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Message Queues
Linked list of msgs stored within K
Create Q / q open msgget()
Msg added at end of Q msgsnd()
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Message Queues
struct msqid_ds {
struct ipc_perm msg_perm;
msgqnum_t msg_qnum; /* # of messages on queue */
msglen_t msg_qbytes; /* max # of bytes on queue */
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 */ . };
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Free BSD
Linux
Mac OS
Size in Bytes of large msg we can send B
8192
Not support
Size of q
2048 B
16384
Max number of messages 40 16
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#include <sys/msg.h>
int msgget(key_t key, int flag);
Returns: message queue ID if OK, 1 on error
Ipc_ perm=0
msgqnum, msg_lspid, msg_lrpid ,msg_stime, msg_rtime
msg_ ctime=current time
int msgctl(int msqid, int cmd, struct msqid_ds *buf );
Returns: 0 if OK, -1 on error
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5. Messages are retrieved from q by
#include<sys/msg.h>
Ssizet msgrcv(int msqid,void *ptr,size bytes,long type,int flag);
Type==0 first msg returned
Type>0 first msg on q whose message type equals type is returned
Type<0 first msg on q whose message type is lowest less than or equal to absolute value type is returned
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Shared memory
Allows two or more processes to share a given region
of memory
Fastest form of IPC
Struct shmid_ds{
Ipc_perm shm_perm;
Size_t shm_size;
Pid_t shm_pid;
Time_t shm_dtime;
Time_t shm_ctime;
Time_t shm_atime; . };
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#include<sys.shm.h>
Int shmget(keyt key,size_t size, int flag);
Return is if ok -1 on error
#include<sys.shm.h>//catch all
Int shmctl(int shmid,int cmd,struct shmids_ds *buf);
Void *shmat(int shmid,const void *addr,int flag);
Int shmdt(void *addr);
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program that prints info on where one particular system places various types of data char array[size]; int main(void) { int shmid; if(ptrmalloc(MALLOC_SIZE)==NULL err_sys(“malloc error”); if (shmid=shmget(ipc_private,size,shm mode)<0) err_sys(“ error”); if (shmptr=shmat(shmid,0,0)) err_sys(“ error”); if (shmctl(shmid,ipc_id,0) err_sys(“ error”); exit(0); }
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semaphores
A semaphore is a counter used to provide access
to a shared data object for
multiple processes.
Example :operation of railroads
Only one rail is allowed.
Guarding this track is semaphore.
Semaphore is simple integer.
Test semaphore that controls the resource.
Semaphore=positive process can use resource
Semaphore=semaphore-1 (since it has used one unit of the resource)
Otherwise semaphore=0, the process goes to sleep
until the semaphore value >0.when the process wakes up, it returns to step1.
Figure: single rail track
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The kernel maintains a semid_ds structure for each semaphore set:
struct semid_ds {
struct ipc_perm sem_perm;
unsigned short sem_nsems; /* # of semaphores in set */
time_t sem_otime; /* last-semop() time */
time_t sem_ctime; /* last-change time */ . };
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The kernel maintains a semid_ds
structure for each semaphore set: minimum
struct semid_ds {
struct ipc_perm sem_perm;
unsigned short sem_nsems; /* # of semaphores in set */
time_t sem_otime; /* last-semop() time */
time_t sem_ctime; /* last-change time */ . };
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The table below lists the system limits that affect semaphore sets.
Description
Typical values
FreeBSD 5.2.1
Linux
Mac OS X 10.3
Solaris 9
The maximum value of any semaphore
32,767
The maximum value of any semaphore's adjust-on-exit value
16,384
The maximum number of semaphore sets, systemwide 10
128
87,381
The maximum number of semaphores, systemwide 60
32,000
The maximum number of semaphores per semaphore set
250 25
The maximum number of undo structures, systemwide 30
The maximum number of undo entries per undo structures 32
The maximum number of operations per semop call
100
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The first function to call is semget to obtain a semaphore ID.
#include <sys/sem.h>
int semget(key_t key, int nsems, int flag);
Returns: semaphore ID if OK, -1 on error
When a new set is created, the following members of the semid_ds structure are initialized.
The ipc_perm structure is initialized. The mode member of this structure is set to the corresponding permission bits of flag.
sem_otime is set to 0.
sem_ctime is set to the current time.
sem_nsems is set to nsems( number of semaphores in the set )
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If a new set is being created (typically in the server), we must specify nsems. If we are referencing an existing set (a client), we can specify nsems as 0.
The semctl function is the catchall for various semaphore operations.
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ... /* union semun arg */);  
The fourth argument is optional, depending on the command requested, and if present, is of type semun, a union of various command-specific arguments:
union semun {
int val; /* for SETVAL */
struct semid_ds *buf; /* for IPC_STAT and IPC_SET */
unsigned short *array; /* for GETALL and SETALL */ };
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Command value
IPC_STAT fetch semid_ds structure for this set
IPC_RMID remove semaphore set from the system
GETVAL return value of semval for the member semnum
SETVAL set the value of semval for the member semnum
GETPID return the value of sempid for the member semnum
GETALL fetch all semaphore values
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Semop performs an array of operations on a semaphore set
#include<sys/sem.h>
Int semop(int semid,struct sembuf semoparray[],size nops);
Return 0 if ok -1 on error
Semoparray argument is a pointer to an array of semaphore operations,represented by sembuf structures
Struct sembuf { unsigned short sem_num;
short sem_op;
short sem_flg; };
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Identifiers and Keys
XSI IPC are message queues, semaphores, and shared memory.
Each IPC structure (message queue, semaphore, or shared memory segment) in the kernel is referred to by a non-negative integer identifier.
To send or fetch a message to or from a message queue, for example, all we need know is the identifier for the queue.
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The identifier is an internal name for an IPC object.
Cooperating processes need an external naming scheme to be able to rendezvous using the same IPC object.
For this purpose, an IPC object is associated with a key that acts as an external name.
Whenever an IPC structure is being created (by calling msgget, semget, or shmget), a key must be specified.
The data type of this key is the primitive system data type key_t, which is often defined as a long integer in the header <sys/types.h>.
This key is converted into an identifier by the kernel.
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