Working with the Unix OS

141
Processes II
segment the kernel maintains.
#include
#include /* defines ipc-perm structure */
struct shrnid_ds {
struct ipc_perm shrn_perm; /* operation perm struct */
int shrn_seqsz; /* segment size */
struct XXX shrn_YYY; /* hardware & implementation dependent information */
ushort shrn_lpid; /* pid of last operation */
ushort shrn_cpid; /* creator pid */
ushort shrn_nattch; /* current # attached */
ushort shrn_cnattch; /* in-core # attached */
time_t shrn_atime; /* last attach time */
time_t shrn_dtime; /* last detach time */
time_t shrn_ctime; /* last change time */
};
int shrnget(key_t key, int size, int shrnflag);
/* used to create or open access to an existing shared memory segment */
shrnflag
0400 SHM_R read by owner
0200 SHM_W write by owner
0040 SHM_R >> 3 read by group
0020 SHM_W >> 3 write by group
0004 SHM_R >> 6 read by world
0002 SHM_W >> 6 write by world
IPC_CREAT
IPC_EXCL
char *shrnat(int shrnid, char *shraddr, int shrnflag);
/* returns the starting address of shared memory segment */
if shrnaddr == 0
system selects address for the caller
else
if value for shrnflag specifies SHM_RND,
shared memory is attached at the address specified by the shrnaddr
argument rounded down by SHMLBA (lower boundary address)
else
shared memory is attached at the address specified by the shrnaddr argument
int shrndt(char *shrnaddr);
/* detaches the segment but does not delete the shared memory segment */
int shrnctl(int shrnid, int cmd, struct shrnid_ds *buf);
/* a cmd of IPC_RMID removes a shared memory segment from the system */
See shared_memory.c
The two process wait for access to the shared memory by waiting for a semaphore's value to become greater than
zero.
This is the most efficient way to wait for the resource, since it is the kernel that does all semaphore operations and
the kernel puts a process to sleep when it has to wait for a semaphore.
! Busy-waiting - instead of sleeping
- keep trying to obtain resource
/* server loop */
mesgptr->mesg_flag = 0; /* signal client */
while (mesgptr->mesg_flag == 0)
; /* wait for client to process */