Malloc returns same address in parent and child process after fork

Question

When calling malloc in the parent and child processes after fork, the result is different depending on the OS.

Here's a simple program to illustrate this:

#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

void malloctest(int cont, int value)
{
    int max = 1;
    int *var1 = malloc(sizeof(int));
    if(var1 == NULL) 
        exit(EXIT_FAILURE);
    
    *var1 = value;
    sleep(value);   
    printf("pid=%d, ppid=%d, address=%p, value=%d\n", getpid(), getppid(), var1, *var1);

    if (cont < max)
    {
        switch (fork())
        {
        case -1: //error
            perror("fork");
            exit(EXIT_FAILURE);
        case 0: //child            
            malloctest(cont + 1,5);
            _exit(EXIT_SUCCESS);
        default: //parent
            sleep(1);
            malloctest(cont + 1,1);

            wait(NULL);
        }
    }
}

int main(int argc, char const *argv[])
{
    malloctest(0,0);
}

In macOS Darwin 20.6.0 Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:31 PDT 2021; root:xnu-7195.141.2~5/RELEASE_X86_64 i386, malloc returns different addresses

pid=63430, ppid=60230, address=0x7f9199405bd0, value=0
pid=63430, ppid=60230, address=0x7f9199504080, value=1
pid=63431, ppid=63430, address=0x7f91995041e0, value=5

whereas in Linux, Linux 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64, malloc returns the same address in parent and child

pid=10314, ppid=3797, address=0x561284e5b260, value=0
pid=10314, ppid=3797, address=0x561284e5b690, value=1
pid=10315, ppid=10314, address=0x561284e5b690, value=5

Is this something to worry about? Or is it just an implementation detail as in, the OS can afford to return the same address because it uses the copy-on-write technique to maintain separate copies of the values pointed to by parent and child? Whereas macOS assigns different addresses straight away

It's nothing to worry about. The two process by definition have seperate address spaces. At the point of the `fork` the address space is more or less the same. But any allocations after that are independent and one should not attempt to interpret any correlation between them. — kaylum, Sep 08 '21 at 23:20
And to be clear, the memory at virtual address `0x561284e5b690` in process 10314 is not the same as the memory at virtual address `0x561284e5b690` in process 10315. Modifying one will not affect the other. They're at different physical addresses (but physical addresses are deliberately kept hidden from ordinary code). — Keith Thompson, Sep 08 '21 at 23:32
Interesting, though. Seems to go a step beyond the more usual notions of "address space randomization". — Steve Summit, Sep 09 '21 at 00:00

score 3 · Answer 1 · answered Sep 08 '21 at 23:20

3

There's nothing to worry about — the processes are separate and have separate address spaces. There is no sharing unless you've set up shared memory (and malloc() won't return pointers to the shared memory).

answered Sep 08 '21 at 23:20

Jonathan Leffler

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Malloc returns same address in parent and child process after fork

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