shmget() is not able to 'find' an existing shared memory segment
unless the 'flags' argument is set to '0'. This is a bug. If
flags = 0666 (or at least compatible with the permissions set
when the segment was created), it should still be able to find
and use the segment.
The test program:
#include <sys/shm.h>
#include <errno.h>
#define CHLD_MODE 0666
int
main(int argc, char **argv)
{
int pid = fork();
int id;
if (pid == 0) {
sleep(5);
id = shmget(1, 100, CHLD_MODE);
printf("child (%d): %d (%d)\n", getpid(), id, errno);
} else {
id = shmget(1, 100, IPC_CREAT | 0666);
printf("parent (%d): %d (%d)\n", getpid(), id, errno);
sleep(10);
shmctl(id, IPC_RMID, 0);
}
return (0);
}
Produces the following output:
parent (35492): 196609 (0)
child (3876): 0 (0)
This shows that:
1. The parent created the shared memory segment and got back its
ID (196609).
2. The child process tried to attach to the parent's shared memory
segment (using the same key = 1), but shmget() returned 0 with no
error!
However, if CHLD_MODE is set to 0 in the code above, then:
parent (34084): 393216 (0)
child (11232): 393216 (0)
Which shows that the child did find the parent's shared memory
segment.
=====
Jerry D. Hedden
<< If you're not having fun, then you're not doing it right! >>
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