inet_addr()和htonl()结合使用引发的connect()超时

Tcp通信使用的是网络字节序，所以一般都需要htonl()把ip地址转换成网络字节序，但如果ip已经是网络字节序了，再调用htonl就会导致不再是网络字节序了，引发严重后果，就是connect或者使用了一个相反的ip

上代码

服务端：

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/types.h>  /* See NOTES */#include <sys/socket.h>#include <netinet/in.h>  #include <arpa/inet.h>  #include <netdb.h>  #include <errno.h>#include <unistd.h>extern int errno;int main(){int domain = AF_INET;int type = SOCK_STREAM;int protocol = 0;int ret  = -1;int nListenFd = -1;int nNewClientFd = -1;short int  port = 2000; struct sockaddr_in addr_in;int backlog = 128; // 默认是128int len = 0;char chBuffer[1024] = {0};int flags = 0;nListenFd = socket( domain,  type,  protocol);if(nListenFd < 0){printf("\n socket failed ! errno[%d]  err[%s]\n", errno, strerror(errno));return -1;}memset(&addr_in, 0, sizeof(struct sockaddr_in));addr_in.sin_family = AF_INET;addr_in.sin_port = htons(port);//htons的返回值是16位的网络字节序整型数   htons尾的字母s代表shortaddr_in.sin_addr.s_addr = htonl(INADDR_ANY);ret = bind(nListenFd, ( struct sockaddr * )(&addr_in), sizeof(struct sockaddr_in));    if(ret < 0)    {    printf("\n bind failed ! errno[%d]  err[%s]\n", errno, strerror(errno));    close(nListenFd); //避免资源泄漏return -1;}    ret = listen(nListenFd, backlog);    if(ret < 0)    {printf("\n listen failed ! errno[%d]err[%s]\n", errno, strerror(errno));close(nListenFd); //避免资源泄漏return -1;}nNewClientFd = accept(nListenFd, ( struct sockaddr *)NULL, NULL); //阻塞模式if(nNewClientFd < 0){printf("\n accept failed ! errno[%d]err[%s]\n", errno, strerror(errno));close(nListenFd); //避免资源泄漏return -1;}printf("\n  new client [%d] \n",nNewClientFd);close(nNewClientFd);close(nListenFd);return 0;}

有问题的客户端：

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <sys/types.h>  /* See NOTES */#include <sys/socket.h>#include <netinet/in.h>  #include <arpa/inet.h>  #include <netdb.h>  #include <errno.h>#include <unistd.h>extern int errno;int main(){int domain = AF_INET;//AF_INETint type = SOCK_STREAM;int protocol = 0;int ret  = -1;int nClientFd = -1;short int  port = 2000; struct sockaddr_in addr_in;int len = 0;char chBuffer[1024] = {0};int flags = 0;char * pchServerIP = "192.168.1.211";nClientFd = socket( domain,  type,  protocol);if(nClientFd < 0){printf("\n socket failed ! errno[%d]  err[%s]\n", errno, strerror(errno));return -1;}    memset(&addr_in, 0, sizeof(struct sockaddr_in));addr_in.sin_family = AF_INET;addr_in.sin_port = htons(port);//htons的返回值是16位的网络字节序整型数   htons尾的字母s代表shortaddr_in.sin_addr.s_addr = htonl(inet_addr(pchServerIP)); //错误的做法//addr_in.sin_addr.s_addr = inet_addr(pchServerIP); ret = connect(nClientFd, ( struct sockaddr * )(&addr_in), sizeof(struct sockaddr_in));    if(ret < 0)    {    printf("\n connect failed ! errno[%d]  err[%s]\n", errno, strerror(errno));    close(nClientFd); //避免资源泄漏return -1;}printf("\n  connect success ! \n");close(nClientFd);return 0;}

编译：
gcc simple_server.c -g -o simple_server
 gcc simple_client.c -g -o simple_client

先启动服务器，并使用strace去追踪调用

strace  ./simple_server

服务器一直阻塞，等待连接

客户端：

本来客户端是要去connect  192.168.1.211 ，结果被搞成了去connect   211.1.168.192，因此活该connect超时啊

改正方法：

客户端的代码

把addr_in.sin_addr.s_addr = htonl(inet_addr(pchServerIP)); 

改成addr_in.sin_addr.s_addr = inet_addr(pchServerIP);即可

顺便说以下这个inet_addr函数

 #include <sys/socket.h>
       #include <netinet/in.h>
       #include <arpa/inet.h>

 in_addr_t inet_addr(const char *cp);

  The inet_addr() function converts the Internet host address cp from IPv4 numbers-and-dots notation  into  binary  data  in
       network  byte  order.  If the input is invalid, INADDR_NONE (usually -1) is returned.  Use of this function is problematic
       because -1 is a valid address (255.255.255.255).  Avoid its use in favor of inet_aton(), inet_pton(3), or  getaddrinfo(3),
       which provide a cleaner way to indicate error return.

 

inet_addr() 函数把字符串ip转换成具有网络字节序的二进制ip地址了，因此不要再画蛇添足，再调用htonl再转一次网络字节序了，后果很严重。

当然文档提示这个函数也不是很好，就是返回-1的时候的ip地址会是255.255.255.255