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Linux多线程同步之消息队列

  消息队列是消息的链表,存放在内核中并有消息队列标示符标示。

  msgget用于创建一个新队列或打开一个现存的队列。msgsnd将新消息加入到消息队列中;每个消息包括一个long型的type;和消息缓存;msgrcv用于从队列中取出消息;取消息很智能,不一定先进先出

  ①msgget,创建一个新队列或打开一个现有队列

  #include

  int msgget ( key_t key, int flag );

  //成功返回消息队列ID;错误返回-1

  ②msgsnd: 发送消息

  #include

  int msgsnd( int msgid, const void* ptr, size_t nbytes, int flag )

  //成功返回0,错误返回-1

  a: flag可以指定为IPC_NOWAIT; 若消息队列已满,则msgsnd立即出错返回EABAIN;

  若没指定IPC_NOWAIT; msgsnd会阻塞,直到消息队列有空间为止

  ③msgrcv: 读取消息:

  ssize_t msgrcv( int msgid, void* ptr, size_t nbytes, long type, int flag );

  a. type == 0; 返回消息队列中第一个消息,先进先出

  b. type > 0 返回消息队列中类型为tpye的第一个消息

  c. type < 0 返回消息队列中类型 <= |type| 的数据;若这种消息有若干个,则取类型值最小的消息

  消息队列创建步骤:

  #define MSG_FILE "."

  struct msgtype {

  long mtype;

  char buffer[BUFFER+1];

  };

  if((key=ftok(MSG_FILE,'a'))==-1)

  {

  fprintf(stderr,"Creat Key Error:%sn", strerror(errno));

  exit(1);

  }

  if((msgid=msgget(key, IPC_CREAT | 0666/*PERM*/))==-1)

  {

  fprintf(stderr,"Creat Message Error:%sn", strerror(errno));

  exit(1);

  }

  msg.mtype = 1;

  strncpy(msg.buffer, argv[1], BUFFER);

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

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

  示例代码:

  #include

  #include

  #include

  #include

  #include

  #include

  #include

  #include

  #include

  #define MSG_FILE "."

  #define BUFFER 255

  #define PERM S_IRUSR|S_IWUSR

  #define IPCKEY 0x111

  struct msgtype {

  long mtype;

  char buffer[BUFFER+1];

  };

  void* thr_test( void* arg ){

  struct msgtype msg;

  int msgid;

  msgid = *((int*)arg);

  printf("msqid = %d IPC_NOWAIT = %dn", msgid, IPC_NOWAIT);

  time_t tt = time(0)+8;

  //while( time(0) <= tt )

  //{

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

  fprintf(stderr,"Server Receive:%sn", msg.buffer);

  msg.mtype = 2;

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

  //}

  pthread_exit( (void*)2 );

  }

  int main(int argc, char **argv)

  {

  struct msgtype msg;

  key_t key;

  int msgid;

  pthread_t tid;

  if(argc != 2)

  {

  fprintf(stderr,"Usage:%s stringn", argv[0]);

  exit(1);

  }

  /*

  char path[256];

  sprintf( path, "%s/", (char*)getenv("HOME") );

  printf( "path is %sn", path );

  msgid=ftok( path, IPCKEY );

  */

  if((key=ftok(MSG_FILE,'a'))==-1)

  {

  fprintf(stderr,"Creat Key Error:%sn", strerror(errno));

  exit(1);

  }

  if((msgid=msgget(key, IPC_CREAT | 0666/*PERM*/))==-1)

  {

  fprintf(stderr,"Creat Message Error:%sn", strerror(errno));

  exit(1);

  }

  pthread_create( &tid, NULL, thr_test, &msgid );

  fprintf(stderr,"msid is :%dn", msgid);

  msg.mtype = 1;

  strncpy(msg.buffer, argv[1], BUFFER);

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

  exit(0);

  }