UNIX for Advanced & Expert Users

I have a process that need two active connections to the same zone of shared memory. But when i do the second call to shmat it give me error 22 (EINVAL). Only works ok the second call to shmat if i disconnect the first connection (shmdt)
In Sun,AIX and Digital, i don´t have this problem.

Steps: I´m going to shared a integer.

1º.- Create shared memory. Work OK:
iIdentificador = shmget(5000,sizeof(int),IPC_CREAT | 0777); //5000 is the key.

2º.-Connection 1 and view the integer shared. Works OK:
piIntegerShared_1=(int *)shmat(iIdentificador,NULL,0))
printf("\n\t =>Dato compartido [%ld]\n",*piIntegerShared_1);


3º.-Connection 2: !!! shmat ERROR !!!:
if((piIntegerShared_2=(int *)shmat(iIdentificador,NULL,0))==(void*)-1)
printf("\n\n\t ERROR shmat memoria2 [%d] [%s]",errno,strerror(errno));

Output: ERROR shmat memoria2 [22] [Invalid argument]

The connection 2 only is ok, if we delete the connection 1 with:
shmdt(piIntegerShared_1);

Which is the problem?

Thanks,
David

The order of the arguments you gave to shmget is wrong...suggest you look at its man page.

5000 is the key_t key, the order is ok.

I dont hardcode the key but rely on the kernel giving me a unique value for it. That's why 5000 threw me off because I took it to be the segment size requested.
Try shmget(IPC_PRIVATE, sizeof(int), 0777) but if you want to hardcode shmid then try...

Code:

shmget(5000, sizeof(int), 0777 & IPC_CREAT);

Don´t worry by the 5000 (key_t), in the original source is not hardcode, this is only the example of the problem.
I have tested your solution: shmget(5000, sizeof(int), 0777 & IPC_CREAT);
and give me error in Step 1: ERROR crear memoria [2] [No such file or directory].


Remember that the first connection (step 2) works correctly, the problem is the second one.

Resolved problem:

cc DemoIPC.c -o -N DemoIPC
DemoIPC
chatr +as mpas DemoIPC

Why is there two thread going on at the same time, in two different forums, with the same topic by the same user?