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Re: Is Forbid() needed for SetMethod()?

Subject: Re: Is Forbid() needed for SetMethod()?
by salass00 on 2019/4/9 12:05:59

Forbid()/Permit() would be needed if the new function acts as a wrapper for the old library function instead of replacing it entirely.


int (*oldFunc)(void);



int newFunc(void) {

    IExec->DebugPrintF("SomeFunc() calledn");

    return oldFunc();

}



/* ... */



    /* Patch SomeFunc() in ISomeLib with newFunc() */

    IExec->Forbid();

    oldFunc = IExec->DoMethod((struct Interface *)ISomeLib, offsetof(struct SomeLibIFace, SomeFunc), newFunc);

    IExec->Permit();

Without the Forbid() in the above example newFunc() might be called before oldFunc has been set, leading to an ISI as it tries to jump to a NULL pointer.

If the function in question is supposed to be callable from interrupts then Disable()/Enable() will need to be used instead of Forbid()/Permit().