CODEWITHSUNDEEP
clinuxmemory-managementmalloc
urzeit
urzeit

Reputation: 2909

Allocation of more than 2gb fails on 64 bit binary

On a 64 bit linux machine I wrote the following small C program:

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

int main (void)
{
    #define BLOCK_SIZE (1024*1024)
    int i;

    for (i = 1; ; i++)
    {
        char *p = (char *)malloc(i * BLOCK_SIZE);
        if (p == 0)
            break;
        *p = 13;
        free(p);
    }

    printf ("Allocated max %d MB\n", (i - 1));

    return 0;
}

If I compile this with -O0 -m64, I can allocate 2047 MB on my desktop system. If I compile the same program with -O0 -m32 3829 MB.

Why is the maximal malloc size for a 32 bit binary larger than for a 64 bit binary on the same machine?

Edit: If I activate the optimizer with -O3, the maximal amount of memory is 20588 on a 64 bit machine.

Upvotes: 1

Views: 1636

Answers (1)

David Heffernan
David Heffernan

Reputation: 612794

You are trying to store a number greater than 2^31 in a signed 32 bit int. It does not fit. Use size_t instead for the size that you pass to malloc. Change the type of i to size_t.

Upvotes: 10

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