【发布时间】：2019-05-27 03:06:11
【问题描述】：
我正在用 C 语言构建一个客户端-服务器应用程序，源代码取自《Unix 环境中的高级编程》一书。
在服务器中它正在执行以下操作：
struct addrinfo hint;
memset(&hint, 0, sizeof(hint));
hint.ai_flags = AI_CANONNAME;
hint.ai_socktype = SOCK_STREAM;
hint.ai_addr = NULL;
hint.ai_next = NULL;
....
if ((n = sysconf(_SC_HOST_NAME_MAX))<0)
{
n = HOST_NAME_MAX;
}
if((host = malloc(n)) == NULL)
{
printf("malloc error\n");
exit(1);
}
if (gethostname(host, n)<0)
{
printf("gethostname error\n");
exit(1);
}
...
if((err = getaddrinfo(host, "ruptime", &hint, &ailist))!=0)
{
syslog(LOG_ERR, "ruptimed: getaddrinfo error %s", gai_strerror(err));
exit(1);
}
for (aip = ailist; aip!=NULL; aip = aip->ai_next)
{
if ((sockfd = initserver(SOCK_STREAM, aip->ai_addr, aip->ai_addrlen, QLEN))>=0)
{
//printf("starting to serve\n");
serve(sockfd);
exit(0);
}
}
据我了解，函数getaddrinfo 用于在主机上查看运行名为ruptime 且类型为SOCK_STREAM 的服务的套接字地址结构。
虽然书中没有指定，但为了工作，我必须在文件/etc/services/ 中运行一个新条目，使用未使用的端口和指定的名称ruptime：
ruptime 49152/tcp #ruptime Unix System Programming
ruptime 49152/udp #ruptime Unix System Programming
虽然未使用，但建议添加 UDP 部分。
但是它说的文档
如果在
hints.ai_flags中指定了AI_PASSIVE标志，并且节点是NULL，那么返回的套接字地址将适合 绑定（2）一个将接受（2）连接的套接字。返回的 套接字地址将包含“通配符地址”（INADDR_ANYIPv4 地址，IN6ADDR_ANY_INIT用于 IPv6 地址）。通配符 应用程序（通常是服务器）使用地址 接受任何主机网络地址上的连接。
所以从这里和其他关于 SO 的讨论来看：
hint.ai_flags |= AI_PASSIVE
...
getaddrinfo(NULL, myserviceport, &hint, &aihint)
似乎更合适。
这两种方法到底有什么区别？第二个是否也在寻找SOCK_DGM？在书中选择第一种方法有什么原因吗？第二种方式，因为我在代码中指定了端口，是否允许避免在/etc/services/中添加新条目？
另一个问题。
对于客户端，我必须传递主机名。我认为环回（客户端和服务器在同一台机器上运行）地址就可以了。相反，主机名类似于./client MBPdiPippo.lan。什么定义了可以使用主机名但不能使用环回地址创建连接这一事实？是我将host 作为第一个参数传递给服务器中的getaddrinfo 吗？
完整代码
server.c
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h> //_SC_HOST_NAME_MAX
#include<string.h>
#include<netdb.h> //Here are defined AF_INET and the others of the family
#include<syslog.h> //LOG_ERR
#include<errno.h> //errno
#include <sys/types.h>
#include"utilities.h"
#include "error.h"
#define BUFLEN 128
#define QLEN 10
#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 156
#endif
int initserver(int type, const struct sockaddr *addr, socklen_t alen, int qlen);
void serve(int sockfd);
int main(int argc, char* argv[])
{
printf("entered main\n");
struct addrinfo *ailist, *aip, hint;
int sockfd, err, n;
char *host;
if (argc != 1)
{
printf("usage: ruptimed\n");
exit(1);
}
if ((n=sysconf(_SC_HOST_NAME_MAX))<0)
{
n = HOST_NAME_MAX;
}
if((host = malloc(n)) == NULL)
{
printf("malloc error\n");
exit(1);
}
if (gethostname(host, n)<0)
{
printf("gethostname error\n");
exit(1);
}
printf("host: %s\n", host);
printf("Daemonizing\n");
int res = daemonize("ruptimed");
printf("%d\n", res);
printf("Daemonized\n");
memset(&hint, 0, sizeof(hint)); //set to 0 all bytes
printf("hint initialized\n");
hint.ai_flags = AI_CANONNAME;
hint.ai_socktype = SOCK_STREAM;
hint.ai_canonname = NULL;
hint.ai_addr = NULL;
hint.ai_next = NULL;
printf("getting addresses\n");
if((err = getaddrinfo(host, "ruptime", &hint, &ailist))!=0)
{
printf("error %s\n", gai_strerror(err));
syslog(LOG_ERR, "ruptimed: getaddrinfo error %s", gai_strerror(err));
exit(1);
}
printf("Got addresses\n");
for (aip = ailist; aip!=NULL; aip = aip->ai_next)
{
if ((sockfd = initserver(SOCK_STREAM, aip->ai_addr, aip->ai_addrlen, QLEN))>=0)
{
printf("starting to serve\n");
serve(sockfd);
exit(0);
}
}
exit(1);
}
void serve(int sockfd)
{
int clfd;
FILE *fp;
char buf[BUFLEN];
set_cloexec(sockfd);
for(;;)
{
/*After listen, the socket can receive connect requests. accept
retrieves a connect request and converts it into a connection.
