GDB and QEMU on Windows

A few weeks ago I started to work on a small operating system for a MIPS-based development motherboard. When thinking about a development toolchain, I immediately looked at one of my favorite emulators – QEMU.

QEMU has a few nice features that make development of operating systems easier than ever. One of these features is the -kernel command line parameter that loads a custom operating system kernel right into memory without the need to write a custom boot loader. Another useful command line option is -s which starts a GDB server inside QEMU so you can connect to it with GDB (with the command target remote :port_number) and debug your loaded kernel with full symbols.

At first, this didn’t work for me. GDB refused to connect to the server for no apparent reason (No connection could be made because the target machine actively refused it.). It took me almost half an hour to figure out where the problem was: QEMU was opening an IPv6-only port and GDB was using IPv4. Quick fix: open up gdbstub.c in QEMU sources, locate the line where the connection string is being created (in QEMU 0.10.2 it’s the line 2300; there is a string that says: tcp::%d,nowait,nodelay,server) and fix the connection string to look like this: tcp::%d,nowait,nodelay,server,ipv4.

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