UnixWare provides several mechanisms that allow processes to exchange data and synchronize execution. The simpler of these mechanisms are pipes, named pipes, and signals. These are limited, however, in what they can do. For instance,
located in the sys/ipc.h
struct ipc_perm
{
uid_t uid; /* owner's user id */
gid_t gid; /* owner's group id */
uid_t cuid; /* creator's user id */
gid_t cgid; /* creator's group id */
mode_t mode; /* access modes */
ulong seq; /* slot usage sequence number */
key_t key; /* key */
long pad[4]; /* reserve area */
};
opcode
| Operation permissions | Octal value |
|---|---|
| Read by user | 00400 |
| Write by user | 00200 |
| Read by group | 00040 |
| Write by group | 00020 |
| Read by others | 00004 |
| Write by others | 00002 |
shmid_ds is located in the sys/shm.h header file.
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission struct */
int shm_segsz; /* segment size */
struct region *shm_reg; /* ptr to region structure */
char pad[4]; /* for swap compatibility */
pid_t shm_lpid; /* pid of last shmop */
pid_t shm_cpid; /* pid of creator */
ushort shm_nattch; /* used only for shminfo */
ushort shm_cnattch; /* used only for shminfo */
time_t shm_atime; /* last shmat time */
time_t shm_dtime; /* last shmdt time */
time_t shm_ctime; /* last change time */
};
int shmget (key, size, shmflg) return shmid parameter: 1.key_t key: key=0 , shmflg=IPC_PRIVATE : new shmid is always returned with an associated shared memory segment data structure key =any value ,shmflg=IPC_CREAT : if the key is not already in use, a new identifier is returned with an associated shared memory segment data structure else it returns an existing shared memory identifier that already has an associated shared memory segment data structure 2.int size input size of datastructure to be shared The system call will fail if the value for the size argument is less than SHMMIN or greater than SHMMAX 3.int shmflg operations permissions(op) control fields (cf) IPC_PRIVATE. will create shared with key 0 . IPC_CREAT Create the segment if it does not already exist in the kernel. IPC_EXCL When used with IPC_CREAT, fail if segment already exists. If IPC_CREAT is used alone, shmget either returns the segment identifier for a newly created segment, or returns the identifier for a segment which exists with the same key value. If IPC_CREAT is used with IPC_EXCL, a new segment is created unless the segment exists, in which case, the call fails with -1. IPC_EXCL guarantees that no existing segment is opened for access. shmflg=op|cf;
int shmctl (shmid, cmd, buf) int shmid, cmd; struct shmid_ds *buf; IPC_STAT return the status information contained in the associated data structure for the specified shmid and place it in the data structure pointed to by the buf pointer in the user memory area IPC_SET for the specified shmid, set the effective user and group identification, and operation permissions IPC_RMID remove the specified shmid with its associated shared memory segment data structure SHM_LOCK lock the specified shared memory segment in memory; must have P_SYSOPS privilege to perform this operation SHM_UNLOCK unlock the shared memory segment from memory; must have P_SYSOPS privilege to perform this operation
void *shmat (shmid, shmaddr, shmflg) int shmid; void *shmaddr; int shmflg; int shmdt (shmaddr) void *shmaddr;
Before a message can be sent or received, a uniquely identified message queue and data structure must be created.
The unique identifier is called the message queue identifier (msqid);
it is used to identify or refer to the associated message queue and data structure located in the sys/msg.h header file.
struct msqid_ds
{
struct ipc_perm msg_perm; /* operation permission struct */
struct msg *msg_first; /* ptr to first message on q */
struct msg *msg_last; /* ptr to last message on q */
ulong msg_cbytes; /* current # bytes on q */
ulong msg_qnum; /* # of messages on q */
ulong msg_qbytes; /* max # of bytes on q */
pid_t msg_lspid; /* pid of last msgsnd */
pid_t msg_lrpid; /* pid of last msgrcv */
time_t msg_stime; /* last msgsnd time */
time_t msg_rtime; /* last msgrcv time */
time_t msg_ctime; /* last change time */
};
int msgget (key_t key, int msgflg); int msgctl (msqid, cmd, buf) int msqid, cmd; struct msqid_ds *buf;
msgp points to a user defined buffer that must contain first a field of type long integer that will specify the type of the
message, and then a data portion that will hold the text of the message
struct
{long mtype; /* message type */
char mtext[]; /* message text */
}msgp;
mtype is a positive integer that can be used by the receiving process for message selection
int msgsnd(int msqid, const void *msgp,size_t msgsz, int msgflg); RETURN : 0:on Successful -1:on unsuccessful Parameters: msgp: pointer to a buffer which to be send as message msgsz : sie of the message msgflg: IPC_NOWAIT 1.set(msgflg=IPC_NOWAIT) 2.notset the operation would block if the total number of bytes allowed on the specified message queue are in use
int msgrcv(int msqid, void *msgp,size_t msgsz, long msgtyp, int msgflg); RETURN: -1:On unsuccessul number of bytes copied from queue PARAMETERS: msgp : pointer to a structure in the user memory area that will receive the message. msgsz length of the message to be received. If its value is less than the message in the array, an error can be returned if desired (see the msgflg argument below). msgtyp : argument is used to pick the first message on the message queue of the particular type specified. If it is equal to zero, the first message on the queue is received; if it is greater than zero, the first message of the same type is received; if it is less than zero, the lowest type that is less than or equal to its absolute value is received. msgflg: argument allows the ``blocking message operation'' to be performed IPC_NOWAIT 1.set(msgflg=IPC_NOWAIT) if the IPC_NOWAIT flag is not set ((msgflg and IPC_NOWAIT) == 0); : the operation would block if there is not a message on the message queue of the desired type (msgtyp) to be received. 2.noset If the IPC_NOWAIT flag is set, the system call will fail immediately when there is not message of the desired type on the queue. MSG_NOERROR 1.set(msgflg=IPC_NOERROR) MSG_NOERROR flag is set, the message is truncated to the length specified by the msgsz argument of msgrcv. 2.noset the system call fail if the message is longer than the size to be received