UDP通讯中数据不能写入文件的问题

写了一个简单的UDP通讯程序，客户端读文件，把数据写到服务器端；服务端读取数据，写到另一个文件里面。

服务器端代码是这样的：

//#include	"unp.h"
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <netinet/udp.h>
#include <strings.h>
#include <stdlib.h>

int
main(int argc, char **argv)
{
	int					sockfd;
	struct sockaddr_in	servaddr, cliaddr;

	sockfd = socket(AF_INET, SOCK_DGRAM, 0);

	bzero(&servaddr, sizeof(servaddr));
	servaddr.sin_family      = AF_INET;
	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
	servaddr.sin_port        = htons(9877);

	bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

	//dg_echo(sockfd, (SA *) &cliaddr, sizeof(cliaddr));

	char buf[500];
	socklen_t len = sizeof(struct sockaddr);
	
	FILE *fp = fopen("/home/charles/code/max-subarray_backup", "w");
	int count = 0;
	for(; ;)
	{
		int nreceived = recvfrom(sockfd, buf, 500, 0, (struct sockaddr *)&cliaddr, &len);
		if(nreceived > 0)
		{
			int nwrite = fwrite(buf, sizeof(char), nreceived, fp);  
			if(nwrite != nreceived)
			{
				printf("expect write: %d, actually write: %d\n", nreceived, nwrite);
				break;
			}
			count += nwrite;
			printf("write: %d bytes\n", count);
			fsync(fileno(fp));

		}
		else
		{
			printf("no data, break\n");
			fsync(fileno(fp));
			break;
			
		}
	}

	fclose(fp);
	return 0;
}

//#include	"unp.h"
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <netinet/udp.h>
#include <strings.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>

int
main(int argc, char **argv)
{
	int					sockfd;
	struct sockaddr_in	servaddr;

//	if (argc != 2)
//		err_quit("usage: udpcli <IPaddress>");

	bzero(&servaddr, sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_port = htons(9877);
	inet_pton(AF_INET, argv[1], &servaddr.sin_addr);

	sockfd = socket(AF_INET, SOCK_DGRAM, 0);

	//dg_cli(stdin, sockfd, (SA *) &servaddr, sizeof(servaddr));
	
	FILE *fp = fopen("/home/charles/code/max-subarray", "r");

	int enable_cork = 1, disable_cork = 0;
	char buf[500];
	int nread = 0;
	
	int count = 0;
	//setsockopt(sockfd, IPPROTO_UDP, UDP_CORK, &enable_cork, sizeof(enable_cork));

	while((nread = fread(buf, sizeof(char), 500, fp)) > 0)
	{
		int nsend = sendto(sockfd, buf, nread, 0, (struct sockaddr *)&servaddr, sizeof(struct sockaddr));
		if(nsend != nread)
		{
			printf("expected send: %d, actually send: %d err: %s\n", nread, nsend, strerror(errno));
	
			break;
		}

		count += nsend;
		printf("write: %d bytes\n", count);
	}

	//setsockopt(sockfd, IPPROTO_UDP, UDP_CORK, &disable_cork, sizeof(disable_cork));
	
	fclose(fp);
	
	exit(0);
}

所以怀疑有数据在buffer里，没有写入到文件。

所以在代码中加了 fsync().

重新编译，测试，还是有同样的问题。

后来在CSDN论坛上问了一下，有高手建议用fflush()试试。

经过验证，用了fflush()就没有问题了。

看来，fflush()和 fsync()还是不一样的。

fflush()能够把FILE stream的 buffer写入到硬盘，但是 fsync就不行。

use fsync() with not streaming files (integer file descriptors)

use fflush() with file streams.

UDP通讯中数据不能写入文件的问题

原文：http://blog.csdn.net/caspiansea/article/details/44537145