Pointer value changes value at an unexpected place

This is a discussion on Pointer value changes value at an unexpected place within the C Programming forums, part of the General Programming Boards category; In an attempt to debug this problem, I have added printf statements to see what is happening when the code ...

  1. #1
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    Unhappy Pointer changes value at an unexpected place

    In an attempt to debug this problem, I have added printf statements to see what is happening when the code is running.

    The printf statement containing “#103D” gives a good value for “FirstPtr” but then its value changes.

    I am using Borland C++ IDE to write 16 bit code for a picoFlash single board computer with a 186 compatible processor and 512K of Ram. I am adding code to an existing program that takes 183 pages to print. I have checked the “check stack overflow” option. The compiler and linker don’t give any error messages.

    I don’t know what other information I should inclue because I have no idea what is happening.

    How can I get to the bottom of this problem?

    Code:
    void DownloadAFile(char *strDirectory,char *strFileName,char *strRxBlock, int intOffset)
    {
     
                    char *FirstPtr;    //Pointer used to parse the incomming Request
                    char strNrChars[4];          //Number of characters to follow
                    int intNrChars;      //Number of characters following next
                    int intLen;                                                            //Length of string
                    char strLength[4];                                            // 4 char string length
                    char *SPtr;                                             //Pointer to the Line Input String
                    static char strLineInput[1024];    //String used for input
                    char *RspPtr;       //Pointer to strResponseString
                    static char strResponseString[120];
                    char *ShortRspPtr;
                    char strShortResponseString[120];           //Without Rec #
                    char strCommandString[100];
                    char strFullFileName[80];
                    static char strOldFullFileName[80];
                    char strRecordNumber[4];                           //Requested Record Number
                    int intRecordNumber;      //Requested Record Number
                    int intSameFileFlag;
                    static FILE *TxtDLFilePtr;
                    char strTmpStr[80];
                    static int intLastRecordRead;
                    int intLastRecordReadPlus1;
                    int intContinueRecs;                                                       //1 => Continue reading records
                    int intBlockNr;
                    char strRecNr[4];
     
    //Skip code here
                    //Pick intRecordNumber
     
                    FirstPtr = &strRxBlock[0];                                                             //to 1
                    printf("#103 ok not term strRxBlock = %s\n", FirstPtr);        //ok
                    printf("#103B s/b=1 Ptr @ strRxBlock[0] = %p\n", FirstPtr);        //ok
     
                    FirstPtr+= intOffset;             //Beginning of # char in Rec #  to 29
                    printf("#103C s/b=29 Ptr @ Nr char in RecordNr = %p\n", FirstPtr);        //ok
     
    //            TempPtr = FirstPtr;
                    strncpy (strNrChars,FirstPtr,4);                  //Nr chars in Record #   ok
    //The value of FirstPtr printed with the following line is ok = 5E7E:0AE4
                    printf("#103D after strncpy FirstPtr = %p\n", FirstPtr);        //ok
     
                    strNrChars[4] = '\0';
     
    //The value of FirstPtr printed with the following line has changed = 5E7E:0A00
                    printf("#103E after strncpy FirstPtr = %p\n", FirstPtr);        //ok
                    printf("#104 ok strNrChars in Record Nr = %s\n",strNrChars);      //Nr chars in Record # 4 ok
                    printf("#103F after strncpy FirstPtr = %p\n", FirstPtr);        //ok
                    intNrChars = atoi(strNrChars);
                    printf("#103G after strncpy FirstPtr = %p\n", FirstPtr);        //ok
                    printf("#105 ok intNrChars = %d\n",intNrChars);                                               // = 4 ok
                    printf("#103H after strncpy FirstPtr = %p\n", FirstPtr);        //ok
     
    //More code follows
    Last edited by Jerry900; 08-02-2012 at 01:50 AM. Reason: Grammer

  2. #2
    and the hat of wrongness Salem's Avatar
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    Simple buffer overrun.

    char strNrChars[4]; //Number of characters to follow
    ...
    strNrChars[4] = '\0';

    The valid subscripts are 0, 1, 2 and 3
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    ZuK
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    Code:
    strNrChars[4] = '\0';
    Overwrites the array bounds.
    Valid indexes are 0 .. 3

    Kurt
    EDIT: I should be typing faster

  4. #4
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    How embarrassing. Thank you for the quick response.

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