linux socket编程 双线程同时读写缓冲流时为什么fprintf被阻塞了

[greaper615]

这是我试着写的一个再简单不过的多线程socket模型

#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>

char *first_msg = "USER user * * :name\r\n", *second_msg = "NICK verylongnickname\r\n";

int connect_to_server(char*, int)
void *thread_send(FILE*);
void *thread_recv(FILE*);

main()
{
	char *server = "irc.freenode.net";
	int port = 6667;

	int fd;
	FILE *fp_send, *fp_recv;
	pthread_t thread;

	if ( (fd = connect_to_server(server, port)) == -1 )
		return 1;
	fp_recv = fdopen(fd, "r");
	fp_send = fdopen(fd, "w");

	pthread_create(&thread, NULL, thread_recv, fp_recv);
	pthread_create(&thread, NULL, thread_send, fp_send);

	while (1)
		sleep(1);
}

void *thread_recv(FILE *fp)
{
	char buf[BUFSIZ];

	while (1) {
		fgets(buf, BUFSIZ, fp);
		printf(buf);
	}
}

void *thread_send(FILE *fp)
{
	fprintf(fp, first_msg);
	fprintf(fp, second_msg);
}

int connect_to_server(char *servname, int portnum)
{
	int fd_sock;
	struct hostent *servp;
	struct sockaddr_in servaddr;

	if ( (fd_sock = socket(AF_INET, SOCK_STREAM, 0)) == -1 )
		return -1;

	bzero((char*)&servaddr, sizeof(struct sockaddr_in));
	if ( (servp = gethostbyname(servname)) == NULL )
		return -1;
	bcopy((char*)servp->h_addr, (char*)&servaddr.sin_addr, servp->h_length);
	servaddr.sin_port = htons(portnum);
	servaddr.sin_family = AF_INET;

	if ( connect(fd_sock, (struct sockaddr*)&servaddr, sizeof(struct sockaddr_in)) == -1 )
		return -1;

	return fd_sock;
}

我以一个irc服务器来测试。编译运行了一下发现问题：发送线程thread_send被阻塞在了fprintf(fp, first_msg);这一句，而接收线程可以正常收到服务器的信息并打印出来（其中没有登录的回应，可见fprintf确实没发送出去）。

如果我仅在以一个单线程进行流读写，先fprintf发送登录报文，然后再fgets接收服务器应答，这样的话一切都没问题，IRC报文可以被发送出去，因为我可以看到服务器发回了有关登录的应答；

如果我把上面的流操作函数换成无缓冲读写，即fprintf换成write，fgets换成read，然后其他一切不变，即仍然是多线程，那么我也可以正常收到服务器的登录信息（说面write发送成功了）

因此我猜测是fprintf在对流写入时被fgets的流读取操作给阻塞了。我搜索了一下，对socket流同时读写的办法，有的只说设置为异步，但我man socket看了一下没看到有关异步设置的信息啊？还有的说需要先fflush，或者以"a+"方式打开socket文件描述符，然后以这个文件指针来读写，都试过了但还是没用。。。。

虽然用read和write肯定是没问题了，但弄不明白为什么无法用缓冲流读写总是有个疙瘩。。。第一次发帖求助，这里先谢谢各位！

[赵4老师]

仅供参考

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifdef WIN32
    #include <windows.h>
    #include <io.h>
#else
    #include <unistd.h>
    #include <sys/time.h>
    #include <pthread.h>
    #define  CRITICAL_SECTION   pthread_mutex_t
    #define  _vsnprintf         vsnprintf
#endif
//Log{
#define MAXLOGSIZE 20000000
#define MAXLINSIZE 16000
#include <time.h>
#include <sys/timeb.h>
#include <stdarg.h>
char logfilename1[]="MyLog1.log";
char logfilename2[]="MyLog2.log";
static char logstr[MAXLINSIZE+1];
char datestr[16];
char timestr[16];
char mss[4];
CRITICAL_SECTION cs_log;
FILE *flog;
#ifdef WIN32
void Lock(CRITICAL_SECTION *l) {
    EnterCriticalSection(l);
}
void Unlock(CRITICAL_SECTION *l) {
    LeaveCriticalSection(l);
}
#else
void Lock(CRITICAL_SECTION *l) {
    pthread_mutex_lock(l);
}
void Unlock(CRITICAL_SECTION *l) {
    pthread_mutex_unlock(l);
}
#endif
void LogV(const char *pszFmt,va_list argp) {
    struct tm *now;
    struct timeb tb;

    if (NULL==pszFmt||0==pszFmt[0]) return;
    _vsnprintf(logstr,MAXLINSIZE,pszFmt,argp);
    ftime(&tb);
    now=localtime(&tb.time);
    sprintf(datestr,"%04d-%02d-%02d",now->tm_year+1900,now->tm_mon+1,now->tm_mday);
    sprintf(timestr,"%02d:%02d:%02d",now->tm_hour     ,now->tm_min  ,now->tm_sec );
    sprintf(mss,"%03d",tb.millitm);
    printf("%s %s.%s %s",datestr,timestr,mss,logstr);
    flog=fopen(logfilename1,"a");
    if (NULL!=flog) {
        fprintf(flog,"%s %s.%s %s",datestr,timestr,mss,logstr);
        if (ftell(flog)>MAXLOGSIZE) {
            fclose(flog);
            if (rename(logfilename1,logfilename2)) {
                remove(logfilename2);
                rename(logfilename1,logfilename2);
            }
        } else {
            fclose(flog);
        }
    }
}
void Log(const char *pszFmt,...) {
    va_list argp;

    Lock(&cs_log);
    va_start(argp,pszFmt);
    LogV(pszFmt,argp);
    va_end(argp);
    Unlock(&cs_log);
}
//Log}
int main(int argc,char * argv[]) {
    int i;
#ifdef WIN32
    InitializeCriticalSection(&cs_log);
#else
    pthread_mutex_init(&cs_log,NULL);
#endif
    for (i=0;i<10000;i++) {
        Log("This is a Log %04d from FILE:%s LINE:%d\n",i, __FILE__, __LINE__);
    }
#ifdef WIN32
    DeleteCriticalSection(&cs_log);
#else
    pthread_mutex_destroy(&cs_log);
#endif
    return 0;
}