attempt to free, allready freed memory .. rfc

From: Grzegorz Jaskiewicz (gj@pointblue.com.pl)
Date: Mon Mar 31 2003 - 13:07:25 EST


Hello !
As allways, i have a problem. Thus i want ask you for comment.

Today i had a strange problem with my customers program. It is very
fresh soft, and they are still strugling with it (developing i meant :-)
). But they were stucked on very strange problem.

[explanation of problem in program- ommited]

Problem it self.

this code will be an example.

struct doom{
        int data1;
        int data2;
        void *mydata;
};

doom doomed;

void dosomething_with_data(doom *d)
{
// .... doing some fancy operations
// then..
        if (d->mydata)
        {
                free(d->mydata);
                d->mydata=NULL;
        }
}

void main()
{
        int i,j;
        void *t;

        // this program is just not doing anything important :-)
        
        t=doomed.mydata=malloc(somenumberofbytes);
        if (t==NULL){
                return;
        }
        
        dosomething_with_data(&doomed);

// plenty of lines and finally ........
        if (doomed.mydata)
        {
                free(doomed.mydata);
                doomed.mydata=NULL;
        }

// now i will get strange behavior in other structures
// seems like it is a memory leak.....
 
}

well, finding this error in somebody software was not easy.
Finaly i tested simmilar behavior on windows, and... it was not better.
But under compilation with m$ comiler in debug mode gived me assert on
first attempt to free allready freed memory.
Is this behavior system has to take ? (i mean segfault)
which options for gcc i have to use to get assertion in case of such
error (i guess malloc seats in glibc, thus debug version of this liblary
might have this). But i am not sure. Waiting for comments.
  

-- 
Grzegorz Jaskiewicz <gj@pointblue.com.pl>
K4 Labs

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