The file returned by accept is a socket descriptor connected to the client that
called connect, haing the same coket type and family type. The original
soket remains available to receive otherconneion requests. If we don't care
about client's identity we can set the second (struct sockaddr *addr)
and third parameter (socklen_t *len) to NULL*/
if((clfd = accept(sockfd, NULL, NULL))<0)
{
/*This generates a log mesage.
syslog(int priority, const char *fformat,...)
priority is a combination of facility and level. Levels are ordered from highest to lowest:
LOG_EMERG: emergency system unusable
LOG_ALERT: condiotin that must be fied immediately
LOG_CRIT: critical condition
LOG_ERR: error condition
LOG_WARNING
LOG_NOTICE
LOG_INFO
LOG_DEBUG
format and other arguments are passed to vsprintf function forf formatting.*/
syslog(LOG_ERR, "ruptimed: accept error: %s", strerror(errno));
exit(1);
}
/* set the FD_CLOEXEC file descriptor flag */
/*it causes the file descriptor to be automatically and atomically closed
when any of the exec family function is called*/
set_cloexec(clfd);
/**pg. 542 Since a common operation is to create a pipe to another process
to either read its output or write its input Stdio has provided popen and
pclose: popen creates pipe, close the unused ends of the pipe,
forks a child and call exec to execute cmdstr and
returns a file pointer (connected to std output if "r", to stdin if "w").
pclose closes the stream, waits for the command to terminate*/
if ((fp = popen("/usr/bin/uptime", "r")) == NULL)
{
/*sprintf copy the string passed as second parameter inside buf*/
sprintf(buf, "error: %s\n", strerror(errno));
/*pag 610. send is similar to write. send(int sockfd, const void *buf, size_t nbytes, it flags)*/
send(clfd, buf, strlen(buf),0);
}
else
{
/*get data from the pipe that reads created to exec /usr/bin/uptime */
while(fgets(buf, BUFLEN, fp)!=NULL)
{
/* clfd is returned by accept and it is a socket descriptor
connected to the client that called connect*/
send(clfd, buf, strlen(buf), 0);
}
/*see popen pag. 542*/
pclose(fp);
}
close(clfd);
}
}
int initserver(int type, const struct sockaddr *addr, socklen_t alen, int qlen)
{
int fd, err;
int reuse = 1;
if ((fd = socket(addr->sa_family, type, 0))<0)
{
return (-1);
}
if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(int))<0)
{
goto errout;
}
if(bind(fd, addr, alen)<0)
{
goto errout;
}
if (type == SOCK_STREAM || type == SOCK_SEQPACKET)
{
if(listen(fd, qlen)<0)
{
goto errout;
}
}
return fd;
errout:
err = errno;
close (fd);
errno = err;
return(-1);
}
utilities.c：包含demonize 和setcloexec 函数。在daemonize 函数中，我没有关闭文件描述符以进行调试。
#include "utilities.h"
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <syslog.h>
#include <sys/time.h>//getrlimit
#include <sys/resource.h>//getrlimit
#include <signal.h> //sigempyset , asigcation (umask?)
#include <sys/resource.h>
#include <fcntl.h> //O_RDWR
#include <stdarg.h>
#include "error.h"
int daemonize(const char *cmd)
{
int fd0, fd1, fd2;
unsigned int i;
pid_t pid;
struct rlimit rl;
struct sigaction sa;
/* *Clear file creation mask.*/
umask(0);
/* *Get maximum number of file descriptors. */
if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
{
err_quit("%s: can’t get file limit", cmd);
}
/* *Become a session leader to lose controlling TTY. */
if ((pid = fork()) < 0)
{
err_quit("%s: can’t fork", cmd);
}
else if (pid != 0) /* parent */
{
exit(0); //the parent will exit
}
setsid();
/* *Ensure future opens won’t allocate controlling TTYs. */
sa.sa_handler = SIG_IGN;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGHUP, &sa, NULL) < 0)
{
err_quit("%s: can’t ignore SIGHUP", cmd);
}
if ((pid = fork()) < 0)
{
err_quit("%s: can’t fork", cmd);
}
else if (pid != 0) /* parent */
{
exit(0);
}
/*
*Change the current working directory to the root so
* we won’t prevent file systems from being unmounted.
