技术背景

ØMQ （也拼写作ZeroMQ，0MQ或ZMQ)，号称号称是“史上最快的消息队列”，基于c语言开发。ZMQ(以下ZeroMQ简称ZMQ)是一个简单好用的传输层，像框架一样的一个socket library，他使得Socket编程更加简单、简洁和性能更高。是一个消息处理队列库，可在多个线程、内核和主机盒之间弹性伸缩。

作为C开发人员有两个选择 CZMQ 或 libzmq （低级zeromq库）。

在这里简单记录 CentOS 下从零安装ZMQ相关依赖、解决各种问题最终实现一个最简单的请求-应答服务器-客户端效果。

ZeroMQ 在 centos下的安装

1）下载ZeroMQ

执行命令：

wget http://download.zeromq.org/zeromq-4.0.4.tar.gz

2）解压ZeroMQ

$ tar zvxf zeromq-4.0.4.tar.gz
$ mv zeromq-4.0.4 zeromq
$ cd zeromq

3）安装依赖

$ yum install libtool
$ ./autogen.sh 

4）编译安装

$ ./configure
......
checking for gcc... no
checking for cc... no
checking for cl.exe... no
configure: error: in `/home/chuser/zeromq':
configure: error: no acceptable C compiler found in $PATH
See `config.log' for more details

提示缺少C编译器，先安装GCC。

$ sudo yum install gcc

安装OK！再次执行

$ ./configure
......
checking whether the C++ compiler works... no
configure: error: Unable to find a working C++ compiler

提示缺少C++编译器，先安装G++。

$ sudo yum install gcc-c++ 

安装OK！再次执行

$ ./configure
$ make
$ sudo make install

ZeroMQ安装成功！

libzmq 在 CentOS 下的安装

libzmq开源仓库： https://github.com/zeromq/libzmq

# 对于 CentOS 8，请以根用户 root 运行下面命令：
cd /etc/yum.repos.d/
wget https://download.opensuse.org/repositories/network:messaging:zeromq:release-stable/CentOS_8/network:messaging:zeromq:release-stable.repo
yum install zeromq-devel

# 对于 CentOS 7，请以根用户 root 运行下面命令：
cd /etc/yum.repos.d/
wget https://download.opensuse.org/repositories/network:messaging:zeromq:release-stable/CentOS_7/network:messaging:zeromq:release-stable.repo
yum install zeromq-devel

# 对于 CentOS 6，请以根用户 root 运行下面命令：
cd /etc/yum.repos.d/
wget https://download.opensuse.org/repositories/network:messaging:zeromq:release-stable/CentOS_6/network:messaging:zeromq:release-stable.repo
yum install zeromq-devel

Demo

(来源:https://zeromq.org/languages/c/#libzmq)

Server:

#include <zmq.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>

int main (void)
{
    //  Socket to talk to clients
    void *context = zmq_ctx_new ();
    void *responder = zmq_socket (context, ZMQ_REP);
    int rc = zmq_bind (responder, "tcp://*:5555");
    assert (rc == 0);

    while (1) {
        char buffer [10];
        zmq_recv (responder, buffer, 10, 0);
        printf ("Received Hello\n");
        sleep (1);          //  Do some 'work'
        zmq_send (responder, "World", 5, 0);
    }
    return 0;
}

Client:

Copy
#include <zmq.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>

int main (void)
{
    printf ("Connecting to hello world server…\n");
    void *context = zmq_ctx_new ();
    void *requester = zmq_socket (context, ZMQ_REQ);
    zmq_connect (requester, "tcp://localhost:5555");

    int request_nbr;
    for (request_nbr = 0; request_nbr != 10; request_nbr++) {
        char buffer [10];
        printf ("Sending Hello %d…\n", request_nbr);
        zmq_send (requester, "Hello", 5, 0);
        zmq_recv (requester, buffer, 10, 0);
        printf ("Received World %d\n", request_nbr);
    }
    zmq_close (requester);
    zmq_ctx_destroy (context);
    return 0;
}

将文件存入以libzmq_server.c，libzmq_client.c命名的文件。

运行下列命令编译：

gcc libzmq_server.c -o libzmq_server -lzmq
gcc libzmq_client.c -o libzmq_client -lzmq

若出现错误，请移步异常问题记录区寻找答案

分别在两个终端运行，若成功，效果如下：

$ ./libzmq_server
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello
Received Hello

$ ./libzmq_client
Connecting to hello world server…
Sending Hello 0…
Received World 0
Sending Hello 1…
Received World 1
Sending Hello 2…
Received World 2
Sending Hello 3…
Received World 3
Sending Hello 4…
Received World 4
Sending Hello 5…
Received World 5
Sending Hello 6…
Received World 6
Sending Hello 7…
Received World 7
Sending Hello 8…
Received World 8
Sending Hello 9…
Received World 9

异常记录

undefined reference to `zmq_sendmsg'

解决方案:
编译时带上 -lzmq

例如：gcc -o hello hello.c -lzmq

error while loading shared libraries: libzmq.so.3

方法1：

找一下动态文件安装到哪里了：

$ whereis libzmq.so.3

$ find / -name libzmq.so.3

将 /usr/local/lib 加到共享库配置文件 /etc/ld.so.conf 中，该方法一劳永逸，但是需要有操作权限

# cat /etc/ld.so.conf
include ld.so.conf.d/*.conf
# echo "/usr/local/lib">>/etc/ld.so.conf
# ldconfig

再执行成功.

方法2：设置环境变量LD_LIBRARY_PATH，适合没有操作权限时使用

# vi ~/.bash_profile 

编辑你当前用户的用户目录下的.bash_profile文件
在文件后面加上：

export LD_LIBRARY_PATH=/usr/local/lib/:$LD_LIBRARY_PATH

保存并关闭文件

# source ~/.bash_profile 

使该文件改动生效（每次登录该用户时也会自动加载该文件，使文件生效）
执行成功

C程序头文件引用问题

一直没有搞清楚头文件引用 "" 和 <> 的区别，今天偶然查到：

• first：include<fileName.h> 引用系统头文件一般用<>。
• second：include"fileName.h" 引用自己定义的头文件一般用" "。

区别是<>首先去系统中去找，" "则在自己当前文件夹找。

一般情况下系统头文件在 usr/include/中。