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Re: VLAs with threads

On 25 May 2008 at 20:15, Aggelidis Nikos wrote:
It is a know fact that pthreads share the same address space. I thought
that this meant that whatever variable i had in main() before creating
a thread, i would be able to use in the thread-itself. Unfortunately
threads have different stacks and that means that any local variable of
main can't be used by threads.
There's a good reason for that. If a variable with automatic storage
could be accessed by any thread, when would you suggest it gets
destroyed?
The problems begin when i decided that i wanted to use a c99 feature:
the variable length arrays {vla}. The problem is that i can't declare a
gloval vla {it isn't permitted by the language}. Also i can't have a vla
inside a structure.... so what can i do?
Create your array on the heap with malloc() instead, protect accesses to
it using a mutex, and decide for yourself when the right time is to
free() it.

Jun 27 '08 #1
5 1258
On Sun, 25 May 2008 20:30:08 +0000, Antoninus Twink wrote:
Create your array on the heap with malloc() instead, protect accesses to
it using a mutex, and decide for yourself when the right time is to
free() it.
but vla's where introduced to solve numeric problems. That's why i want to
use vla...

Jun 27 '08 #2
On 25 May 2008 at 20:44, Aggelidis Nikos wrote:
On Sun, 25 May 2008 20:30:08 +0000, Antoninus Twink wrote:
>Create your array on the heap with malloc() instead, protect accesses to
it using a mutex, and decide for yourself when the right time is to
free() it.

but vla's where introduced to solve numeric problems.
What do you mean? The only possible advantage I can think of of a VLA is
that if you have a heavily-called function then the time savings from
twiddling the stack pointer instead of calling malloc()/free() might be
significant. A big disadvantage of VLAs is that it isn't possible to
detect and attempt recovery if the allocation fails.
That's why i want to use vla...
Use the appropriate tool for the job. A VLA is not a good vehicle for
inter-thread communication.

Jun 27 '08 #3
On Sun, 25 May 2008 21:38:38 +0000, Antoninus Twink wrote:
Use the appropriate tool for the job. A VLA is not a good vehicle for
inter-thread communication.
thanks for the advice Antnoninus, it seems that you are right, i will
revert back to classic malloced arrays...

-nicolas
Jun 27 '08 #4
In article <sl*******************@nospam.invalid>,
Antoninus Twink <no****@nospam.invalidwrote:
>but vla's where introduced to solve numeric problems.
>What do you mean?
I don't know if it's what's being referred to here, but there's
an obvious use for C99's VLAs in numerical work:

int invert(int n, double matrix[n][n]);

- a construct which many programmers had lamented the lack of over a
couple of decades.

In this case of course there is no allocation.

-- Richard
--
In the selection of the two characters immediately succeeding the numeral 9,
consideration shall be given to their replacement by the graphics 10 and 11 to
facilitate the adoption of the code in the sterling monetary area. (X3.4-1963)
Jun 27 '08 #5
On Sun, May 25, 2008 at 09:38:38PM +0000, Antoninus Twink wrote:
On 25 May 2008 at 20:44, Aggelidis Nikos wrote:
On Sun, 25 May 2008 20:30:08 +0000, Antoninus Twink wrote:
Create your array on the heap with malloc() instead, protect accesses to
it using a mutex, and decide for yourself when the right time is to
free() it.
but vla's where introduced to solve numeric problems.

What do you mean? The only possible advantage I can think of of a VLA is
Well, if the OP wants variable length arrays, let's do that, he might
have a good reason: e.g. he has a well tested set of routines for vlas,
and he is trying to port it to a multicore system.

Posix threads are not standard C, but let's not cancel the discussin at this
point. Posix threads allow one single void* to pass to a function executed
in a separate thread. The OP wants now to use this to pass a vla.

One typically creates a struct that contains all the desired data to pass,
and one gives the pointer to this struct (converted to void*) to
pthread_create().

If the question was about a one-dimensional array, I'd suggest the flexible
array member. With two dimensions, I'd better let the OP declare the array
in the main, and put a pointer to it (or the its first subarray) into the
sturct of parameters.

Let me show an example:

#include <stdio.h>
#include <pthread.h>

struct parameters {
int n;
void *array;
};

void* thread(void*P)
{
int n=((struct parameters*)P)->n;
double (*A)[n]=((struct parameters*)P)->array;
/* now you have n and A exactly as if you had a prototype, like
thread(int n,double A[n][n])
*/
for(int i=0;i<n;i++,puts("")) for(int j=0;j<n;j++) printf("%g ",A[i][j]);
return NULL;
}

int main()
{
int n=4;
double A[n][n];
for(int i=0;i<n;i++) for(int j=0;j<n;j++) A[i][j]=10*i+j;
struct parameters P={.n=n,.array=(void*)A[0]};
pthread_t T;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_create(&T,&attr,thread,&P); /* error checking ... */
pthread_join(T,NULL);
}

Szabolcs
Jun 27 '08 #6

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