[Newbie] Threads duplicating memory : normal behaviour ?

Hi everyone,

I'm currently developping a project on a SBC9261 board, with 64MB of RAM available.

I'm using two threads (from the Pthread library) in addition to the main program.

Due to recent memory leaks, I had to check how much memory the processes need (using top
) and was surprised to see that the three of them use the same amount of RAM : ~10MB.
So when they reach ~20MB (1/3 of total RAM), everything slows down and my program finally crashes.

Is this a normal behaviour ? Is there a way to make the thread share the memory, as it seems to be the same datas (same size anyway) ?

Or any other suggestions to save memory ?

Thank you for your answers !

Threads can indeed share the same memory. However, you may need to protect access to this memory area using a mutex.

Here's a trivial example:

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

pthread_mutex_t mutex;


typedef struct
{
  int val1;
  int val2;
} Data;


void* thread(void* arg)
{
   Data* data = (Data*) arg;

   pthread_mutex_lock(&mutex);

   printf("data->val1 = %d\tdata->val2 = %d\n", data->val1, data->val2);

   data->val1 = data->val2 = 1;

   pthread_mutex_unlock(&mutex);

   return 0;
}


int main()
{
   Data* data = malloc(sizeof(Data));
   pthread_t tid1, tid2;

   pthread_create(&tid1, NULL, thread, data);
   pthread_create(&tid2, NULL, thread, data);

   pthread_join(tid1, NULL);
   pthread_join(tid2, NULL);

   free(data);

   return 0;
}

That's a lot of memory for a thread. Since the memory model with a threaded app is a shared address space, all the threads will share all of the memory. I'm not sure what would cause so much memory to be taken, but my guess is you have some initialization routine that is misbehaving.

Measuring memory per-thread can be tricky - because all of the threads share memory, the entire "process" gets allocated X MB, and each thread may also report that it holds X MB - because each thread "does". It just so happens that they all hold the same X MB.

Hopefully this was helpful for you.

Originally Posted by orgcandman Measuring memory per-thread can be tricky - because all of the threads share memory, the entire "process" gets allocated X MB, and each thread may also report that it holds X MB - because each thread "does". It just so happens that they all hold the same X MB.

The reason why my threads use so much memory might be that I'm working with Qt for a GUI, and this library is quite big. Actually, the main program uses Qt, but the two other threads don't. That why I would've expected to see the main program using the 10MB, and the threads using a few ko...

The top output :

Mem: 27252K used, 34488K free, 0K shrd, 0K buff, 18224K cached
Load average: 0.29 0.07 0.02
  PID USER     STATUS   RSS  PPID %CPU %MEM COMMAND
  888 root     R      10176     1  8.8 16.3 MyProgram
  899 root     S      10176   888  0.0 16.3 MyProgram
  900 root     S      10176   899  0.0 16.3 MyProgram

Thank you both for your answers and don't hesitate to post if you have other suggestions !