Enable Semihosting in ARM DS 2024, ARM Compiler

Hi again

The C/C++ libraries that are provided with Arm Compiler for Embedded 6 are semihosted by default, so all the usual C stdio functions will just work.  See

https://developer.arm.com/documentation/101470/2024-1/Controlling-Target-Execution/Using-semihosting-to-access-resources-on-the-host-computer

For example:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    const char *filename = "/home/stheobal/test/out.txt";

    FILE* fp = fopen(filename, "wt");
    if (!fp) {
        printf("Could not open file %s for writing", filename);
    }
    else {
        printf("fopen success\n");
    }

    char str[] = "Hello World!\n";

    fprintf(fp, "%s", str);
    fflush(fp);
}

Be aware that semihosting allows code running on a target platform to access files and execute system commands on a host machine, potentially allowing sensitive data on the host to be exposed.  Before you use semihosting, you must ensure that you assess and mitigate the security implications.  For example, a semihosted application running on an FVP model may access files on its host machine, unless semihosting is explicitly disabled in the FVP model.  Similarly, a semihosted application running on a hardware target may access files on a debugger host that is connected by a debug agent such as DSTREAM-ST.  For the latter case, the debugger has new controls to configure the semihosting security policy.  In Arm DS 2024.1 and later, by default, if semihosting is enabled in the debugger, the debugger prevents semihosting from accessing files and executing system commands on the debugger host. The policy can be adjusted with the "set semihosting policy warn|allow|block" command.  See:

https://developer.arm.com/documentation/101471/6-4-0armds/Arm-Debugger-commands/Arm-Debugger-commands-listed-in-alphabetical-order/set-semihosting

Hope this helps

Stephen

Hi Stephen and thank you for the information.

I tried similar code, and I tried yours, too.

Although I enabled Semihosting and Policy, 

set semihosting enabled on
set semihosting policy allow

It still cannot read and write the file, I get the error:
Could not open file C:\test\out.txt for writing

Is there anything that should be changed in ARM DS or in the code?

Hi again

Apologies, the example I gave yesterday contains a couple of errors:

1) For the fopen mode, the non-ANSI option 't' is not supported.  So "wt" should be just "w" (or "wb" for a binary file). See

https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst

2) The fprintf call should (obviously!) have been inside the else clause

Here's the corrected version, which works for me on Windows, when the debugger is connected to real hardware, and "set semihosting policy allow":

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    const char *filename = "C:\\Temp\\semihosting\\out.txt";

    FILE* fp = fopen(filename, "w");
    if (!fp) {
        printf("Could not open file %s for writing", filename);
    }
    else {
        printf("fopen success\n");
        char str[] = "Hello World!\n";

        fprintf(fp, "%s", str);
        fflush(fp);
    }
}

You shouldn't need to "set semihosting enabled on" explicitly when using the Arm Compiler for Embedded 6 toolchain, however, it is harmless to do so.  The Debugger enables semihosting automatically if either symbols __auto_semihosting or __semihosting_library_function are present in an image. Arm Compiler for Embedded 6 adds __semihosting_library_function automatically to an image at link time if that image uses any semihosting-using functions. If compiling with GCC or LLVM, use an alias symbol definition such as void __auto_semihosting(void) __attribute__ ((alias("main"))); or turn on semihosting support in the Debugger explicitly with "set semihosting enabled on".

Hope this helps

Stephen

Hi Stephen

I noticed "wt" and changed it to "w" before I run the file.
However, I started debugging using your code and another files that I had been created.
Although I get "fopen success Hello World!" print in the app console, I can see no folder is created, and even if I create the folder and the file, it would not write anything on that.

Is there any options that I have to check in ARM DS? (for example in debugging section)

As you know I am using Arm Compiler for Embedded 6 and I did these changes on ARM DS commands:
set semihosting enabled on
set semihosting policy allow


Thank you in advance for your kind cooperation.

Hi again

Apologies for the delay in responding.  Are you still having problems with this?

I've re-tested the code above and it works for me.  Here's what I did (on my Windows laptop):

1) Launch Arm DS
2) Import the startup_Cortex-M7_AC6 example
3) Replace its main() with the main() I gave above
4) Compile the project
5) Create a folder C:\Temp\semihosting
6) Modify startup_Cortex-M7_AC6-MPS2.launch to use a known-good Cortex-M7 board via DSTREAM
7) Connect/Debug
8) Enter "set semihosting policy allow"
9) Run

"fopen success" is printed in the App Console, and "Hello World!" is written to out.txt.

Can you try repeating the above with your own board?

Stephen

Hi and thank you for your support! highly appreciated!

After we discussed I tried the procedure until I found that I have to Enter "set semihosting policy allow" just before run.
Then I put this command in run debug initialization, so the problem is solved.

Warm regards