POSIX Message Queue

Posix message queue is way to communicate data between process.
This could use for producer-consumer kind of algorithm.

whenever use this code in daemon, use syslog and strerror(errno) function instead of perror.

Compiling and linking

Header file
#include <mqueue.h> library -lrt use for linking

open a message queue with name.

bool open_queue(mqd_t& mqid,const string& queue_name,unsigned int maxqueue,unsigned int msgsize) {

// First we need to set up the attribute structure

struct mq_attr attr; attr.mq_maxmsg =maxqueue; attr.mq_msgsize = msgsize; attr.mq_flags = 0;

// Open a queue with the attribute structure

mqid = mq_open (queue_name.c_str(), O_RDWR | O_CREAT | O_NONBLOCK,  0666, &attr); if(mqid==(mqd_t)-1){ perror("Error:mq_open: ");  // use syslog and strerror(errno) for daemon return false; } return true; }

closing a message

inline void close_queue(mqd_t& mqid) { mq_close(mqid); }

Send or write message to queue

void send_message(mqd_t& mqid, const std::string& str_msg,unsigned int priority) { if (mq_send (mqid, str_msg.c_str(), str_msg.length(), priority) == -1) perror ("Error:mq_send: ");

// use syslog and strerror(errno) for daemon

}

Get count of message available in queue.

int get_message_count(mqd_t& mqid) { int ret=0 struct mq_attr attr; if(mq_getattr (mqid, &attr)==0){ ret= attr.mq_curmsgs; } else perror("Error:mq_getattr: "); return ret; }

Get the max size of messages.

int get_message_size(mqd_t& mqid) { int ret=0; struct mq_attr attr; if(mq_getattr (mqid, &attr)==0){ ret= attr.mq_msgsize; } else perror("Error:mq_getattr: "); return ret; }

Get a message from message queue.

bool get_message(mqd_t& mqid,const std::string& str_msg) { bool ret=false; int msgsize = get_message_size(mqid); if(msgsize && get_message_count(mqid)){ char* buf= new buf[msgsize+1]; memset(buf,0,msgsize+1); if(mq_receive (mqid, buf, msgsize, &prio) != -1){ str_msg=std::string(buf); ret=true; } delete[] buf; } return ret; }