How many GB can malloc allocate for your program

Question

I used the following code to find it out but I always get 1 as the answer. is there something wrong. Thanks

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

int main(){
    int mult = 0;
    int chk =8;
    do{
        mult+=1;
        int *p = (int*)malloc(1024*1024*1024*mult);
        if(p==0){
            chk =0;

        }else{
            free(p);
        }
    }while(chk !=0);
    mult = mult -1;
    printf("The number of gigs allocated is : %d\n",mult);
    return 0;
}

Just to help, I have a 64 bit system with both windows and linux installed. Thus, is the above logic correct even though I am getting just 1 gb as the answer on a 64 bit system?

It depends on your o/s. It may depend on the limits set by the o/s, which may be adjusted by appropriately privileged users. Have you tried allocating 1023*1024*1024*2? — Jonathan Leffler, Feb 02 '11 at 16:16
I think this code risks suffering from integer overflow, if int is a signed 32-bit variable. Try making the constants unsigned long (or unsigned long long if your compiler has it). — unwind, Feb 02 '11 at 16:20
@unwind That worked for me. I was able to get past 500 GBs before shutting it off. (Mac OS X uses `mmap()` beyond a certain size.) Make your comment an answer so I can up vote it. — chrisaycock, Feb 02 '11 at 16:29

score 3 · Accepted Answer · answered Feb 02 '11 at 16:09

3

If it is a 32-bit OS, then it is not surprising that the largest contiguous block would be 1GB (or somewhere between that and 2GB). On a 64-bit OS, larger blocks would be possible.

If you change your code to allocate smaller individual pieces, you will likely be able to allocate more than 1GB total.

answered Feb 02 '11 at 16:09

Mark Wilkins

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Sometimes, you can get up to about 3, 3.5 GB on 32-bit system. 4 GB is the hard limit, of course. It is some value smaller than *that*. – Jonathan Leffler Feb 02 '11 at 16:15
@Jonathan: I was going to mention numbers in my answer and then thought better of it since I didn't know what platform he was using. Plus there are other issues such as fragmentation to consider. Allocating 1MB blocks in a loop would "overstate" the maximum compared to most real world applications (at least in my experience). – Mark Wilkins Feb 02 '11 at 16:23
@Jonathan: If a 32-bit OS allows you to allocate more than 2gb contiguous and yet has a 32-bit `ptrdiff_t`, it is **broken** and **nonconformant** (it will yield the wrong sign for pointer differences, which could be extremely dangerous!). – R.. GitHub STOP HELPING ICE Feb 02 '11 at 21:40
@R..: not sure about one block of contiguous memory, but there are options on Windows to allow you to get more then 2 GiB total shared memory. I don't know what the options are - I don't work enough on Windows to care, either. But the products I work on (for the Unix ports) are able to use more than 2 GiB on Windows (and on Unix) on 32-bit systems. More than 2 GiB in aggregate...but less than 4 GiB. – Jonathan Leffler Feb 02 '11 at 23:13
1

If the memory is obtained via some method outside of the functions specified by the C standard, then I suppose an implementation can document that pointer subtraction on such memory does not necessarily work. But `ptrdiff_t` is supposed to be large enough to store any pointer difference whose result is defined, and subtracting pointers within a single chunk obtained by `malloc` is certainly well-defined. – R.. GitHub STOP HELPING ICE Feb 02 '11 at 23:44
We used to use the [/3GB] flag in boot.ini to tell the 32-bit Windows kernel to limit itself to 1GB, and allow applications access to 3GB. That doesn't mean you can get a pointer to one 3GB chunk in 32-bit windows of course! But as memory gets allocated and fragmented, it gets harder and harder to get even a 1GB chunk (which must of course be contiguous). – moodboom Dec 12 '13 at 15:02

score 1 · Answer 2 · edited May 23 '17 at 11:59

1

These questions might help you a bit: How much memory was actually allocated from heap for an object? and How do I find out how much free memory is left in GNU C++ on Linux

edited May 23 '17 at 11:59

Community

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answered Feb 02 '11 at 16:12

jwir3

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Jeffrey.W.Dong · Answer 3 · 2011-02-08T00:34:40.020

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int main(void){
    int MB = 0;
    while(malloc(1<<30)){
        ++MB;
    }
    printf("The number of gigs allocated is : %d\n",MB);
    return EXIT_SUCCESS;
}

edited Feb 08 '11 at 00:34

answered Feb 08 '11 at 00:22

Jeffrey.W.Dong

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What is this supposed to show? You can very easily come into a situation where malloc can return without error but allocate more space than you actually have. Take a look at "memory overcommit" – Falmarri Feb 08 '11 at 00:34

How many GB can malloc allocate for your program

3 Answers3