书：
在unpbook 官网下载了源码，在Mac OS上make了里面的源码，我做的步骤：
1）chmod u+x configure
./configure
2)cd lib
make
3)cd ../libfree
make （报错了）
4)cd ../libgai
make （报错了）
然后试着去gcc一个TCP echo回文Server程序,然后一堆报错
#include "unp.h" int main(int argc, char **argv) { int listenfd, connfd; pid_t childpid; socklen_t clilen; struct sockaddr_in cliaddr, servaddr; void sig_chld(int); listenfd = Socket(AF_INET, SOCK_STREAM, 0); bzero(&servaddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(SERV_PORT); Bind(listenfd, (SA *) &servaddr, sizeof(servaddr)); Listen(listenfd, LISTENQ); Signal(SIGCHLD, sig_chld); for ( ; ; ) { clilen = sizeof(cliaddr); if ( (connfd = accept(listenfd, (SA *) &cliaddr, &clilen)) < 0) { if (errno == EINTR) continue; /* back to for() */ else err_sys("accept error"); } if ( (childpid = Fork()) == 0) { /* child process */ Close(listenfd); /* close listening socket */ str_echo(connfd); /* process the request */ exit(0); } Close(connfd); /* parent closes connected socket */ } } } 稍微gcc下，报一堆错误
Undefined symbols for architecture x86_64: "_Bind", referenced from: _main in tcpserv03-11de6f.o "_Close", referenced from: _main in tcpserv03-11de6f.o "_Fork", referenced from: _main in tcpserv03-11de6f.o "_Listen", referenced from: _main in tcpserv03-11de6f.o "_Signal", referenced from: _main in tcpserv03-11de6f.o "_Socket", referenced from: _main in tcpserv03-11de6f.o "_err_sys", referenced from: _main in tcpserv03-11de6f.o "_sig_chld", referenced from: _main in tcpserv03-11de6f.o "_str_echo", referenced from: _main in tcpserv03-11de6f.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) 
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