Thread: Browsing through a Pointer to Array of Structures

Hello, I'm working on some wacky shared memory using application for Unix.

Here is the thing, I use shmget and shmat to create and attach a shared memory block, but making it an array of structures.

The structure:

Code:

struct object{
   char nick[16];
   pid_t pid;
   key_t queue;
} *mem, *p;

SHM declaration:

Code:

if( (mem = (struct object*)shmat((key_t)argv[1], (void*)0, 0)) == (void*)-1 )        {
                  shmget((key_t)argv[1], SHMSIZE*sizeof(struct object), IPC_CREAT | 0666);
                  mem = (struct object*)shmat( (key_t)argv[1], (void*)0 , 0);
        }

And now the problematic part. I'm using this loop to browse through this thing.

Code:

for(p = mem; p< &mem[SHMSIZE]); p++)        {
            if(!p->nick)
            {
                key = ftok(".", randomize());  //randomize just provides a random integer from dev/random
                msgid = msgget(key, IPC_CREAT);


                p->nick = getenv("USERNAME");
                printf("%s", p->nick);
                p->pid = getpid();
                p->queue = msgid;
            }
            printf("lalala\n");
        }

But this one just doesn't work. Any suggestions please?

Well I tested the following:

Code:

for(p = mem; p< &mem[SHMSIZE]); p++)         {
             printf("lalala\n");
         }

also:

Code:

for(p = mem; p<=(mem+1023); p++)         {
             printf("lalala\n");
         }

neither did anything.

Ah, the code compiles just fine with -Wall.

Code:

if( (mem = (struct object*)shmat((key_t)argv[1], (void*)0, 0)) == (void*)-1 )        {
                  shmget((key_t)argv[1], SHMSIZE*sizeof(struct object), IPC_CREAT | 0666);
                  mem = (struct object*)shmat( (key_t)argv[1], (void*)0 , 0);
        }

Shouldn't you first call shmget() to get an id and then use that id as the first parameter for shmat()?

Code:

if ((id = shmget(key, size, flags)) != -1)
    mem = (struct object *) shmat(id, NULL, 0);

Bye, Andreas