Debugging help, pointer forwarding

This is a discussion on Debugging help, pointer forwarding within the C Programming forums, part of the General Programming Boards category; There's the code under, but I'm getting an invalid read size of 4, and it seems because I forwarded the ...

  1. #1
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    Debugging help, pointer forwarding

    There's the code under, but I'm getting an invalid read size of 4, and it seems because I forwarded the pointer a little far ahead... I think.
    It's just a little part of my network code, but it's pretty easy to understand what I'm doing under right?

    LSP_buffer is a char *, that's why I have to cast it.

    allocated memory for a struct LSP and how many counts edges there is around this router(nodes).

    Not so sure if I thought it out right, by adding sizeof(struct LSP) to pointer it would mean forwarding the pointer by so many bytes as the size of struct right?, why then would I get "Invalid write of size 4" ?

    thanks for replying.


    Code:
       LSP_buffer = malloc((sizeof(struct LSP) + (sizeof(struct EDGE) * count))); 
        lsp = (struct LSP*) LSP_buffer;                                        /*cast it to struct LSP and write in the necessary info about this LSP*/
        lsp->source = htonl(own);
        lsp->seq = htonl(LSP_generation++);                                    /*set which generation this LSP is, and increment it*/
        lsp->nodes = htonl(count);
        node = (struct NODE *) (LSP_buffer + sizeof(struct LSP));                /*cast LSP_buffer to struct EDGE and move the pointer forward so we don't overwrite any data*/
       
        fprintf(stderr, "Own address: %d LSP generation is %d\n", own , LSP_generation);
        
    
        /*store info about the edges around this router*/
        for(i = 0; i < MAX_ADDRESSES; i++){     
          if(distance[own][i].cost > 0){
        node->address = htonl(i);   <---------------------------    
        node->cost = htonl(distance[own][i].cost);
        node = node + sizeof(struct NODE);                                /*forward pointer by sizeof struct NODE so we don't overwrite old NODE data*/
          }
        }

  2. #2
    - - - - - - - - oogabooga's Avatar
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    I see two things.

    1. On line 6 you cast it as a (struct NODE *), but maybe it should be (struct EDGE *).

    2. On line 16, you add in sizeof(struct NODE). But since node is of type (struct NODE *), simply adding 1 will move it forward by sizeof(struct NODE) bytes. Instead you are moving it forward by sizeof(struct NODE *) * sizeof(struct NODE *) bytes!
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  3. #3
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    Your're right, struct EDGE has only an int and NODE to have two integers, an ID number and cost, so NODE was twice the size of EDGE, no wonder I got invalid write size by 4 byte (integer -> 4 byte).
    and because it's pointer, simply adding 1 will move it forward, gotta go back to the basic:P.

    Thanks for the fast reply! Appreciate it.

    What happens with the thread now when my problem is solved?

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