Posted by tarek bouchkati on 2016-02-11 11:42

Yes, this incomplete configuration is to enable semihosting (not SWO/SWV) :
you have missed the : -lc -lrdimon
the complete linker flags are :

-specs=nano.specs -u _printf_float -specs=rdimon.specs -lc -lrdimon

To use SWO/SWV:
in syscalls.c, add #include “stm32f4xx.h” (depends on your device) & (you should include CMSIS folder)
and modify the _write function :

int _write(int file, char *ptr, int len)
{
	int DataIdx;

	for (DataIdx = 0; DataIdx < len; DataIdx++)
	{
	   ITM_SendChar( *ptr++ );
	}

	return len;
}

After that, you can use printf to output message via printf.

To visualize the ITM output, you can use ST-Link Utility (menu : ST-LINK > Printf via SWO Viewer)

Best Regards,
Tarek