程序通信之消息通信

消息通信 message

消息是程序間通信的一種重要方式，通常由用戶端和伺服器端組成。伺服器以特定的鍵值建立一個消息隊列，根據約定的消息格式填充要發送的消息和資料類型，把他插入消息隊列。而在用戶端，根據已知的鍵值和指定地點類型編碼，接收對應的資料。和共享記憶體的方式相比，不用自己維護消息隊列，可以集中精力關注資料傳輸，但不足的是，對于需要共享或者傳輸比較複雜資料結構的應用，它就顯得不太靈活。

消息通信重要的函數組包括：

int msgget(key_t key, int msgflg);

The msgget()  system  call returns the System V message queue identifier associated with the value of the key argument.  A

new message queue is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no message queue with the given key

key exists, and IPC_CREAT is specified in msgflg.

If  msgflg specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno

set to EEXIST.  (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

Upon creation, the least significant bits of the argument msgflg define the permissions of the message queue.   These  per‐

mission  bits have the same format and semantics as the permissions specified for the mode argument of open(2).  (The exe‐

cute permissions are not used.)

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,int msgflg);

DESCRIPTION

The  msgsnd() and msgrcv() system calls are used, respectively, to send messages to, and receive messages from, a System V

message queue.  The calling process must have write permission on the message queue in order to send a  message,  and  read

permission to receive a message.

The msgp argument is a pointer to a caller-defined structure of the following general form:

struct msgbuf {

long mtype;       /* message type, must be > 0 */

char mtext[1];    /* message data */

};

The  mtext field is an array (or other structure) whose size is specified by msgsz, a nonnegative integer value.  Messages

of zero length (i.e., no mtext field) are permitted.  The mtype field must have a strictly positive  integer  value.   This

value can be used by the receiving process for message selection (see the description of msgrcv() below).

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

msgctl() performs the control operation specified by cmd on the System V message queue with identifier msqid.

The msqid_ds data structure is defined in <sys/msg.h> as follows:

struct msqid_ds {

struct ipc_perm msg_perm;     /* Ownership and permissions */

time_t        msg_stime;    /* Time of last msgsnd(2) */

time_t        msg_rtime;    /* Time of last msgrcv(2) */

time_t        msg_ctime;    /* Time of last change */

unsigned long   __msg_cbytes; /* Current number of bytes in

queue (nonstandard) */

msgqnum_t      msg_qnum;     /* Current number of messages

in queue */

msglen_t       msg_qbytes;   /* Maximum number of bytes

allowed in queue */

pid_t          msg_lspid;    /* PID of last msgsnd(2) */

pid_t          msg_lrpid;    /* PID of last msgrcv(2) */

The ipc_perm structure is defined as follows (the highlighted fields are settable using IPC_SET):

struct ipc_perm {

key_t         __key;       /* Key supplied to msgget(2) */

uid_t         uid;         /* Effective UID of owner */

gid_t         gid;         /* Effective GID of owner */

uid_t         cuid;        /* Effective UID of creator */

gid_t         cgid;        /* Effective GID of creator */

unsigned short mode;        /* Permissions */

unsigned short __seq;       /* Sequence number */

Valid values for cmd are:

IPC_STAT

Copy information from the kernel data structure associated with msqid into the msqid_ds structure pointed to by buf.

The caller must have read permission on the message queue.

IPC_SET

Write the values of some members of the msqid_ds structure pointed to by buf to the kernel data structure associated

with this  message  queue, updating also its msg_ctime member.  The following members of the structure are updated:

msg_qbytes, msg_perm.uid, msg_perm.gid, and (the least significant 9 bits of) msg_perm.mode.  The effective  UID  of

the calling  process  must  match  the owner (msg_perm.uid) or creator (msg_perm.cuid) of the message queue, or the

caller must be privileged.  Appropriate privilege (Linux: the CAP_SYS_RESOURCE capability) is required to raise  the

msg_qbytes value beyond the system parameter MSGMNB.

IPC_RMID

Immediately remove  the  message queue, awakening all waiting reader and writer processes (with an error return and

errno set to EIDRM).  The calling process must have appropriate privileges or its effective user ID must  be  either

that of the creator or owner of the message queue.  The third argument to msgctl() is ignored in this case.

