c,osx,mmap,copy-on-write , Implementing copy-on-write buffer with mmap on Mac OS X


Implementing copy-on-write buffer with mmap on Mac OS X

Question:

Tag: c,osx,mmap,copy-on-write

I've been playing around with copy-on-write buffers on Linux and the following example seems to work as intended:

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define SIZE 4096

#define SHM_NAME "foobar"

int main(void)
{
    int fd = shm_open(SHM_NAME, O_RDWR | O_CREAT, 0666);
    int r = ftruncate(fd, SIZE);

    char *buf1 = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
                     MAP_SHARED, fd, 0);

    strcpy(buf1, "Original buffer");

    char *buf2 = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
                      MAP_PRIVATE, fd, 0);

    // At this point buf2 is aliased to buf1

    // Now modifying buf2 should trigger copy-on-write)...

    strcpy(buf2, "Modified buffer");

    // buf1 and buf2 are now two separate buffers

    strcpy(buf1, "Modified original buffer");

    // clean up

    r = munmap(buf2, SIZE);
    printf("munmap(buf2): %i\n", r);
    r = munmap(buf1, SIZE);
    printf("munmap(buf1): %i\n", r);
    r = shm_unlink(SHM_NAME);
    printf("shm_unlink: %i\n", r);

    return EXIT_SUCCESS;
}

However under OS X (10.10) the second mmap call returns MAP_FAILED, with errno = 22 (EINVAL). The OS X man page for mmap seems to suggest that this should work (it even mentions copy-on-write in the description of the MAP_PRIVATE flag), and I've experimented with various different flags for the calls to mmap, but nothing seems to work. Any ideas ?


Answer:

It appears that using shm_open with MAP_SHARED and MAP_PRIVATE does something undesirable with the file descriptor. Using open is a possible workaround:

int fd = open(SHM_NAME, O_RDWR | O_CREAT, 0666);
...

Result:

munmap(buf2): 0
munmap(buf1): 0
shm_unlink: -1

Using shm_open with MAP_SHARED and MAP_PRIVATE results in an Invalid file descriptor, although using it with MAP_SHARED and MAP_SHARED for example does not. It's unclear to me whether this is a bug, or by design - the behavior does not seem correct though.


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