My stupid code adventure

exvor · 01-16-2005, 04:25 PM

Hello everyone

Ive recently been playing with some malloc code in windows xp
"Actually trying to see if i can break the memory manager"

i wanna know what you guys think of my memory allocator forever

Code:

/* This program will allocate memeory untill there is no more memory left */ 
/* on the system ********************************************************/


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


int main (void)
{ 
    unsigned long count = 5; 
    
  while ( count != 0)
        {  
          malloc(count); 
          count++;
          
           }
    return 0; 
}

What perplexes me while this is purpously bad code :P is when i compile and run the code it allocates memory more and more then windows just seams to set it to 10 mb and stops allocating any more.

Is this cause there is a limit on malloc or because the system memory manager is protecting the system from this memory leek ??

Also would linux detect this buggy program and protect the system from utliziing all of the availible memory.

I know this is probably a stupid question.

wapcaplet · 01-16-2005, 05:29 PM

Perhaps a C programmer can elaborate (and correct me if I'm wrong), but when you use malloc(), you are requesting some memory from the memory manager. If the memory is not available, malloc fails. My guess is that you're hitting the limit of available memory, and malloc is failing. A smart OS isn't going to just let you allocate memory until you completely run out.

You could put in a test, print a message and exit whenever malloc fails, if you want to test this theory.

aluser · 01-16-2005, 06:06 PM

I know in openbsd there's an adjustable per-process limit on heap size. You can even set it according to what group or user the process is. For root it's unlimited by default, iirc.

On my defaultish debian install I seem to be able to go more than a gigabyte into swap with one non-root process; i'd guess it's unlimited.

itsme86 · 01-16-2005, 11:36 PM

Linux supplies ulimit

Code:

itsme@dreams:~$ ulimit -a
core file size (blocks)     unlimited
data seg size (kbytes)      unlimited
file size (blocks)          unlimited
max locked memory (kbytes)  unlimited
max memory size (kbytes)    unlimited
open files                  1024
pipe size (512 bytes)       8
stack size (kbytes)         8192
cpu time (seconds)          unlimited
max user processes          6144
virtual memory (kbytes)     unlimited
itsme@dreams:~$

exvor · 01-17-2005, 09:54 AM

Yea thats what is happening it just failes to allocate the memory on the computer.

here is the revised code

Code:

/* This program will allocate memeory untill there is no more memory left */ 
/* on the system ********************************************************/


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


int main (void)
{ 
    unsigned long count = 5; 
  char *testbit; 
    
  while ( count != 0)
        {  
         testbit = malloc(count); 
          count++;
          if (testbit == NULL) 
             { 
                      printf("Cannot allocate space\n");
                      }

          
           }
    return 0; 
}

i havent tried this program on linux but on windows xp it goes till it allocates 14 mb of memory then i get the Cannot allocate space message

keefaz · 01-17-2005, 10:35 AM

Could you output the value of count in testbit == NULL condition ?

Code:

if(testbit == NULL) {
    printf("Cannot allocate space\n");
    printf("count = %l\n", count);
}

exvor · 01-17-2005, 10:48 AM

are you asking me to impliment this and give the output here ?

jeorj_euler · 01-18-2005, 02:24 AM

Interesting. Such rapid allocation on Linux results in consumption of free swap space. I am not sure how swap works precisely, but from what I can tell, the kernel will move pages already in physical memory to swap to make room for a process such as this. The more rapidly a process reads and writes to memory, the more precedence it has in using physical memory. When such a process finally exits or is killed, a lot of physical memory is suddenly freed.

How much Virtual Memory do you have XP configured to use? Could you figure out how to run the process as SYSTEM?

jeorj_euler · 01-18-2005, 02:35 AM

This can happen with linked structures as you well know.

Code:

struct node
{
        node_t data;
        struct node *next;
};
void form_vector (struct node **const x, int const n)
{
        if (n > 0x0)
        {
                int i;
                // cursor pointer
                struct node1 *r;
                // allocation node zero
                *x = (struct node*) malloc(sizeof(struct node));
                r = *x;
                                                                                                                                                       
        // allocation node subsequent
        for (i = 0x1; i < n; i++)
        {
                r->next = (struct node*) malloc(sizeof(struct node));
                r = r->next;
        }
        }
}

Should n be a very large number or should node_t be a massive data type, more memory will be consumed.

exvor · 01-18-2005, 12:26 PM

I had widnows xp manageing the swap space on the parttion from what i can tell on my work computer here the system is using about 200 mb of swap on the harddrive. When i run this program i get a message that says " Windows is running out of virutal memory and increased the size of the page file " So windows does the same thing it moves alot of running processes into the swap space and when the program is finially killed or closed alot of physical memory is then freed. I dont know how this could be usefull in a program but maybe it can be used to temporarly force the system to free physical memrory.