Guidelines and HOWTOs/Debugging/Debugging IOSlaves

This page describes how you can debug a KIO ioslave.

How to Get Debug Output

GUI (Qt5/KF5 instructions)

  1. Launch the kdebugsettings tool, either from terminal or from KRunner (the latter can be invoked with Alt + F2)
  2. Type the name of the KIO Slave (e.g. ftp, http, ...). (If it's not listed, it doesn't use categorized debug output, so either its output is always on, or commented out in the code, in which case it needs to be ported to qCDebug)
  3. From the drop-down menu next to each KIO Slave you're interested in, select Full Debug
  4. Press OK to close the dialog and the save your changes
  5. You can either run kdeinit5 in terminal, in which case the output from any IO slave that's started afterwards will show up in that terminal OR
  6. Alternatively log out then log back in, and additional debug info will typically end up in ~/.X.err or ~/.xsession-errors


  • If you have rebuilt an IO Slave from source with some changes you'll need ensure that kdeinit5 will pick it up; this works with KIO from the KDE repos:
cd <path to build dir>/bin
    • in the first cd command you change to the directory containing the compiled binaries, in KIO case it's <build dir>/bin.
  • Instead of using kdebugsettings, you can change the Qt logging categories rules temporarily, in terminal:
export QT_LOGGING_RULES="*kio*=true"
    • for more information see this.

GUI (Qt4/kdelibs4 instructions)

  1. Press ALT+F2.
  2. Enter 'kdebugdialog --fullmode' without the quotes and press enter.
  3. Select the desired number in the "Debug area", e.g. 7103 for http.
  4. In the [Information] box, select "File" as the output.
  5. Enter the desired file name, e.g. /tmp/kio_http.log.
  6. Press OK to close the dialog.
  7. Press ALT+F2, type kdeinit4 and press enter or alternatively log out of KDE and log back in.

This will print additional debug info to the stderr of your kdeinit process, which typically ends up in ~/.X.err or ~/.xsession-errors

Manual (Qt4/kdelibs4 instructions)

It is useful to redirect the debug output of your particular slave to a file instead of stderr. E.g. I myself use the following lines in $KDEDIR/share/config/kdebugrc.


This redirects all debug info for areas 7103 and 7113 (as used by kio_http) to the file /tmp/http.

To get debug information from the SMB slave you can add the following to kioslaverc:


This will print additional debug info to the stderr of your kdeinit process, which typically ends up in ~/.X.err or ~/.xsession-errors

How does an io-slave get started?

Your application requests 'klauncher' via DBus for a slave. If 'klauncher' does not have an idle slave ready, it will ask kdeinit to start a new one. kdeinit forks and dlopens the library that contains the io-slave. Then it calls a function called kdemain() in the library.

Attaching gdb to an io-slave

Due to the above sequence it is rather hard to get an io-slave in your debugger. But wait there is hope. You can start klauncher in such a way that slaves for a certain protocol (the first parameter of KIO::SlaveBase() constructor of the slave class) are started in debug mode.

E.g. to start all 'http' slaves in debug mode, you type:

KDE_SLAVE_DEBUG_WAIT=http kdeinit5

This will restart 'kdeinit' and 'klauncher'.

Note: The string after the equal signal designates the name of a service, not the name of the slave! E.g. if you want to debug the kio_imap4, you must use:

KDE_SLAVE_DEBUG_WAIT=imap kdeinit5

For example, these commands don't work:

KDE_SLAVE_DEBUG_WAIT=imap4 kdeinit5

When your application now requests a http slave, the slave will be started by kdeinit, but before it calls kdemain() (cq. main()) it will suspend the slave by sending it a SIGSTOP signal.

In the terminal from which you started kdeinit you will get the following message:

kdeinit: Suspending process
kdeinit: 'gdb kdeinit 16779' to debug
kdeinit: 'kill -SIGCONT 16779' to continue

You can now debug your slave by typing (or pasting) 'gdb kdeinit 16779' in a terminal.

Note that modern linux kernels disable ptrace. If gdb says "ptrace: Operation not permitted." then you need this:

echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

If you don't want to debug a slave you can let it continue by sending it a SIGCONT by typing 'kill -SIGCONT 16779'. Be aware that slaves will not be killed while they are suspended.

Once you have started gdb, you can set e.g. breakpoints and then resume the slave by typing 'continue'. The debugger will return immediate with a message that a SIGSTOP has been received so you will have to type 'continue' a second time.

See also Windows-specific notes on debugging io-slaves.

Debugging io-slaves with valgrind

KLauncher can be told to run certain io-slaves through valgrind. The following command can be used to let klauncher run all https io-slaves via valgrind:

KDE_SLAVE_VALGRIND=https kdeinit5

The valgrind output will appear as the stderr output of the kdeinit process. The $VALGRIND_OPTS environment variable can be used to pass options to valgrind. If you want to use a different skin:

KDE_SLAVE_VALGRIND_SKIN=callgrind      ( for example )

specific kioslaves

This page was last edited on 13 May 2020, at 12:29. Content is available under Creative Commons License SA 4.0 unless otherwise noted.