Segmentation fault with input test file

This is a discussion on Segmentation fault with input test file within the C Programming forums, part of the General Programming Boards category; OK, there's a bug here: Code: void add_student(int loopnum) { char unit_name[101]; int low, student_ID; Position P; low=0; scanf("%d %s", ...

  1. #16
    End Of Line Hammer's Avatar
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    OK, there's a bug here:

    Code:
    void add_student(int loopnum)
    {
        
        
        char unit_name[101];
        int low, student_ID;
        Position P;
        low=0;
        
        scanf("%d %s", &student_ID, unit_name);
    
        units[loopnum] = (unit_struct*) malloc(sizeof(unit_struct));
        units[loopnum]->name = malloc(sizeof(unit_name)+1);
       
        while(strcmp(units[low]->name, unit_name) !=0)
        {
           
           low++;
        }  
       
        if(units[low]->IDs == NULL)
        {
           units[low]->IDs = MakeEmpty(NULL);
           P = Header(units[low]->IDs);
        }
        
        InsertInOrder(student_ID, units[low]->IDs);
    
        printf ("AddStud: pointer P pointing to: %p\n", P);
    
        P = Advance(P);
                
    }
    The problem is that P is only assigned a value if (units[low]->IDs == NULL). If that condition is not true, P does not get assigned.

    In either case, you use P in the call : P = Advance(P); which is wrong!

    I put a printf statement in to display the contents of the pointer to prove it:

    Code:
    1
    a
    4
    1
    a
    AddStud: pointer P pointing to: 0x970c10
    4
    2
    a
    AddStud: pointer P pointing to: 0x0
    Segmentation fault (core dumped)
    Also, the Advance function does nothing for this variable as it's local to the add_student function. I'm not sure what you're trying to do with it.

    Have fun fixing......
    Last edited by Hammer; 04-28-2002 at 07:24 PM.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  2. #17
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    ..but if i remove that if statement, then the list is just going to be emptied every time that function is called isn't it?

  3. #18
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    Originally posted by bentles
    ..but if i remove that if statement, then the list is just going to be emptied every time that function is called isn't it?
    Memory allocated via calls to malloc will remain in existance until you free() it. You have to keep a pointer that points to the start of the list, but that can exist globally or be passed from function to function.

    I haven't checked to see if your memory management is correct or not!

    That particular call to the advance func certainly does nothing. P is declared within add_student, and is therefore destroyed when you leave that function. If it was containing a value you needed, you would have to either assign it to an external variable, or return it to the caller.

    As a quick fix, I run the prog without that line, and it appeared to work OK.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  4. #19
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    I dont even need the p variable, its not serving any purpose whatsoever..

  5. #20
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    ....doesnt fix the problem tho..

  6. #21
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    Oh thank you lord 1!! I figured it out. Thanks Hammer ! I fixed it by making a global variable position P in the header and used the following: P=First(units[low]->IDs after the InsertInOrder call in add_student to move the position to the next position.
    WOOHOO!!

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