Why is there a limit in accessing array in C?

Question

The OSX system can allocate the memory just fine from malloc (return non-NULL). However when accessing position pass 1073741823, Segmentation fault: 11 happened. Can anyone please educate me the reason?

I can access any points below 1073741824 just fine (as shown in the code.) I did try accessing random positions above 1073741824 point and results were still the same error while any random access below 1073741824 worked just fine.

    void dead(){
        size_t pos = 0;
        size_t max =2147483647;
        max *= 2;
        printf("%ld\n",max);
        int* data = malloc(max);
        if(data == NULL){exit(1);}
        pos = 2147483647/2;
        pos-=3;
        for (; pos < 2147483647; pos++) {
            printf("%ld\n",pos);
            data[pos] = 10;
            printf("%i\n",data[pos]);
        }
    }

    4294967294
    1073741820
    10
    1073741821
    10
    1073741822
    10
    1073741823
    10
    1073741824
    Segmentation fault: 11

Theoretically it should not die there.

I think some implementations of malloc have an undocumented behavior on certain inputs like (size_t)-2, and do something other than actually allocate memory. — aschepler
– aschepler, Commented Mar 25, 2019 at 3:41

Jonathan Leffler · Accepted Answer · 2019-03-25 04:20:13Z

2

The malloc() function allocates chars, and sizeof(int) is typically much larger than 1 char.

For example; if you allocate memory for 2147483647 chars but a sizeof(int) is "4 chars"; then you've only allocated enough memory for "2147483647/4 = 536870911 ints".

Note that malloc() typically asks the operating system for a larger amount of memory and splits it up into smaller pieces. This means that when you access something you didn't allocate with malloc() you may still access the larger amount of memory that malloc() asked the OS for, so you don't get a page fault ("segmentation fault") as soon as you access something you didn't allocate (more specifically, you don't get "segmentation fault" as soon as you try to access the 536870912th int that you didn't allocate memory for).

edited Mar 25, 2019 at 4:20

Jonathan Leffler

759k145 gold badges961 silver badges1.3k bronze badges

answered Mar 25, 2019 at 3:47

Brendan

37.7k2 gold badges45 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Why is there a limit in accessing array in C?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related