How to LIMIT share memory in C ???

[tritong]

With 2 c files, they use 1 memory area. How to set limit memory for them ???
I use library shm.h and its func (shmget, shmat, shmctl ... ). When i share memory for another, i don't know how to limit the memory ???

[tritong]

Size of segment :

#define SHMSZ 27

   if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) 
   {
        perror("shmget");
        exit(1);
    }

When SHMSZ > 27.

shmget: Invalid argument

Why ?

[itsme86]

It's implementation-specific. Have you looked at the man page?

On my system it says:

       SHMMAX     Maximum size in bytes for a shared memory  seg­
                  ment:  implementation dependent (currently 4M).

       SHMMIN     Minimum size in bytes for a shared memory  seg­
                  ment:  implementation  dependent  (currently  1
                  byte, though PAGE_SIZE is the effective minimum
                  size).

Is this your first shmget() call with that key?

       EINVAL      if  a new segment was to be created and size <
                   SHMMIN or size > SHMMAX, or no new segment was
                   to  be  created,  a  segment  with  given  key
                   existed, but size is greater than the size  of
                   that segment.

[tritong]

I use FreeBSD 6.0
The man page do not show that.

[quzah]

Maybe not, but it does say this:

When creating a new shared memory segment, size indicates the desired
size of the new segment in bytes. The size of the segment may be rounded
up to a multiple convenient to the kernel (i.e., the page size).

And this:

ERRORS
The shmget() system call will fail if:

[EINVAL]

Size specified is greater than the size of the previ-
ously existing segment. Size specified is less than
the system imposed minimum, or greater than the system
imposed maximum.

Quzah.

[tritong]

Thanks so much.
Because I use it but not remove it after run. So I change the seg size and compile, it occurs error.
I have solved it.

Can I write one struct to share memory ??

struct test
{
    int a;
    int b;
}

I insert it 10 times in loop. And then insert 1 time with this memory. It replace the first record in share memory ??? How can I add it in the end of the share memory ???

MEM Before :  {0,0} {1,1} {2,2} {3,3} {4,4} {5,5} {6,6} {7,7} {8,8} {9,9} 
After insert {10, 10}
MEM after   :  {10,10} {1,1} {2,2} {3,3} {4,4} {5,5} {6,6} {7,7} {8,8} {9,9} 

Can add the end of shared memory to have this result : {0,0} {1,1} {2,2} {3,3} {4,4} {5,5} {6,6} {7,7} {8,8} {9,9} {10,10}  ???