Question

I have a strange problem with shared memory.

Method shmget returns -1 and errno is set to EEXIST. According to man, it is only possible when both flag IPC_EXCL and IPC_CREAT are provided.

My code:

int main()
{
        int shmid = shmget(0xABCD, MAX_SIZE, IPC_CREAT | 0x660);
        int shmid2 = shmget(0xABCD, MAX_SIZE, IPC_CREAT | 0x660 );
        if(shmid == -1)
        {
                if(errno == EEXIST)
                        perror("Error");
                return -1;
        }
        if(shmid2 == -1)
        {
                if(errno == EEXIST)
                        perror("Error2");
                return -1;
        }
        shmctl(shmid, IPC_RMID, NULL);
        return 0;
}

It compiles with -Wall without warnings, I check with ipcs if the segment is already present (and remove it if needed). The output is Error2: File exists.
It works when I change the second shmget to:

int shmid2 = shmget(0xABCD, MAX_SIZE, 0 );

Quote from man shmget:

EEXIST     IPC_CREAT | IPC_EXCL was specified and the segment exists. 

And one more question: is it true, that mode_flags (i.e. 0x660) are not used when trying to execute shmget?

Answer 1

Permission modes need to be specified in octal, not hexadecimal. 0x660 (hex) = 03140 (octal). And the IPC_EXCL flag has the octal value 02000 in the Linux ABI — so by using 0x660 instead of 0660 you are accidentally setting IPC_EXCL, which is why you get the error.

If I change both instances of 0x660 to 0660 in your program and fix the other things that make it not compile (notably, you left out all the headers and the definition of MAX_SIZE) it works as expected.