shmat返回分段错误（核心已转储）

Question

Im trying to write into shared memory but for some reason after i call shmat() and strcpy i get segmentation fault(core dumped) why is that?

This is my code:

int fd,shmid;
key_t shmkey;
char *shm_add;
pid_t pid,pid1=0,pid2=0;

shmkey=ftok("shmdemo.c",'j');
if ( shmkey == (key_t)-1 )
 {
    printf("main: ftok() for shm failed\n");
    return -1;
}
shmid=shmget(shmkey, 50, 0666 | IPC_CREAT | IPC_EXCL);
if (shmid == -1)
{
    printf("main: shmget() failed\n");
    return -1;
}
shm_add=(char *)shmat(shmid,0,0);
if ( shm_add==NULL )
{
    printf("main: shmat() failed\n");
    return -1;
 }

strcpy(shm_add,"hello");

edit: I have file name shmdemo.c on the directory, and the errno of shmget say "File exists" but when i delete "shmdemo.c" from the directory, a new errno comes in the ftok that say "No such file or directory".

Thank you, Asaf.

Answer 1

Answerifying the various comments. When you call:

shmget(..., IPC_CREAT | IPC_EXCL);

what you're saying with the flags is: Create a new shared memory segment for this key, and make sure no shared memory segment exists with that key already.

If you only use:

shmget(..., IPC_CREAT);

you're saying: If a shared memory segment already exists with this key, return it; otherwise create a new one for this key and return that.

Generally, you probably don't want the second variant of the call. Concurrency is hard enough without adding any nondeterminism of your own. (Ie just letting whichever of a bunch of cooperating processes comes first create the shared memory; as opposed to having a designated "owner" process.)

When using the first call, if a shared memory segment already exists at the given key, the syscall fails with the errno value EEXISTS . SysV IPC are persistent, so they don't get automatically cleaned up after your program exits like, say, file descriptors. (In fact, they're more analogous to temporary files.)

You should clean them up in your program using something like shmctl(shmid, IPC_RMID, NULL); . You can also check for stray IPC objects using ipcs and remove the ones left behind with ipcrm or cleanipcs .

Answer 2

尝试删除IPC_EXCL标志并检查程序。