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IPC Programming (cont)

Faculty of Computer Science and Engineering
Ho chi Minh city University of Technology

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IPC
 Communication
Transferring message
Sharing information
Mechanisms:
Pipe
Signal
Message queue
Shared memory
Socket
RPC/RMI


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Synchronization
Solving confliction
Processing order
Mechanisms:
Lock file
Semaphore
Mutex (pthread)

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Faculty of Computer Science and Engineering - HCMUT


IPC Programming
 Shared memory
 Semaphore

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SystemV IPC
$ipcs
------Shared Memory Segments -------key
shmid owner perms bytes
0x00000000 65536 root
644
110592

nattch
11

------Semaphore Arrays -------key
semid owner perms

nsems



------Message Queues -------key
msqid owner perms

used-bytes

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status
dest

messages


Shared memory
xy

Process 1

Process 1

Process 2

Process 2
xy

xy
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Shared memory
 See all shared memory segments
 ipcs
 ipcs –a
 ipcs -m

 Remove a shared memory segment
 ipcrm shm shm_id
 ipcrm -m shm_id

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Shared memory
 Allow a lot of processes using the same memory segment
 Minimum/Maximum shared memory segment size is
1byte/4MB
 Maximum number of shared memory segments: 4096
 Usage
 Shared memory segment must be created first
 Attach shared memory segment to process’s address space
before using
 Detach shared memory segment from process’s address
space after finishing using it
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Operations on shared
memory
 shmget()
 shmat()
 shmdt()
 shmctl()

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shmget()
#include
#include
#include
int shmget(key_t key,int size,int shmflg);
 key: key of shared memory segment
 size: size of shared memory segment (bytes)
 shmflg: IPC_CREAT, IPC_EXCL or with its permission

 Example
shm_id = shmget(123, 4096, IPC_CREAT | 0660)

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Create IPC object key
#include
#include
key_t ftok(const char *path, int id);
 path: refer to an existing, accessible file
 id: project identifier
Return value:
 key_t value: if successful
 -1: if fail

 Example
key = ftok(“/tmp/file_123”, 321);
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shmat()
#include
#include
#include
void *shmat(int shmid,void *shmaddr,int shmflg);
 shmid: shared memory ID returned from shmget() function
 shmaddr: attaching address
 shmflg: SHM_RDONLY or 0

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shmdt()
#include
#include
#include
int shmdt(void *shmaddr);
 shmaddr: shared memory address (returned from shmat()

function)

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shmctl()
#include
#include
#include
int shmctl(int shmid,int cmd,struct shmid_ds *buf);
 shmid: shared memory ID returned from shmget() function
 cmd: IPC_STAT, IPC_SET and IPC_RMID

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Example
 Create a shared memory segment with 128 bytes size
 Create two processes to use this shared memory segment
 The first process writes 2 integer numbers into this shared

memory
 The second process reads from this shared memory, adds
these two numbers and writes the sum back to shared
memory
 The first process reads from shared memory to get the sum
and print out this sum to screen
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Example
int main() {
int *shm, shmid, k;
shmid = shmget(IPC_PRIVATE,128,IPC_CREAT|0666);
shm = (int*) shmat(shmid,0,0);
if(fork()==0) { /*child*/
shm[0]=111;
shm[1]=999;
sleep(3);
pintf("Process %d reads: Sum = %d“,getpid(),shm[2]);
shmdt((void *)shm);
shmctl(shmid, IPC_RMID, (struct shmid_ds *)0);
}
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Example
else {
/*parent*/
sleep(1);
printf("Process %d writes to shared memory ...\n",
getpid());
shm[2]=shm[0]+shm[1];
shmdt((void *)shm);
}
return(0);
}

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Faculty of Computer Science and Engineering - HCMUT


IPC Programming
 Shared memory
 Semaphore

SinhVienZone.com

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Faculty of Computer Science and Engineering - HCMUT


SystemV IPC
$ipcs
------Shared Memory Segments -------key
shmid owner perms bytes
0x00000000 65536 root
644
110592

nattch
11

------Semaphore Arrays -------key
semid owner perms

nsems

------Message Queues -------key
msqid owner perms

used-bytes

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Faculty of Computer Science and Engineering - HCMUT

status
dest

messages


Semaphore
 See all shared semaphore
 ipcs
 ipcs –a
 ipcs -s

 Remove a semaphore
 ipcrm sem semid
 ipcrm -s semid

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Operations on semaphore
 semget()
 semop()
 semctl()

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semget()
#include
#include
#include
int semget(key_t key, int nsems, int semflg);
 key: key of semaphore
 nsems: number of semaphore in the semaphore set
 semflag: IPC_CREAT, IPC_EXCL or with its permission

 Example
sem_id1=semget(IPC_PRIVATE,8,IPC_CREAT|0600);
sem_id2=semget(123,1,IPC_CREAT|IPC_EXCL|0660);
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Hàm semop()
#include
#include
#include
int semop(int semid, struct sembuf *sops,
size_t nsops);
 semid: semaphore set ID returned from semget() function
 sops: a pointer to an array of nsops sembuf structures, each

sembuf specifies an operation on a specific semaphore
 nsops: number of struct sembuf to be performed

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struct sembuf
struct sembuf {
ushort sem_num;
short sem_op;
short sem_flg;
}

/* semaphore number */
/* semaphore operation */
/* operation flags */

 sem_num: the order of this semaphore in the semaphore set
 sem_op: change the semaphore value
 sem_flg:



IPC_NOWAIT: non-blocking mode
SEM_UNDO: undo operation
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semctl()
#include
#include
#include
int semctl(int semid, int semnum, int cmd);
int semctl(int semid, int semnum, int cmd,
union semun arg);
union semun{
int val;
struct semid_ds *buf;
ushort *array;
};
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Faculty of Computer Science and Engineering - HCMUT


semctl() - cmd argument
 Commands on semaphore set
 IPC_STAT: get semaphore information
 IPC_SET : set semaphore’s ownership and permissions
 IPC_RMID: remove the semaphore set immediately
 Commands on individual semaphore
 GETVAL: get the value of the semnum-th semaphore
 SETVAL: set value of the semnum-th semaphore to arg.val
 GETPID: get PID of process that executed the last semop call for the
semnum-th semaphore of the set
 GETNCNT : get number of processes waiting for semval to increase
 GETZCNT: get number of processes waiting for semval to become zero
 Commands on all semaphores
 SETALL: set values for all semaphores of the set
 GETALL: get values of all semaphores of the set

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