Semihosting on ARM with GCC and OpenOCD

One of the many nice features of the ARM Cortex microcontrollers is the ability to use the JTAG debug interface as a sink for printf messages. This capability is known as semihosting. It’s actually quite straightforward to configure with open source tooling (the newlib C standard library and OpenOCD JTAG implementation).

You’ll need to add the following to LDFLAGS,

LDFLAGS += --specs=rdimon.specs -lc -lrdimon

Note that librdimon takes the place of libnosys.

Ensure that initialise_monitor_handles() is called in main before calling printf,

extern void initialise_monitor_handles(void);

int main(void) {
    initialise_monitor_handles();

    printf("hello world!\n");
    // do things
}

If you are using the toolchain’s crt0 initialization function then initialise_monitor_handles has already been called for you.

At runtime enable semihosting in OpenOCD with arm semihosting enable before initialise_monitor_handles is called. Failure to do this will result in a HardFault due to an unexpected debug event. Also be aware that semihosting is disabled on core reset. This effectively means that an image compiled with semihosting enabled will be unable to run in the absence of a debugger (unless you make the call to initialise_monitor_handles contingent on a runtime flag).

You should now see your printf messages show up in the OpenOCD console. You can

Resources

• Very helpful guide by Andrey Yurovsky: https://plus.google.com/+AndreyYurovsky/posts/5rupuziHKGC
• Original openocd-devel thread: http://thread.gmane.org/gmane.comp.debugging.openocd.devel/10941/focus=10958
• libopencm3 example: https://github.com/libopencm3/libopencm3-examples/tree/master/examples/stm32/l1/stm32l-discovery/usart-semihosting
• OpenOCD manual section (search for “semihosting”): http://openocd.sourceforge.net/doc/html/Architecture-and-Core-Commands.html#Architecture-and-Core-Commands