maximum memory which malloc can allocate

I was trying to figure out how much memory I can malloc to maximum extent on my machine (1 Gb RAM 160 Gb HD Windows platform).

I read that the maximum memory malloc can allocate is limited to physical memory (on heap).

Also when a program exceeds consumption of memory to a certain level, the computer stops working because other applications do not get enough memory that they require.

So to confirm, I wrote a small program in C:

int main(){  
    int *p;
    while(1){
        p=(int *)malloc(4);
        if(!p)break;
    }   
}

I was hoping that there would be a time when memory allocation would fail and the loop would break, but my computer hung as it was an infinite loop.

I waited for about an hour and finally I had to force shut down my computer.

Some questions:

Does malloc allocate memory from HD also?

What was the reason for above behaviour?

Why didn't loop break at any point of time?

Why wasn't there any allocation failure?

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I read that the maximum memory malloc can allocate is limited to physical memory (on heap).

Wrong: most computers/OSs support virtual memory, backed by disk space.

Some questions: does malloc allocate memory from HDD also?

malloc asks the OS, which in turn may well use some disk space.

What was the reason for above behavior? Why didn't the loop break at any time?

Why wasn't there any allocation failure?

You just asked for too little at a time: the loop would have broken eventually (well after your machine slowed to a crawl due to the large excess of virtual vs physical memory and the consequent super-frequent disk access, an issue known as "thrashing") but it exhausted your patience well before then. Try getting eg a megabyte at a time instead.

When a program exceeds consumption of memory to a certain level, the computer stops working because other applications do not get enough memory that they require.

A total stop is unlikely, but when an operation that normally would take a few microseconds ends up taking (eg) tens of milliseconds, those four orders of magnitude may certainly make it feel as if the computer had basically stopped, and what would normally take a minute could take a week.

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I know this thread is old, but for anyone willing to give it a try oneself, use this code snipped

#include <stdlib.h>

int main() {
int *p;
while(1) {
    int inc=1024*1024*sizeof(char);
    p=(int*) calloc(1,inc);
    if(!p) break;
    }
}

run

$ gcc memtest.c
$ ./a.out

upon running, this code fills up ones RAM until killed by the kernel. Using calloc instead of malloc to prevent "lazy evaluation". Ideas taken from this thread: Malloc Memory Questions

This code quickly filled my RAM (4Gb) and then in about 2 minutes my 20Gb swap partition before it died. 64bit Linux of course.

---

malloc does its own memory management, managing small memory blocks itself, but ultimately it uses the Win32 Heap functions to allocate memory. You can think of malloc as a "memory reseller".

The windows memory subsystem comprises physical memory (RAM) and virtual memory (HD). When physical memory becomes scarce, some of the pages can be copied from physical memory to virtual memory on the hard drive. Windows does this transparently.

By default, Virtual Memory is enabled and will consume the available space on the HD. So, your test will continue running until it has either allocated the full amount of virtual memory for the process (2GB on 32-bit windows) or filled the hard disk.

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