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unix system programming....shmget error

Hello experts,

I have a really urgent question. I'm doing a system programming for my os class. When I use shmget function, I get an error saying invalid argument....I don't understand. Can anyone help~~~?

struct mySharedMem{
...
...
}

mySharedMem* shm;

int main(int argc, char* argv[]){
mySharedMem* shm;
int shmId;
key_t key = KEYVALUE;

int numProcesses;
pid_t pid;

if((shmId = shmget(key, sizeof(mySharedMem), IPC_CREAT|0666)) <0){
perror("Error in main: shmget"); //I get error message from this line!!!!
exit(1); // error is "Error in main:shmget: invalid argument"
}

help~~~

ASKER CERTIFIED SOLUTION

PCableGuy

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shinymoon

ASKER

Thanks for responding.

at the begining of my program, I defined KEYVALUE like below
/#define KEYVALUE 5678

but, I changed my code as your advice and run it again..I get the exactly same error
my code looks like .....

int main(int argc, char* argv[]){
mySharedMem* shm;
int shmId;
key_t key = 5678;

if((shmId = shmget(key, sizeof(mySharedMem), IPC_CREAT|0666)) <0){
perror("Error in main: shmget");
exit(1);
}

which argument is invalid !?

shinymoon

ASKER

and the error is...
Error in main: shmget: Invalid argument

shinymoon

ASKER

oh, I just tried a different value for key that is 123. It worked. Thanks ^^
Is there any limitation to assign the value for the key???

PCableGuy

Hi shinymoon,

Most of the code I've seen usually uses IPC_PRIVATE as the key. Otherwise, I think you might need to use the ftok function to get a key. In other words, I don't think it can be an arbitrary number. Perhaps, that's a good question for your programming class instructor.

http://www.csl.mtu.edu/cs4411.ck/www/NOTES/process/shm/key.html