[ale] Clones and printf()

[Joe Steele]

On Thu 3/21/02 6:58 PM, Chris Fowler wrote:
>
> I think I found one problem.  Here is a snip
> 
> int *stack = (int *)malloc(CHILD_STACK_SIZE);  // 4096
> 
> pid  = clone(execTask, stack, SIGCHLD|CLONE_VM, task);
> 
> The manpages states that a stack grows down.  

Correct (at least on ix86 architecture), which means the stack 
pointer should initially point past the end of the malloc'ed block:

   int *stack = (int *)malloc(CHILD_STACK_SIZE);  // 4096
   stack = (int *)((char *)stack + CHILD_STACK_SIZE);

The first item pushed on the stack will be stored at stack - 1, the 
next item pushed will be at stack - 2, etc.  All items pushed will 
fall inside the malloc'ed block.

> Since I'm
> passing a pointer to the beginning of the stack it is possible for
> the thread to write into the parents process.  Corupting my
> program.  Is this correct?  I looked at some sampels on the
> internet and they do not show doing that.
> 
> http://www.linuxjournal.com/modules.php?op=modload&name=NS-articles/misc/ima
> ges&file=5211l2
>
> Maybe that example is wrong.  

No doubt (at least on ix86 architecture).

> Is there any *good* documentation
> online on using threads effectively.  Maybe a book? It is just awesome
> to have multiple threads doing tasks and sharing data.  It opens
> up many possibilites.  I have some great planes with this.

One place to look would be the Linux Threads FAQ:

http://www.linuxdoc.org/FAQ/Threads-FAQ/index.html

Although it was last revised in '97, it still has useful info, along 
with links to other useful resources.

I suspect that most people don't use clone() directly, but instead 
use library functions which handle the cloning for them.  
Consequently, finding examples which use clone() may be challenging.

--Joe


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