Question

This is my first time posting a question to the forum, by the way.
I’m having problems implementing an algorithm, and I’ve narrowed it down to the following lines of code:

int Jacobi( double** A, double* b, int n, double* x0,
double tol, int maxInt ) {
    int i = 0;
    int j = 0;
    int done = 0;
    int loopCount = 0;

    /*previous x variable*/
    double* xPrev = 0;

    /*update information*/
    double** T = 0;
    double* c = 0;

    /*initialize x previous to a very large value*/
--->xPrev = ( double* )malloc( sizeof( double ) * n );
    for( i = 0; i < n; i++ ) {
            xPrev[ i ] = 5000.0;
    }
    ...
}

By inspection via gdb, I have found that the line with the arrow pointing to it is the one that is causing the trouble. Before that line is executed, x0[ 1 ] = 1. Afterwards, it is somehow changed to x0[ 1 ] = (an extremely small number that I think is the minimum double precision value). I can’t figure out why this is happening, or how it is possible. Does anyone have any insight?

Here is the gdb run to prove it:

(gdb) break 88
Breakpoint 1 at 0x804881f: file linsys.c, line 88.
(gdb) run
Starting program: /home/stu1/s11/gaw9451/Courses/AP/hw4/linsys_test

Breakpoint 1, Jacobi (A=0x804b008, b=0x804b048, n=2, x0=0x804b060,
tol=9.9999999999999998e-13, maxInt=8) at linsys.c:88
88              xPrev = ( double* )malloc( sizeof( double ) * n );
(gdb) display x0[ 1 ]
1: x0[ 1 ] = 1
(gdb) next
89              for( i = 0; i < n; i++ ) {
1: x0[ 1 ] = 5.3049894774131808e-313

On a possibly related note, I get an error at run time when I free the variable xPrev at the end of the function. I had to comment it out to see any output from my program.

Summary: Does anyone have any idea how malloc can edit data in a completely different variable field?

Thanks in advance,
phoenixheart6

Answer 1

I am pretty sure you messed up a previous malloc, something like allocating less than needed and now malloc overwrites what never belonged to you.

Picture the malloc memory like this.

   +-----------------------------------------------------+
   |xxxxxxxxxx|!!!!!!!|??????????????????????????????????|
   +-----------------------------------------------------+

• The X region represents what you asked from malloc
• The ! region represents what you wrote past the legal size
• The ? region represents unused memory

Now when you do a second malloc, it will feel perfectly entitled to give away “your” ! part.