Thread: dynamic buffers

Code:

if (out) { str_cat(out, FRED);

This doesn't update 'out'. You need a pointer to a pointer if you're going to try and do it that way. You should get rid of the if check all together. At the very least, every single call to str_cat should be assigning the return value. Otherwise, your pointer ends up pointing to memory you've gotten rid of with your realloc call. Then the next call to realloc ends up trying to reallocate a pointer which has already been freed.

See, there is actually a reason I suggested you post your code.


Quzah.

You can use a pointer to update the contents of a passed variable. For example:

Code:

void foo( int bar )
{
    bar = 5; /* This will not change 'bar' outside the function. */
}

Now to be able to update the variable we pass, we need a pointer instead.

Code:

void foo( int *bar )
{
    *bar = 5; /* This will change 'bar' outside the function. */
}

We can do the same thing with actual pointers. If we have a pointer, and inside a function we pass it to, we want to change what it points to, we use a pointer to that. Like so:

Code:

void foo( int **bar )
{
    *bar = malloc( 20 * sizeof( int ) );
}

Now outside the function, the pointer we have passed, will end up pointing to newly allocated 20 integers. Called like so:

Code:

int * bar;

foo( &bar );
/* bar now points at our newly allocated memory */
...
free( bar );

Quzah.

Since you appear to be running on a Linux box, I'd suggest you try

Code:

gcc -g prog.c -lefence

Electric fence is a memory debug tool which tries to tell you the point when you first trash memory, not when you first find out about it.
Then run the executable inside the debugger.

Example
char *p = malloc( 10 );
strcpy( p, "hello world" );
Now you know this is bad, but there's no reason why these next calls would ever fail, except by chance.

free ( p );
char *q = malloc( 100 );
char *r = realloc ( q, 200 );

With electric fence in there, you'd see the problem at the strcpy, not some random (and seemingly correct) unrelated code elsewhere in your program.