Why is this free corruption the heap?

This is a discussion on Why is this free corruption the heap? within the C Programming forums, part of the General Programming Boards category; Hi there, Doing something very simple here, not sure why I'm getting heap corruption at run time. This function was ...

  1. #1
    Registered User
    Join Date
    Jan 2010
    Posts
    5

    Why is this free corruption the heap?

    Hi there,

    Doing something very simple here, not sure why I'm getting heap corruption at run time.
    This function was much longer but I stripped it down to figure out what's wrong::

    Code:
    bool DeleteRecord(void){
    
    	StudentRecord** delTemp; // temporary copy of DB used to resize array when adding
    
    	delTemp = (StudentRecord**)malloc(g_numRecords * sizeof(StudentRecord*));
       for(count=0;count<g_numRecords-1;count++){
    		delTemp[count]=(StudentRecord *)malloc(sizeof(StudentRecord));
    		delTemp[count]->firstname=(char *)malloc(sizeof(char));//malloc fname 
    		delTemp[count]->lastname=(char *)malloc(sizeof(char));//malloc lname
       }
    	printf("malloced\n");
    	delTemp[0]->firstname="........yous";
    	printf("delTemp[0]->firstname=%s\n", delTemp[0]->firstname);
    	delTemp[0]->lastname="youtwo";
    	printf("delTemp[0]->lastnamename=%s\n", delTemp[0]->lastname);
    	delTemp[0]->id=22;
    	delTemp[0]->mark=33;
    	printf("deltemp 0 id, mark: %d, %f\n", delTemp[0]->id,delTemp[0]->mark); 
    
    	free(delTemp[0]->firstname);
    	printf("freed fname\n");
    	free(delTemp[0]->lastname);
    	printf("freed lname\n");
    	free(delTemp);
    	printf("freed deltemp\n");
    
    
    
    	return true;
    }
    Code:
    struct StudentRecord
    {
    	char* firstname;
    	char* lastname;
    	int id;
    	float mark;
    };
    I get heap corruption trying to free the memory I just malloced?
    Any suggestions?
    Thanks!!!

  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Posts
    14,826
    You don't need to have a "-1" in there. Why are you using a pointer to a pointer?
    Code:
    struct foo *bar;
    bar = malloc( N * sizeof *bar );
    for( x = 0; x < N; x++ )
    {
        bar[ x ].a = b;
        bar[ x ].y = z;
    }
    ...
    
    free( bar );
    I'm not sure why you're using a pointer to a pointer here.
    delTemp[count]->firstname=(char *)malloc(sizeof(char));//malloc fname
    delTemp[count]->lastname=(char *)malloc(sizeof(char));//malloc lname
    Why are you allocating 1 char for each of their first and last names? Did you mean to actually have more than one character?


    Quzah.
    Hope is the first step on the road to disappointment.

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