Pointers - Passing to function

This is a discussion on Pointers - Passing to function within the C Programming forums, part of the General Programming Boards category; Hi, I need help with the following: I am using a C API available in a network simulator. Using the ...

  1. #1
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    Pointers - Passing to function

    Hi,

    I need help with the following:

    I am using a C API available in a network simulator. Using the op_ima_obj_svar_get function it takes an object id and the name of the state variable that the user wishes to retrieve and returns a pointer to the variable. Because the state variable can be any type (void*) it must be cast to the appropriate type. DhtT_Key_Bytes is defined in a header file as typedef unsigned char* DhtT_Key_Bytes;

    Code:
    DhtT_Key_Bytes*		nodesNodeID;
    DhtT_Key_Bytes		predec;
    Int				intpredec;
    
    nodesNodeID = (DhtT_Key_Bytes *) op_ima_obj_svar_get(rem_proc_objid, "myNodeID");
    predec = dht_sup_get_node_predecessor_idealised (*nodesNodeID);
    intpredec = sha_get_int_id(predec);
    The above works perfectly. Now here’s the problem: I am passing the same to a different function but get an invalid memory access error:

    Code:
    DhtT_Key_Bytes*		predecessorPtr;
    Int				intPredecessorPtr;
    
    predecessorPtr = (DhtT_Key_Bytes *) op_ima_obj_svar_get(rem_proc_objid, "predecessor");
    intPredecessorPtr = sha_get_int_id(*predecessorPtr);
    The state variables “myNodeID” and “predecessor” are both of type DhtT_Key_Bytes.

    The only difference that I can make out is that the dht_sup_get_node_predecessor_idealised expects a parameter of type DhtT_Key_Bytes whereas the sha_get_int_id expects a parameter of unsigned char digest[ShaC_Bytes].

    I have used sha_get_int_id function before and it’s always worked just fine when passing in a variable of type DhtT_Key_Bytes.

    I think I’m confused with my pointers. Can someone please help me with this?

    Thanks in advance.

  2. #2
    cas
    cas is offline
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    Because the state variable can be any type (void*) it must be cast to the appropriate type.
    You do not need to cast void* when assigning to any object pointer type. C does the conversion for you automatically.

    As for the stated problem: I'm confused about sha_get_int_id(). You say it takes unsigned char[] as an argument, but you're passing DhtT_Key_Bytes. Does your compiler complain when you do this? If so, don't do that! If not, then it would appear that DhtT_Key_Bytes is a typedef for a pointer to unsigned char, which means they're interchangeable.

    At any rate, when you get a memory access error, your first step should be to use a debugger to find out what's going on. If it's available for your platform, Valgrind is a fantastic debugger. Otherwise, you probably have some other sort of debugger available, such as gdb or, if you're using an IDE, some built-in debugging tool.

  3. #3
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    It's the case of the "wild pointer", the double indirect pointer has been initialized to point to an object but not the single indirect one i.e.
    Code:
    char **dp, *sp
    dp->sp
    sp->?

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