I just ran into the same bug (Ubuntu 16.04.4 LTS, fully updated at time of the comment).
I've played with cmd arguments a little and it seems that the bug is triggered at some length of the vnc socket path. This is running okay: $ qemu-system-x86_64 -enable-kvm -vnc unix:./service/vnc-sock Also okay: $ qemu-system-x86_64 -enable-kvm -vnc unix:../test-vm/service/vnc-sock But this fails: $ qemu-system-x86_64 -enable-kvm -vnc unix:../test-vm/../test-vm/service/vnc-sock *** Error in `qemu-system-x86_64': free(): invalid pointer: 0x00007f1cfc939b78 *** ======= Backtrace: ========= /lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7f1cfc5ec7e5] /lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7f1cfc5f537a] /lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7f1cfc5f953c] qemu-system-x86_64(+0x4b228d)[0x55fcafa3a28d] qemu-system-x86_64(visit_type_VncServerInfo+0xa2)[0x55fcafa4c2c2] qemu-system-x86_64(qapi_free_VncServerInfo+0x30)[0x55fcafa39890] qemu-system-x86_64(+0x44184a)[0x55fcaf9c984a] qemu-system-x86_64(+0x446953)[0x55fcaf9ce953] qemu-system-x86_64(+0x446b35)[0x55fcaf9ceb35] qemu-system-x86_64(aio_dispatch+0x68)[0x55fcaf9f0988] qemu-system-x86_64(+0x45bc1e)[0x55fcaf9e3c1e] /lib/x86_64-linux-gnu/libglib-2.0.so.0(g_main_context_dispatch+0x2a7)[0x7f1cfd2cd197] qemu-system-x86_64(main_loop_wait+0x18b)[0x55fcaf9ef28b] qemu-system-x86_64(main+0x17b4)[0x55fcaf769714] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f1cfc595830] qemu-system-x86_64(_start+0x29)[0x55fcaf76eda9] Versions: ii ipxe-qemu 1.0.0+git-20150424.a25a16d-1ubuntu1.2 all PXE boot firmware - ROM images for qemu ii qemu-block-extra:amd64 1:2.5+dfsg-5ubuntu10.25 amd64 extra block backend modules for qemu-system and qemu-utils ii qemu-kvm 1:2.5+dfsg-5ubuntu10.25 amd64 QEMU Full virtualization ii qemu-system-common 1:2.5+dfsg-5ubuntu10.25 amd64 QEMU full system emulation binaries (common files) ii qemu-system-x86 1:2.5+dfsg-5ubuntu10.25 amd64 QEMU full system emulation binaries (x86) ii qemu-utils 1:2.5+dfsg-5ubuntu10.25 amd64 QEMU utilities -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1705743 Title: qemu-system-x86 crashes when VNC connection is established To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1705743/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs