posix 消息队列

注意

在涉及到posix消息的函数时, gcc 编译时要加-lrt参数, 如
gcc -lrt unpipc.c mqpack.c send.c -o send
gcc -lrt unpipc.c mqpack.c receive.c -o receive

posix消息的name必须以/开头, 例
./send /msgname

基本函数

#include <mqueue.h>
mqd_t mq_open(const char *name,int oflag, ...
    /* mode_t mode, struct mq_attr *attr */);
int mq_close(mqd_t mqdes);
int mq_unlink(const char *name);
int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
int mq_setattr(mqd_t mqdes,const struct mq_attr *attr,struct mq_attr *oattr);
int mq_send(mqd_t mqdes, const char *ptr, size_t len, unsigned int prio);
ssize_t mq_receive(mqd_t mqdes, char *ptr, size_t len, unsigned int *priop);
int mq_notify(mqd_t mqdes, const struct sigevent *notification);
 
#include <signal.h>
int sigwait(const sigset_t *set, int *sig);

posix消息队列的读写可以想像成文件的读写:
mq_close用于关闭mqd/关闭文件fd
mq_unlink用于删除消息队列/删除文件
mq_send发送消息到消息队列/写入文件
mq_receive从消息队列中取出消息/读取文件
mq_getattr获取消息属性/获取文件信息
mq_notify当队列中的消息数量由0到1时产生指定的信号/IO同步

mq_notify:
1.任意时候只有一个进程可以被注册为接收通知
2.当通知被发送到注册进程时,其注册即被撤销,就像老式的signal一样
3.当消息到达队列后再进行注册的话,不会产生消息
4.只有在消息队列的数量从0到1时才产生

sigwait:
1.sigwait是与当前进行的屏蔽信号函数sigprocmask搭配使用的
2.sigwait挂起线程,直到有感兴趣的信号到达,而这个信号也属于当前进程的屏蔽信号集
3.sigwait将信号从未决信号中删除,且不会调用信号处理函数,第二个参数返回该信号值
4.与sigsuspend不同的是,sigwait并不改变当前进程的信号屏蔽集,也不调用处理函数

包裹函数

mqpack.h

#ifndef _MQPACK_H
#define _MQPACK_H
 
#include "unpipc.h"
#include <mqueue.h>
mqd_t Mq_open(const char *name,int oflag,mode_t mode,struct mq_attr *attr);
void Mq_close(mqd_t mqd);
void Mq_unlink(const char *name);
void Mq_getattr(mqd_t mqd,struct mq_attr *attr);
void Mq_setattr(mqd_t mqd,struct mq_attr *attr,struct mq_attr *oattr);
void Mq_send(mqd_t mqd,const char *ptr,size_t len,unsigned int prio);
ssize_t Mq_receive(mqd_t mqd,char *ptr,size_t len,unsigned int *priop);
void Mq_notify(mqd_t mqd,const struct sigevent *notification);
 
#endif

mqpack.c

#include "mqpack.h"
 
mqd_t Mq_open(const char *name,int oflag,mode_t mode,struct mq_attr *attr){
    mqd_t mqd=mq_open(name,oflag,mode,attr);
    if(mqd == -1)
        err_quit("mq_open error");
    return(mqd);
}
void Mq_close(mqd_t mqd){
    if(mq_close(mqd) == -1)
        err_quit("mq_close error");
}
void Mq_unlink(const char *name){
    if(mq_unlink(name) == -1)
        err_quit("mq_unlink error");
}
void Mq_getattr(mqd_t mqd,struct mq_attr *attr){
    if(mq_getattr(mqd,attr) == -1)
        err_quit("mq_getattr error");
}
void Mq_setattr(mqd_t mqd,struct mq_attr *attr,struct mq_attr *oattr){
    if(mq_setattr(mqd,attr,oattr) == -1)
        err_quit("Mq_setattr error");
}
void Mq_send(mqd_t mqd,const char *ptr,size_t len,unsigned int prio){
    if(mq_send(mqd,ptr,len,prio) == -1)
        err_quit("mq_send error");
}
ssize_t Mq_receive(mqd_t mqd,char *ptr,size_t len,unsigned int *priop){
    size_t n=mq_receive(mqd,ptr,len,priop);
    if((-1 == n) && (errno != EAGAIN))
        err_quit("mq_receive error");
    return (n);
}
void Mq_notify(mqd_t mqd,const struct sigevent *notification){
    if(mq_notify(mqd,notification) == -1)
        err_quit("mq_notify error");
}

例子

编译和执行步骤就是文章开头的那三条命令
send.c

#include "unpipc.h"
#include "mqpack.h"
 
int main(int argc,char *argv[]){
    if(argc != 2)
        err_quit("usage: /send <name>");
    char buff[1024];
    mqd_t mqd;
    struct mq_attr attr;
    mqd=Mq_open(argv[1],O_WRONLY|O_CREAT,0644,NULL);
 
    if(Fork() == 0){
        if(execl("./receive","./receive",argv[1],(char *)0) == -1)
            err_quit("execl error");
    }
 
    sleep(2);
 
    while(1){
        Fgets(buff,1024,stdin);
        if(buff[strlen(buff)-1] == '
') //去掉buff中的换行符
            buff[strlen(buff)-1]='';
        Mq_send(mqd,buff,strlen(buff),0);
    }
    Mq_close(mqd);
    exit(0);
}

receive.c

#include "unpipc.h"
#include "mqpack.h"
 
void print_info(mqd_t mqd,struct mq_attr *attr){
    if(mq_getattr(mqd,attr) == -1)
        err_quit("mq_getn error");
    printf("max num of maxmsg = %ld,max size of msg = %ld
",attr->mq_maxmsg,attr->mq_msgsize);
    printf("current mum of maxmsg = %ld
",attr->mq_curmsgs);
}
 
int main(int argc,char *argv[]){
    int signo;
    mqd_t mqd;
    void *buff;
    ssize_t n;
    sigset_t newmask;
    struct mq_attr attr;
    struct sigevent sigev;
 
    if(argc != 2)
        err_quit("usage mqtest2 <name>");
    mqd=Mq_open(argv[1],O_RDONLY|O_NONBLOCK,0,NULL);
    Mq_getattr(mqd,&attr);
    buff=Malloc(attr.mq_msgsize);
    print_info(mqd,&attr);
 
    Sigemptyset(&newmask);
    Sigaddset(&newmask,SIGUSR1);
    Sigprocmask(SIG_BLOCK,&newmask,NULL);
 
    sigev.sigev_notify=SIGEV_SIGNAL;
    sigev.sigev_signo=SIGUSR1;
    Mq_notify(mqd,&sigev);
 
    for(;;){
        Sigwait(&newmask,&signo);
        if(signo == SIGUSR1){
            Mq_notify(mqd,&sigev);
            while((n=Mq_receive(mqd,buff,attr.mq_msgsize,NULL))>0)
                printf("read %ld bytes
",(long)n);
            //if(errno != EAGAIN)
            //   err_quit("mq_receive error");
        }
    }
}