What is the maximum acceptable size of an array?

From: John Zoidberg <zohn.joidberg_at_gmail.com>
Date: Mon, 9 Jul 2007 18:02:41 +0200

I've encountered a strange problem in one of my macros. I managed to reduce it to an array size problem.

This macro runs without errors when compiled (.L toto.C+) with "#define MAX 1972432" and exits root without any message if compiled with "#define MAX 1972433":
//includes

#include <iostream>
using namespace std;

//OK

#define MAX 1972432
//not OK
//#define MAX 1972433

//main function

int toto()
{
  char a[MAX];

  cout<<"MAX="<<MAX<<endl;
  cout<<"total size="<<MAX*sizeof(char)<<" bytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/1024.<<"

Kilobytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/(1024.*1024.)<<" Megabytes"<<endl;
  cout<<"total size="<<MAX*sizeof(char)*8<<" bits"<<endl;

  return(0);
}

You might have to change the limit values of MAX to see this problem on your PC.
I noticed that it's quite random sometimes. Sometimes it worked with 1972432 and sometimes not.
I also recommend quitting and restarting ROOT after each test because it seems to always work if you stay in the ROOT shell even after recompiling.

Other tests I made:

This macro runs without error when not compiled (.x toto2.C):
//includes

#include <iostream>
using namespace std;

//OK

#define MAX 10000000
//not OK
//#define MAX 1972433

//main function

int toto2()
{
  char a[MAX];

  cout<<"MAX="<<MAX<<endl;
  cout<<"total size="<<MAX*sizeof(char)<<" bytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/1024.<<"

Kilobytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/(1024.*1024.)<<" Megabytes"<<endl;
  cout<<"total size="<<MAX*sizeof(char)*8<<" bits"<<endl;

  return(0);
}

This C++ program compiles and runs without error:
//includes

#include <iostream>
using namespace std;

//OK

#define MAX 10000000
//not OK
//#define MAX 1972433

//main function

int main()
{
  char a[MAX];

  cout<<"MAX="<<MAX<<endl;
  cout<<"total size="<<MAX*sizeof(char)<<" bytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/1024.<<"

Kilobytes"<<endl;
  cout<<"total size="<<(float)MAX*(float)sizeof(char)/(1024.*1024.)<<" Megabytes"<<endl;
  cout<<"total size="<<MAX*sizeof(char)*8<<" bits"<<endl;

  return(0);
}

So, is there any rational explanation to this? Should I use TCloneArrays or TNtuples instead? Received on Mon Jul 09 2007 - 18:02:50 CEST