printf Through Semihosting Under SW4STM32

Most embedded systems does not come with a display. To print/dump useful information, like log, for debugging purpose can be a bit tricky. However GCC, GDB, and OpenOCD provide semihosting to enable dumping of the information onto the the debugging console instead. The feature must be manually enabled. The following steps show how to do it (I used the idea from here).

First, we need to tell the linker to link with libc library (from nanolib C) that provides printf function and also to link with librdimon library that does the semihosting. This is done by configuring the the linker flag. To do that under SW4STM32 IDE, click on Project Properties, select C/C++ Build, then Settings. In the MCU GCC Linker menu, select Miscellaneous, then update the Linker flags field with:

-specs=nosys.specs -specs=nano.specs -specs=rdimon.specs -lc -lrdimon

In Debug Configurations add the following option in the Startup tab. To get there, click on Run and then Debug Configurations...:

monitor arm semihosting enable

The initialization script will be run everytime the debugger (GDB) is fired up. The command tells the OpenOCD to enable ARM semihosting.

Add the following function prototype and function call. The initialise_monitor_handles() call must be before any printf call. The function initializes the hardware and software to handle semihosting:

Include stdio.h header file in main.c. The header is required to use printf():

Add printf() call to print any message desired. Note that \n is important to force the message to be flushed to the console. Otherwise the message will not appear.

Run the program under debug mode. The Hello, world! should be printed in the OpenOCD console like the following:

It is fine with printing integer values. But it does not play well with floating point value:

To allow the printing of floating point values, in Debug Configurations remove the following option, as suggested here:

-specs=nano.specs

Now, it should be able to get printed:

The only catch is that the code size and the data size are increased by about 20KB and 2KB, respectively.

sscanf Integer, Float, and Double

With the above change to the linker flag, we can also convert integer, float, and double values in a string and store them into some variables using sscanf().

Note that for reading a double, sccanf() requires %lf instead of %f as the type field. However, printing a double by printf() uses %f as the type field, which is the same for printing a float. Also the code size is increased by an additional of about 20KB.

Debugging

Resetting Program

[Update (27/06/2017): New version of SW4STM32 IDE has a button on the toolbar to reset the MCU. However, the following serves as informational text.] To reset the program under debugging mode, click on the GDB console (in Debug perspective):

and type:

tb main
monitor reset halt
c

The following shows how it looks like:

The tb instruction is to create a temporary breakpoint at main(). Note that temporary breakpoint means that once the breakpoint has been triggered, it is automatically discarded. To create a permanent break point, use b instead. The next line asks the monitor to reset and halt the MCU. The last line, requests GDB to continue (resume running).

To view the list of breakpoints, type info breakpoints and you get something like the below:

Num     Type           Disp Enb Address    What
13      breakpoint     del  y   <PENDING>  main; monitor reset halt; c
15      breakpoint     keep y   0x08000e2a in main at ../Src/main.c:102
16      breakpoint     keep y   0x08000e1e in main at ../Src/main.c:76
17      breakpoint     keep y   0x08000d9c in SystemClock_Config at ../Src/main.c:121

The numbers on the leftmost are the IDs of the breakpoints. To delete a breakpoint, say breakpoint 15, type del 15.

Other Useful GDB Commands

There are a number of useful GDB commands that you might want to try out:

info registers
list main
x                         # Step 1 instruction    
x/5i                      # Step 5 instructions
p/x $pc                   # Print PC (Program Counter) in hex   

References

1. http://bgamari.github.io/posts/2014-10-31-semihosting.html
2. https://community.st.com/thread/41124-floating-point-support-in-sscanf
3. http://wiki.stm32duino.com/index.php?title=Blue_Pill
4. http://wiki.stm32duino.com/index.php?title=STM32_Smart_V2.0
5. https://mcuoneclipse.com/2014/09/11/semihosting-with-gnu-arm-embedded-launchpad-and-gnu-arm-eclipse-debug-plugins/