maximum memory which malloc can allocate

Question

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?

Answer 1

I don't actually know why that failed, but one thing to note is that `malloc(4)" may not actually give you 4 bytes, so this technique is not really an accurate way to find your maximum heap size.

I found this out from my question here.

For instance, when you declare 4 bytes of memory, the space directly before your memory could contain the integer 4, as an indication to the kernel of how much memory you asked for.

Answer 2

Try this

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

main() {
    int Mb = 0;
    while (malloc(1<<20)) ++Mb;
    printf("Allocated %d Mb total\n", Mb);
}

Include stdlib and stdio for it.
This extract is taken from deep c secrets.

Answer 3

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.

Answer 4

As per C90 standard guarantees that you can get at least one object 32 kBytes in size, and this may be static, dynamic, or automatic memory. C99 guarantees at least 64 kBytes. For any higher limit, refer your compiler's documentation.

Also, malloc's argument is a size_t and the range of that type is [0,SIZE_MAX], so the maximum you can request is SIZE_MAX, which value varies upon implementation and is defined in <limits.h>.

Answer 5

when first time you allocate any size to *p, every next time you leave that memory to be unreferenced. That means

at a time your program is allocating memory of 4 bytes only

. then how can you thing you have used entire RAM, that's why SWAP device( temporary space on HDD) is out of discussion. I know an memory management algorithm in which when no one program is referencing to memory block, that block is eligible to allocate for programs memory request. That's why you are just keeping busy to RAM Driver and that's why it can't give chance to service other programs. Also this a dangling reference problem.

Ans : You can at most allocate the memory of your RAM size. Because no program has access to swap device.

I hope your all questions has got satisfactory answers.

Answer 6

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 e.g. 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 (e.g.) 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.