IPC_INFO (Linux-specific)

Return information  about system-wide message queue limits and parameters in the structure pointed to by buf.  This

structure is of type msginfo (thus, a cast is required), defined in <sys/msg.h> if  the  _GNU_SOURCE  feature  test

macro is defined:

struct msginfo {

int msgpool; /* Size in kibibytes of buffer pool

used to hold message data;

unused within kernel */

int msgmap;  /* Maximum number of entries in message

map; unused within kernel */

int msgmax;  /* Maximum number of bytes that can be

written in a single message */

int msgmnb;  /* Maximum number of bytes that can be

written to queue; used to initialize

msg_qbytes during queue creation

(msgget(2)) */

int msgmni;  /* Maximum number of message queues */

int msgssz;  /* Message segment size;

int msgtql;  /* Maximum number of messages on all queues

in system; unused within kernel */

unsigned short int msgseg;

/* Maximum number of segments;

The msgmni, msgmax, and msgmnb settings can be changed via /proc files of the same name; see proc(5) for details.

MSG_INFO (Linux-specific)

Return a  msginfo  structure  containing the same information as for IPC_INFO, except that the following fields are

returned with information about system resources consumed by message queues: the msgpool field returns the number of

message queues  that  currently  exist  on the system; the msgmap field returns the total number of messages in all

queues on the system; and the msgtql field returns the total number of bytes in all messages in all  queues  on  the

system.

MSG_STAT (Linux-specific)

Return a msqid_ds structure as for IPC_STAT.  However, the msqid argument is not a queue identifier, but instead an

index into the kernel's internal array that maintains information about all message queues on the system.

RETURN VALUE

On success, IPC_STAT, IPC_SET, and IPC_RMID return 0.  A successful IPC_INFO or MSG_INFO operation returns the index of the

highest used entry in the kernel's internal array recording information about all message queues.  (This information can be

used with repeated MSG_STAT operations to obtain information about all queues on the system.)  A successful MSG_STAT opera‐

tion returns the identifier of the queue whose index was given in msqid.

On error, -1 is returned with errno indicating the error.

依賴的頭檔案包括：

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/msg.h>

下面以具體可用的一個基于消息通信的服務端和用戶端的代碼為例：

服務端：test_msg_svc.c

#include<stdio.h>

#include<assert.h>

#include<inttypes.h>

#include <string.h>

#include <stdlib.h>

#include <errno.h>

#include <unistd.h>

#include <sys/stat.h>

#define MSG_FILE "test_msg_clnt"

#define BUFFER 255

#define PERM S_IRUSR|S_IWUSR

struct msgtype {

  long mtype;

  char buffer[BUFFER+1];

int main()

{  

  struct msgtype msg;

  key_t key;

  int msgid;

  if((key=ftok(MSG_FILE,'a'))==-1)

  {                              

{

      fprintf(stderr,"Creat Key Error：%s\a\n",strerror(errno));

      exit(1);

  }  

  if((msgid=msgget(key,PERM|IPC_CREAT|IPC_EXCL))==-1)

  //if((msgid=msgget(key,PERM|IPC_EXCL))==-1)

  {

      fprintf(stderr,"Creat Message Error：%s\a\n",strerror(errno));

  while(1)

      msgrcv(msgid,&msg,sizeof(struct msgtype),1,0);

      fprintf(stderr,"Server Receive：%s\n",msg.buffer);

      msg.mtype=2;

      msgsnd(msgid,&msg,sizeof(struct msgtype),0);

      memcpy(msg.buffer, "Hello", strlen("Hello") + 1);

  exit(0);

}  

用戶端： test_msg_clnt.c:

struct msgtype {                                                                                                                    

int main(int argc,char **argv)

  if(argc!=2)

      fprintf(stderr,"Usage：%s string\n\a",argv[0]);

  }

  if((msgid=msgget(key,PERM))==-1)

  msg.mtype=1;

  strncpy(msg.buffer,argv[1],BUFFER);

  msgsnd(msgid,&msg,sizeof(struct msgtype),0);

  memset(&msg,'\0',sizeof(struct msgtype));

  msgrcv(msgid,&msg,sizeof(struct msgtype),2,0);

  fprintf(stdout,"Client receive：%s\n",msg.buffer);

}

編譯運作及其結果：

[kxyx@localhost testcases]$ gcc -o test_msg_svc test_msg_svc.c

[kxyx@localhost testcases]$ gcc -o test_msg_clnt test_msg_clnt.c

[kxyx@localhost testcases]$ ./test_msg_svc

Server Receive：HaHi

Server Receive：GdLuCK

[kxyx@localhost testcases]$ ./test_msg_clnt HaHi

Client receive：HaHi

Client receive：Hello

[kxyx@localhost testcases]$ ./test_msg_clnt GdLuCK

Client receive：GdLuCK

本文轉自存儲之廚51CTO部落格，原文連結：http://blog.51cto.com/xiamachao/1793985 ，如需轉載請自行聯系原作者