ARM: semi-hosting support for stderr

Hello,

I propose this small patch so that ARM semi-hosting handles stderr as expected when linking with Newlib/libgloss.

index 9549e6c..6874036 100644
@@ -211,8 +211,11 @@ uint32_t do_arm_semihosting(CPUState *env)
         if (strcmp(s, ":tt") == 0) {
             if (ARG(1) < 4)
                 return STDIN_FILENO;
-            else
+            else if (ARG(1) == 4)
                 return STDOUT_FILENO;
+           else
+             return STDERR_FILENO; /* See newlib/libgloss
+                                      implementation.  */
         }
         if (use_gdb_syscalls()) {
             gdb_do_syscall(arm_semi_cb, "open,%s,%x,1a4", ARG(0),

Christophe

Ping?

Apologies for this rather belated reply, but I only recently managed to confirm the relevant information...

Unfortunately there is no means in the semihosting ABI to distinguish app stdout from stderr. Effectively semihosting defines a single input channel and a single output channel, not three stdin/stdout/stderr channels. All the implementations of semihosting that I am aware of work this way, so it's not just an omission from the semihosting documentation.

Although newlib/libgloss use different modes for opening stdout and stderr, the ARM C library implementation does not, for instance; so your patch is relying on a detail of implementation of a particular semihosting user. So I don't think we can apply this patch, because it could break other (nonlibgloss) users of semihosting.

-- PMM

Thanks for your answer.

I think we could argue it's a bug in the ARM semihosting spec, so maybe we could request them to change this. However, I am unsure how to do this efficiently, as I recently tried with little success to have them change the "angel_SWIreason_ReportException" such that at least ADP_Stopped_AplicationExit can have the program return code as an extra parameter... (anyway I have another small Qemu patch to handle this situation by returning the contents of r0 when the program entered exit(). I can submit it if someone is willing to review it).

Christophe.