使用C将文件从服务器发送到Linux上的客户端

Question

I have a problem with this code. The problem is that I have to develop a client-server model with this futures:

1. Client type: "GETfile_name.txt";
2. Server receives the request and send file's size;
3. Server send the file;

The problem is that Server doesn't send the file to the client. Thanks to all for your help.

The code is this:

Server:

f = fopen(file_name,"r");

/* file's size */
fseek(f,0L,SEEK_END);
size = ftell(f);
fseek(f,0L,SEEK_SET);

/* convert int to string */
sprintf(file_size,"%d",size); 

/* send file's size to the client */
Send(s,file_size,sizeof(file_size)); 

/* send file to the client */
buffer = (char *) malloc(sizeof(char)*size);  
bzero(buffer,sizeof(char)*size);
x = fread(binoltro,sizeof(char),sizeof(char)*dimensione,f);
Send(s,binoltro,x);

Client:

/* wait file's size */
recv(s,buffer,MAX,0); 
printf("Remote server will send: %s Bytes\n",buffer);
size = atoi(buffer);

/* execute malloc */
buffer = (char *) malloc(sizeof(char)*size);
bzero(buffer,size*sizeof(char));

Recv(s,buffer,size*sizeof(char));
fwrite(buffer,sizeof(char),size*sizeof(char),f);
printf("File: %s",buffer);

/* clean the buffer */
bzero(buffer,MAX);

/* do free */
free(buffer);
fclose(f);

With:

int Recv (int s, char *ptr, size_t maxlen){
size_t n;
ssize_t nread;
char c;

for (n=1; n<maxlen; n++)
{
    nread=recv(s, &c, 1, 0);
    if (nread == 1)
    {
        *ptr++ = c;
        if (c == '\n')
            break;
    }
    else if (nread == 0)    
    {
        *ptr = 0;
        return (n-1); 
    }
    else
        return (-1); 
}
*ptr = 0;
return (n);
}

And:

int Send(int s, char *ptr, size_t nbytes){
size_t  nleft;
ssize_t nwritten;

for (nleft=nbytes; nleft > 0; )
{
    nwritten = send(s, ptr, nleft, 0);
    if (nwritten <=0)
        return (nwritten);
    else
    {
        nleft -= nwritten;
        ptr += nwritten;
    }
}
return (nbytes - nleft); 
}

Answer 1

The problem I see is that your Recv() function breaks when either it receives a '\\n' or there is no more data or maxlen is reached. None of these conditions match after your client received the last byte of the file length so it reads on and the file data is lost.

I would suggest you fill the file length to a fixed number of digits( eg 10) by using

sprintf( file_size, "%010s", size );

and use a modified Recv() function like this:

int Recv (int s, char *ptr, size_t nbytes ){
    size_t n;
    ssize_t nread;


    while( nbytes > 0 ) {
        if( (nread = recv( s, ptr, nbytes, 0 )) <= 0 )
            return -1; /* Error */

        ptr += nread;
        nbytes -= nread;
    }
    +ptr = '\0';
    return 0; /* OK */
}

Then your client can first read the file size using

Recv( s, buffer, 10 )

and then the data using

Recv( s, ..., atoi( buffer ) )

Make sure to alloc file size + 1 because of the '\\0' that is added by Recv()