Programming shared memory – more head banging.

I was trying to use shared memory (to look at Java Shared Classes), and it took me a day to get it working – better documentation would have helped.

I had two basic problems

1. Using smctl to display information about the shared memory, gave the size as 0 bytes, even though the ipcs command showed me there were megabytes of data in the shared memory area.
2. Trying to attach the shared memory gave me “invalid parameters” return code – even though the documented reasons for this error code did not apply to my program.

I tried many things, from using different userids, to running with a different storage key, running APF authorised….

I eventually got it to work by compiling my C program in 64 bit mode rather than 31 bit mode. There is no discussion about 31 bit/64 bit in the documentation. If the shared memory in 64 bit mode, you will need 64 bit addressability, so you need a 64 bit program. But there is no way of determining that the shared memory is 64 bit!

My basic program

{ 
//struct shmid_ds  buf; 
  struct shmid_ds64 buf; 
  memset(&buf   ,0,sizeof(buf)); 
  int shmid    =  8197; 
  int rc = 0; 
  long l; 
  int cmd = IPC_STAT; 
  char * fn = "COLIN"; 
  int shmflg =0; 
    shmflg =   IPC_STAT; 
//  rc =shmctl(shmid, cmd,        &buf); 
    rc =shmctl64(shmid, cmd,        &buf); 
    perror("shmctl      "    ); 
    printf("shctl rc %i\n",rc); 
    l = buf.shm_segsz;
    printf("size     %ld\n",l); 
    printHex(stdout,&buf,sizeof(buf)); 
    /////////////////////////////////////////////// 
    //  shmat 
    /////////////////////////////////////////////// 
    char * pData = NULL; 
    pData = shmat(shmid, NULL , 0          ); 
    printf("Address %ld\n",pData); 
    printHex(stdout,pData+4096*1024,1024*1024); 
    int e = errno; 
    perror("shmat "); 
    printf("Errno: %s\n",strerror(e)); 
   return 0; 
}  

Originally I was using EDCCB to compile and bind this.

The EINVAL error return code was (from the documentation) for cases where the pointer in shmat was non NULL. I was passing NULL – so none of this made sense.

The reason code 0717014A was

JRInvalidAmode: An incorrect access mode was specified on the access service
Action: The access mode specified on the access service has unsupported bits turned on. Reissue the request and specify a valid access mode.

It turned out that my program was 31 bit. When I used 64 bit – it magically started working.

I compiled it with EDCQCB, and had to change a few things to be 64 bit mode.

• shmid_ds buf -> shmid_ds64
• shmctl -> shmctl64

When I ran it in 31 bit mode, the length of the storage returned was 0. In 64 bit mode, it gave the correct length. This looks like a way of telling what mode the shared memory is!