In what way doesn't this work?

The following (cheesy) program worked for me on an omap5912 (ARM926) board.
Compiling it to generate a 'shm' executable gave me the following output:

# shm &
57353 running shm
# ptr = 0x80100000
setting 0 to 0xdeadbeef
hanging around...

# shm 0x80000000
using ptr=80000000
read deadbeef
#

This is the source:

#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
         void    *ptr;
         int             fd;

         if (argc == 2) {
                 ptr = (void *)strtoul(argv[1], 0, 0);
                 printf("using ptr=%x\n", ptr);
                 printf("read %x\n", *(unsigned *)ptr);
                 exit(0);
         }

         fd = shm_open("/shmfile", O_CREAT|O_EXCL|O_RDWR, 0666);
         if (fd == -1) {
                 perror("shm_open");
                 exit(1);
         }
         shm_unlink("/shmfile");
         if (shm_ctl(fd, SHMCTL_ANON|SHMCTL_GLOBAL|SHMCTL_LOWERPROT, 0, 4096) == -1) {
                 perror("shm_ctl");
                 exit(1);
         }
         ptr = mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
         if (ptr == MAP_FAILED) {
                 perror("mmap");
                 exit(1);
         }
         printf("ptr = 0x%x\n", ptr);
         printf("setting %x to 0xdeadbeef\n", *(unsigned *)ptr);
         *(unsigned *)ptr = 0xdeadbeef;
         printf("hanging around...\n");
         while (1)
                 ;
         exit(0);
}

Michael Cranston wrote:
> Hello,
> 
> I am running an ARM architecture and trying to use shared memory. I have read the docs on the 'special' cases for ARM 
(specifically ARM9) memory management.
> 
> What I would like to do, and I thought the docs indicated this was possible, was to have process #1 create some shared
 memory and map it using the following commands and flags.
> 
> fd = shm_open(fileNameArg, O_RDWR | O_CREAT | O_EXCL, 0777);
> shm_ctl(fd, SHMCTL_ANON | SHMCTL_GLOBAL | SHMCTL_LOWERPROT, 0, sizeInBytes);
> 
> ptr = mmap( NULL, length, prot, flags, fd, offset)
> 
> My understanding is that the combination of SHMCTL_ANON | SHMCTL_GLOBAL | SHMCTL_LOWERPROT will place this memory in a
 'global' address pool that will be accessible by other processes
> 
> In other words, Can I just pass the 'ptr' to process #2 and have it be able to access the memory at that address 
without the need for process #2 to call mmap() ???
> 
> I have tried this in my system, and it doesn't seem to work, so maybe I am misunderstanding the docs.
> 
> The OS version is 6.4.0
> 
> Thanks for the help.
> - Mike
> 
> 
> _______________________________________________
> OSTech
> http://community.qnx.com/sf/go/post28982
>