[Qemu-devel] [Bug 1216845] Re: qemu-system-arm semihosting always calls exit(0)

[Karl Zimmerman]

Had a similar problem with my emulation environment.  However, I did
some inspection of the assembly code generated by newlib for ARM semi-
hosting.  While it initially appears that exit() and _exit() discard the
status code, upon careful inspection one finds that it is pushed on the
stack, with the SP pointing right to it at the point at which the SWI is
executed.

Thus, if the code passed to 0x18 is 0x20026, you can fetch the status
code passed to exit() from the stack.

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Title:
  qemu-system-arm semihosting always calls exit(0)

Status in QEMU:
  New

Bug description:
  In my embedded ARM project I have a bunch of unit tests that I run in
  a POSIX synthetic environment, and, as usual for POSIX processes,
  these tests return 0 for success and !=0 for error.

  Now I expanded the testing environment to run some of these tests
  compiled for ARM, under QEMU, with the tracing messages forwarded via
  the semihosting API.

  Up to now everything is fine with the emulation.

  However I have a problem with passing the failure code back to the
  operating system, to drive the continuous integration framework.

  I checked the arm-semi.c code and for SYS_EXIT and I discovered that
  the parameter passed is ignored and it always calls exit(0):

      case SYS_EXIT:
          gdb_exit(env, 0);
          exit(0);

  To solve my problem I temporarily made a patch, and for cases that
  should return non zero codes, I call an unsupported BKPT instruction,
  which makes QEMU abort, and pass an non zero code (1) back to the
  operating system.

      qemu: Unsupported SemiHosting SWI 0xf1

  This kludge is more or less functional, but is quite inconvenient.

  After checking the ARM manuals, I discovered that SYS_EXIT is not
  standard, and the 0x18 code used for it originally was used for
  angel_SWIreason_ReportException, which has a slightly different
  purpose.

  Now the question:

  Would it be possible to no longer ignore the code passed to 0x18, and
  if it is non zero, to call exit() with a different value?

  The suggested rule would be:

  if (code ==0 || code == 0x20026)
    exit(0);
  elif (code < 256)
    exit(code);
  else
    exit(1);

  The value 0x20026 means ADP_Stopped_ApplicationExit, and, if I
  understood it right, it means that the program terminated
  successfully. If this is not true, it can be removed from the first
  conditional statement.

  What do you think? Can this be added to arm-semi.c?

  
  Regards,

  Liviu

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