Question

我将在这里重述整个问题，以便它是可以回答的。

我可以在不使用套接字的同一台机器上完美地复制二进制文件，但只是制作一个简单的复制功能。尝试实现此代码以复制到TCP / IP连接但无法使其工作。

FILE *filehandle = fopen("imagefile.jpg", "rb");
FILE *dest  =fopen("imagecopy.jpg", "wb");    // copied image file
fseek(filehandle, 0, SEEK_END);
unsigned long filesize = ftell(filehandle);
char *buffer = (char*)malloc(sizeof(char)*filesize);
rewind(filehandle);
int bytesread = fread(buffer, sizeof(char), filesize, filehandle);
for( int i=0; i<filesize; i++ )
{
    fputc(buffer[i], filehandle);    // copies all the contents to dest
}

上面的代码非常适合在计算机中复制图像文件，但实现在服务器上复制时很难实现。

我正在尝试将图像文件从服务器发送到客户端，这些文件都是在C中手动生成的。文件的长度由服务器只有在发送文件时才知道服务器，因此缓冲区是在服务器中动态生成的，如下所示：

服务器

fseek(filehandle, 0, SEEK_END);
long filesize = ftell(filehandle);    // file could be 11000bytes
char *buffer = (char*)malloc(sizeof(char)*filesize);    // char buffer with 11000 bytes to store the data from the file.
// then I call the send() function
rewind(filehandle);    // go back to beginning
send(clientsocket, buffer, filesize, 0);    // this is being sent perfectly, no errors because in the actual code, I am checking for errors

客户端

// here is where I don't understand how to dynamically allocate the 11000 bytes to store the data in a client buffer
// the filesize is not necessarily going to be 11000 so need to dynamically allocate
// I did the following:
#define BUFSIZE 10
FILE *filehandle = fopen("imagefile.jpg", "wb");    // image file created by client
char *buffer = (char*)malloc(sizeof(char)*BUFSIZE);
int bytesread = recv(buffer, 1, strlen(buffer), 0);
if( bytesread > 0 )
{
    printf("Bytes read: %d\n", bytesread);    // bytes read is 5
    printf("Buffer: %s\n", buffer);    // but buffer shows all the binary text like it normally would
    // when I try to store buffer in a file, it doesn't put full buffer because only 5 characters are written
    for( int i=0; i<bytesread; i++ )
    {
        fputc(buffer[i], filehandle);    // this doesn't create full image
    }
}

如何动态分配服务器发送的11000字节？

Answer 1

您需要循环发送和接收。 send()和recv()都不能保证发送/读取您请求的字节数。

您还应该在文件数据之前发送文件大小，以便接收方知道预期的字节数和停止读取的时间。

尝试更像这样的事情：

SERVER

bool senddata(SOCKET sock, void *buf, int buflen)
{
    unsigned char *pbuf = (unsigned char *) buf;

    while (buflen > 0)
    {
        int num = send(sock, pbuf, buflen, 0);
        if (num == SOCKET_ERROR)
        {
            if (WSAGetLastError() == WSAEWOULDBLOCK)
            {
                // optional: use select() to check for timeout to fail the send
                continue;
            }
            return false;
        }

        pbuf += num;
        buflen -= num;
    }

    return true;
}

bool sendlong(SOCKET sock, long value)
{
    value = htonl(value);
    return senddata(sock, &value, sizeof(value));
}

bool sendfile(SOCKET sock, FILE *f)
{
    fseek(f, 0, SEEK_END);
    long filesize = ftell(f);
    rewind(f);
    if (filesize == EOF)
        return false;
    if (!sendlong(sock, filesize))
        return false;
    if (filesize > 0)
    {
        char buffer[1024];
        do
        {
            size_t num = min(filesize, sizeof(buffer));
            num = fread(buffer, 1, num, f);
            if (num < 1)
                return false;
            if (!senddata(sock, buffer, num, 0))
                return false;
            filesize -= num;
        }
        while (filesize > 0);
    }
    return true;
}

FILE *filehandle = fopen("imagefile.jpg", "rb");
if (filehandle != NULL)
{
    sendfile(clientsocket, filehandle);
    fclose(filehandle);
}

客户端

bool readdata(SOCKET sock, void *buf, int buflen)
{
    unsigned char *pbuf = (unsigned char *) buf;

    while (buflen > 0)
    {
        int num = recv(sock, pbuf, buflen, 0);
        if (num == SOCKET_ERROR)
        {
            if (WSAGetLastError() == WSAEWOULDBLOCK)
            {
                // optional: use select() to check for timeout to fail the read
                continue;
            }
            return false;
        }
        else if (num == 0)
            return false;

        pbuf += num;
        buflen -= num;
    }

    return true;
}

bool readlong(SOCKET sock, long *value)
{
    if (!readdata(sock, value, sizeof(value)))
        return false;
    *value = ntohl(*value);
    return true;
}

bool readfile(SOCKET sock, FILE *f)
{
    long filesize;
    if (!readlong(sock, &filesize))
        return false;
    if (filesize > 0)
    {
        char buffer[1024];
        do
        {
            int num = min(filesize, sizeof(buffer));
            if (!readdata(sock, buffer, num))
                return false;
            int offset = 0;
            do
            {
                size_t written = fwrite(&buffer[offset], 1, num-offset, f);
                if (written < 1)
                    return false;
                offset += written;
            }
            while (offset < num);
            filesize -= num;
        }
        while (filesize > 0);
    }
    return true;
}

FILE *filehandle = fopen("imagefile.jpg", "wb");
if (filehandle != NULL)
{
    bool ok = readfile(clientsocket, filehandle);
    fclose(filehandle);

    if (ok)
    {
        // use file as needed...
    }
    else
        remove("imagefile.jpg");
}

Answer 2

我们可以避免包含图像大小的标题，但我们只是读到发送数据的末尾。关于缓冲区大小，我们可以使用固定数字，例如10 * 1024，当我们从服务器收到一些数据时，我们只是根据实际接收的数据长度将它保存到文件中。

// please open a file ...
FILE * fp;
// ...
const int LENGTH = 10 * 1024;

int len = 0;
char * buffer = (char *)malloc(LENGTH);
while ((len = recv(socket, buffer, LENGTH, 0)) > 0) {
    fwrite(buffer, 1, len, fp);
}
free(buffer);
// close the file

@ T.C：我想我们不能根据服务器发送的大小来分配缓冲区，以防图像太大而无法保存在客户端的内存中。没有提到服务器是假的，并且打算进行任何攻击。

通过TCP / IP连接发送二进制文件

2 个答案: