Working with the Unix OS

Processes II
System V IPC
- message queues
- semaphores
- shared memory0
Message queue Semaphore Shared memory
include file sys/msg.h sys/sem.h sys/shm.h
system calls
to create or open msgget semget shmget
for control operations msgct1 semct1 shmct1
for IPC operations msgsnd semop shmat
msgrcv
shmdt
/* */
struct ipc_perm { /* */
ushort uid; /* owner's user id */
ushort gid; /* owner's group id */
ushort cuid; /* creator's user id */
ushort cgid; /* creator's group id */
ushort mode; /* access modes */
ushort seq; /* slot usage sequence number */
ket_t key; /* key */
};
! Generating IPC ids
char *path key_t key msgget() int id
ftok()
semget()
char proj
shrnget()
! Logic flow for opening
an IPC channel
start here
OK create new entry return ID
| | no
key==IPC PRIVATE? yes system table full?
| no | yes
Key already exists?
no
IPC_CREAT set?
| yes
yes error return
CREAT & EXCL set?
errno=EEXIST
| no
access permission? no error return
| yes
errno=EACCES
OK return ID
136
yes
errno= ENOSPC
no
errno= ENOENT
Message queues
There is no requirement that any process be waiting for a message to arrive on queue before some other process is
allowed to write a message to that queue
For every message queue in the system, the kernel maintains the following structure of information:
#include
#include /* defines ipc_perm structure */
struct msqid_ds {
struct ipc_perm msg_perm; /* operation perm struct */
struct msg *msg_first; /* ptr to first msg on q */
struct msg *msg_last; /* ptr to last msg on q */
ushort msg_cbytes; /* current # of bytes on q */
ushort msg_qnuro; /* current # of messages on q */
ushort msg_qbytes; /* max # of bytes allowed on q */
ushort msg_lspid; /* pid of last msgsnd */
ushort msg_lrpid; /* pid of last msgrcv */
time_t msg_stime; /* time of last msgsnd */