Trying to extend shared memory region without losing current info - shared-memory

Question

I am trying to create a shared memory segment that will hold an initial set of data, but will need to be extended at some later time when another block of data becomes available. I've read several posts here and tried a few things, but I seem to be missing some magic and could use some advice.
Here is a test program that illustrates the problem I'm encountering:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
#include <signal.h>
#include <sys/mman.h>
#include <fcntl.h>
int main(int argc, char **argv)
{
void *addr, *resaddr;
int *iarray;
int fd;
/* reserve a large address space - note that this doesn't ALLOCATE
* anything. It simply tells the OS to reserve this much space
* for us to eventually/potentially use. For this test case, we
* will ask for 1GByte of space. We include the MAP_NORESERVE
* flag to indicate that we don't want anything allocated, but
* people report that this flag is ignored as PROT_NONE is
* apparently sufficient for that purpose. */
fd = open(path, O_RDWR | O_CREAT | O_TRUNC, 0660);
addr = mmap(NULL, 1U << 30, PROT_NONE, MAP_ANONYMOUS | MAP_SHARED | MAP_NORESERVE, -1, 0);
fprintf(stderr, "GOT ADDRESS %p\n", addr);
iarray = (int*)addr;
/* now get resources for a piece of that file */
ftruncate(fd, 1U << 16);
resaddr = mmap(addr, 1U << 16, PROT_WRITE | PROT_READ, MAP_FIXED | MAP_SHARED, fd, 0);
fprintf(stderr, "RESERVED ADDRESS %p\n", resaddr);
/* put somthing into it */
iarray[1024] = 1;
fprintf(stderr, "iaddr[1024]: %d\n", iarray[1024]);
/* increase the size */
ftruncate(fd, 1U << 18);
fsync(fd);
resaddr = mmap(addr, (1U << 18) - (1U << 16), PROT_WRITE | PROT_READ, MAP_FIXED | MAP_SHARED, fd, 1U << 16);
fprintf(stderr, "EXTENDED RESERVED ADDRESS %p\n", resaddr);
/* add something to that region */
iarray[31000] = 2;
fprintf(stderr, "\tiarray[1024]: %d\n\tiarray[31000]: %d\n", iarray[1024], iarray[31000]);
exit(0);
}
Running this yields the following result:
GOT ADDRESS 0x111396000
RESERVED ADDRESS 0x111396000
iaddr[1024]: 1
EXTENDED RESERVED ADDRESS 0x111396000
iarray[1024]: 0
iarray[31000]: 2
Things work as I expected, with the sole caveat being the loss of the initial stored data (apparently reset to zero). Does anyone have a suggestion of what I should do differently to have that initial stored data retained?

Answer 1

In my understanding, the third invocation of mmap() is incorrect.
resaddr = mmap(addr, (1U << 18) - (1U << 16), PROT_WRITE | PROT_READ, MAP_FIXED | MAP_SHARED, fd, 1U << 16);
should be replaced with
resaddr = mmap(addr + (1U << 16), (1U << 18) - (1U << 16), PROT_WRITE | PROT_READ, MAP_FIXED | MAP_SHARED, fd, 1U << 16);
(the offset argument is the offset in the file, not in memory)