*/
if (chdir("/") < 0)
{
err_quit("%s: can’t change directory to /", cmd);
}
/* Close all open file descriptors. */
if (rl.rlim_max == RLIM_INFINITY)
{
rl.rlim_max = 1024;
}
printf("closing file descriptors\n");
/*for (i = 0; i < rl.rlim_max; i++)
{
close(i);
}*/
/* *Attach file descriptors 0, 1, and 2 to /dev/null.*/
//printf not working
/*printf("closed all file descriptors for daemonizing\n");*/
/*fd0 = open("/dev/null", O_RDWR);
fd1 = dup(0);
fd2 = dup(0);*/
/* *Initialize the log file. Daemons do not have a controlling terminal so
they can't write to stderror. We don't want them to write to the console device
because on many workstations the control device runs a windowing system. They can't
write on separate files either. A central daemon error-logging facility is required.
This is the BSD. 3 ways to generate log messages:
1) kernel routines call the log function. These messages can be read from /dev/klog
2) Most user processes (daemons) call syslog to generate log messages. This causes
messages to be sent to the UNIX domain datagram socket /dev/log
3) A user process on this host or on other host connected to this with TCP/ID
can send log messages to UDP port 514. Explicit network programmin is required
(it is not managed by syslog.
The syslogd daemon reads al three of log messages.
openlog is optional since if not called, syslog calls it. Also closelog is optional
openlog(const char *ident, int option, int facility)
It lets us specify ident that is added to each logmessage. option is a bitmask:
LOG_CONS tells that if the log message can't be sent to syslogd via UNIX
domain datagram, the message is written to the console instead.
facility lets the configuration file specify that messages from different
facilities are to be handled differently. It can be specified also in the 'priority'
argument of syslog. LOG_DAEMON is for system deamons
*/
/*
openlog(cmd, LOG_CONS, LOG_DAEMON);
if (fd0 != 0 || fd1 != 1 || fd2 != 2)
{*/
/*This generates a log mesage.
syslog(int priority, const char *fformat,...)
priority is a combination of facility and level. Levels are ordered from highest to lowest:
LOG_EMERG: emergency system unusable
LOG_ALERT: condiotin that must be fied immediately
LOG_CRIT: critical condition
LOG_ERR: error condition
LOG_WARNING
LOG_NOTICE
LOG_INFO
LOG_DEBUG
format and other arguments are passed to vsprintf function forf formatting.*/
/*syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);
exit(1);
}*/
return 0;
}
/*The function set the FD_CLOEXEC flag of the file descriptor already open that
is passed to as parameter. FD_CLOEXEC causes the file descriptor to be
automatically and atomically closed when any of the exec family function is
called*/
int set_cloexec(int fd)
{
int val;
/* retrieve the flags of the file descriptor */
if((val = fcntl(fd, F_GETFD, 0))<0)
{
return -1;
}
/* set the FD_CLOEXEC file descriptor flag */
/*it causes the file descriptor to be automatically and atomically closed
when any of the exec family function is called*/
val |= FD_CLOEXEC;
return (fcntl(fd, F_SETFD, val));
}
错误函数我用过
/* Fatal error unrelated to a system call.
* Print a message and terminate*/
void err_quit (const char *fmt, ...)
{
va_list ap;
va_start (ap, fmt);
err_doit (0, 0, fmt, ap);
va_end (ap);
exit(1);
}
/*Print a message and return to caller.
*Caller specifies "errnoflag"*/
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
{
char buf [MAXLINE];
vsnprintf (buf, MAXLINE-1, fmt, ap);
if (errnoflag)
{
snprintf (buf+strlen(buf), MAXLINE-strlen(buf)-1, ": %s",
strerror (error));
}
strcat(buf, "\n");
fflush(stdout); /*in case stdout and stderr are the same*/
fputs (buf, stderr);
fflush(NULL); /* flushes all stdio output streams*/
}
【问题讨论】：
标签： c unix server posix getaddrinfo