or break path/to/file.cpp:1234 to break when the line in the given file is reached.

• Type c (short for continue) to let the Julia process continue.

• You are good to go! Follow these debugging tips to get started.

The easier way: launching Julia from gdb

The method above sure works, but it requires two terminals (might be annoying) and sudo (might be problematic). You can also simply type gdb julia, starting a new gdb session. Set your breakpoints, and then run will actually start Julia and you will switch to its REPL. When encountering a breakpoint, you switch back to the gdb console. You can go back and forth like this.

The lack of simultaneous control on gdb and julia can be annoying, but this is quicker to set up. The choice is yours!

Some tips

Printing variables

More info here.

• For jl_value_t * variables, you can type call jl_(my_var) in GDB to print my_var in the Julia REPL.

• For LLVM variables, you can type call my_var->dump() in GDB to print my_var in the GDB terminal.

GDB caveats

• Typing Enter with an empty line in GDB repeats the last command. And commands are buffered! Meaning, if you type Enter 5 times after running continue, it will continue 5 times after hitting a breakpoint, effectively ignoring your 5 next breakpoints. Do not rage type Enter!