【发布时间】：2017-02-05 03:54:05
【问题描述】：
这是针对Linux系统的，用C语言编写的。它涉及网络编程。这是一个文件传输程序。
我一直遇到这个问题，这段代码无法预测。它要么完全成功，要么客户端中的 while 循环永远不会结束。我发现这是因为 fileLength 变量有时会是一个巨大的（负或正）值，我认为这是由于 ntohl 犯了一些错误。当我输入一个打印语句时，它似乎工作得很好，没有错误。
这是客户端代码：
//...here includes relevant header files
int main (int argc, char *argv[]) {
//socket file descriptor
int sockfd;
if (argc != 2) {
fprintf (stderr, "usage: client hostname\n");
exit(1);
}
//...creates socket file descriptor, connects to server
//create buffer for filename
char name[256];
//recieve filename into name buffer, bytes recieved stored in numbytes
if((numbytes = recv (sockfd, name, 255 * sizeof (char), 0)) == -1) {
perror ("recv");
exit(1);
}
//Null terminator after the filename
name[numbytes] = '\0';
//length of the file to recieve from server
long fl;
memset(&fl, 0, sizeof fl);
//recieve filelength from server
if((numbytes = recv (sockfd, &fl, sizeof(long), 0)) == -1) {
perror ("recv");
exit(1);
}
//convert filelength to host format
long fileLength = ntohl(fl);
//check to make sure file does not exist, so that the application will not overwrite exisitng files
if (fopen (name, "r") != NULL) {
fprintf (stderr, "file already present in client directory\n");
exit(1);
}
//open file called name in write mode
FILE *filefd = fopen (name, "wb");
//variable stating amount of data recieved
long bytesTransferred = 0;
//Until the file is recieved, keep recieving
while (bytesTransferred < fileLength) {
printf("transferred: %d\ntotal: %d\n", bytesTransferred, fileLength);
//set counter at beginning of unwritten segment
fseek(filefd, bytesTransferred, SEEK_SET);
//buffer of 256 bytes; 1 byte for byte-length of segment, 255 bytes of data
char buf[256];
//recieve segment from server
if ((numbytes = recv (sockfd, buf, sizeof buf, 0)) == -1) {
perror ("recv");
exit(1);
}
//first byte of buffer, stating number of bytes of data in recieved segment
//converting from char to short requires adding 128, since the char ranges from -128 to 127
short bufLength = buf[0] + 128;
//write buffer into file, starting after the first byte of the buffer
fwrite (buf + 1, 1, bufLength * sizeof (char), filefd);
//add number of bytes of data recieved to bytesTransferred
bytesTransferred += bufLength;
}
fclose (filefd);
close (sockfd);
return 0;
}
这是服务器代码：
//...here includes relevant header files
int main (int argc, char *argv[]) {
if (argc != 2) {
fprintf (stderr, "usage: server filename\n");
exit(1);
}
//socket file descriptor, file descriptor for specific client connections
int sockfd, new_fd;
//...get socket file descriptor for sockfd, bind sockfd to predetermined port, listen for incoming connections
//...reaps zombie processes
printf("awaiting connections...\n");
while(1) {
//...accepts any incoming connections, gets file descriptor and assigns to new_fd
if (!fork()) {
//close socket file discriptor, only need file descriptor for specific client connection
close (sockfd);
//open a file for reading
FILE *filefd = fopen (argv[1], "rb");
//send filename to client
if (send (new_fd, argv[1], strlen (argv[1]) * sizeof(char), 0) == -1)
{ perror ("send"); }
//put counter at end of selected file, and find length
fseek (filefd, 0, SEEK_END);
long fileLength = ftell (filefd);
//convert length to network form and send it to client
long fl = htonl(fileLength);
//Are we sure this is sending all the bytes??? TEST
if (send (new_fd, &fl, sizeof fl, 0) == -1)
{ perror ("send"); }
//variable stating amount of data unsent
long len = fileLength;
//Until file is sent, keep sending
while(len > 0) {
printf("remaining: %d\ntotal: %d\n", len, fileLength);
//set counter at beginning of unread segment
fseek (filefd, fileLength - len, SEEK_SET);
//length of the segment; 255 unless last segment
short bufLength;
if (len > 255) {
len -= 255;
bufLength = 255;
} else {
bufLength = len;
len = 0;
}
//buffer of 256 bytes; 1 byte for byte-length of segment, 255 bytes of data
char buf[256];
//Set first byte of buffer as the length of the segment
//converting short to char requires subtracting 128
buf[0] = bufLength - 128;
//read file into the buffer starting after the first byte of the buffer
fread(buf + 1, 1, bufLength * sizeof(char), filefd);
//Send data too client
if (send (new_fd, buf, sizeof buf, 0) == -1)
{ perror ("send"); }
}
fclose (filefd);
close (new_fd);
exit (0);
}
close (new_fd);
}
return 0;
}
注意：我已经稍微简化了代码，希望它更清晰。 任何以 //... 开头的东西都代表一堆代码
【问题讨论】：
-
我没有看到任何同步机制。对等点交换原始二进制数据，而没有任何指示该数据是什么。所以你的
name的一部分可以很容易地解释为长度。你需要定义一个protocol。 -
numbytes, sin_size类型未知，因为未在帖子中声明。 AKAIK，他们有一种导致问题的类型。最好发布他们的声明。示例socklen_t sin_size = sizeof their_addr; -
在这种情况下，这将如何导致问题？ （抱歉，我是新手。）我告诉它以正确的顺序以正确的大小发送和接收数据，不是吗？
-
recv正在接收当前可用的尽可能多的数据。想想如果在调用recv时只有第一条消息的一部分可用会发生什么。 -
随着post change，当
send (new_fd, argv[1], strlen (argv[1]) * sizeof(char), 0)被调用时new_fd的值是多少？它似乎未初始化。
标签： c linux network-programming printf endianness