通过 TCP/IP 发送文件

Question

i'm trying to send a file via TCP/IP using a C++ program on windows. The send and the reception works fine. but in the received file two commas appear after every 40 character. If I send for example :

123456789
123456789
123456789
123456789
123456789
123456789
23456789
123456789
123456789
123456789
123456789
123456789
123456789
123456789

The recepted file is :

123456789
123456789
123456789
123456789
123456789
„123456789
123456789
123456789
123456789
123456789
„123456789
123456789
123456789
123456789

Here is the server code for sending a file :

static int send_client_file(SOCKET sock, char *Sbuffer){

int n = 0;
char* f_name = "send.txt";
//char* f_name = "server.c";

cout << endl << "Waiting for the client message" << endl;
FILE *fp = fopen(f_name, "r");
if (fp == NULL)
{
    cout << "ERROR : File " << f_name << " not found ";
    perror("fopen()");
    exit(errno);
}

//bzero(Sbuffer, 50); // on vide le buffer d'envoie
memset(Sbuffer, 0, 50);
while((n = fread(Sbuffer, sizeof(char), 50, fp)) > 0)
{
    if((n = send(sock, Sbuffer, strlen(Sbuffer), 0)) < 0)
    {
        perror("send()");
        exit(errno);
    }
    memset(Sbuffer, 0, 50);
}

cout << "file sent" << endl;

return n;
}

And here is the client code to receive it :

int static recv_server_file(SOCKET sock, char *buffer){

int n = 0;
int m = 0;

cout << endl << "Receiving file from the server..." << endl;
char* f_name = "receive.txt";
FILE *fp = fopen(f_name, "a");
if (fp == NULL)
{
    cout << "ERROR : File " << f_name << "not found " << endl;
    perror("fopen()");
    exit(errno);
}

else {

while(n = recv(sock, buffer, Rbuffer_size, 0))
{
    if (n < 0)
    {
        perror("recv()");
        exit(errno);
    }
    m = fwrite(buffer, sizeof(char), n, fp);

    if(m < n)
    {
        cout << "File write failed." << endl;
        break;
    }

    break;
    memset(buffer, '/0', Rbuffer_size);
}
cout << "ok!" << endl;

fclose(fp);
}
cout << "The received data is : " << buffer << endl ;

return m;

}

As you can see, there is ",," which appears in the recepted file. Any idea of how to resolve this ?

Answer 1

By all appearances, it looks like you have a buffer overrun. Note that the unwanted character occurs after 50 bytes (not 40 as you said in the question). Your buffer is 50 characters long.

You cannot call strlen() on the buffer that you have read in, because strlen requires a terminating null character to work. That is, you'd need the buffer to be 51 characters.

That may not be the only problem, but it is certainly part of it.

Answer 2

So many problems here.

Here is the server code for sending a file:

static int send_client_file(SOCKET sock, char *Sbuffer){

int n = 0;
char* f_name = "send.txt";
//char* f_name = "server.c";

cout << endl << "Waiting for the client message" << endl;
FILE *fp = fopen(f_name, "r");
if (fp == NULL)
{
    cout << "ERROR : File " << f_name << " not found ";
    perror("fopen()");
    exit(errno);
}

//bzero(Sbuffer, 50); // on vide le buffer d'envoie
memset(Sbuffer, 0, 50);

Unnecessary. Remove.

while((n = fread(Sbuffer, sizeof(char), 50, fp)) > 0)
{
    if((n = send(sock, Sbuffer, strlen(Sbuffer), 0)) < 0)

The send() part of that should be send(sock, Sbuffer, n, 0) . This is the underlying problem. You are sending the wrong length. You are also assuming that the data just read actually contains a null, for which there is no guarantee. The strlen() call could overrun the buffer and cause undefined behaviour.

    {
        perror("send()");
        exit(errno);
    }
    memset(Sbuffer, 0, 50);

The memset() is unnecessary. Remove.

}

cout << "file sent" << endl;

return n;
}

Here you are returning zero. I don't see the point.

And here is the client code to receive it :

int static recv_server_file(SOCKET sock, char *buffer){

int n = 0;
int m = 0;

cout << endl << "Receiving file from the server..." << endl;
char* f_name = "receive.txt";
FILE *fp = fopen(f_name, "a");
if (fp == NULL)
{
    cout << "ERROR : File " << f_name << "not found " << endl;
    perror("fopen()");
    exit(errno);
}

else {

while(n = recv(sock, buffer, Rbuffer_size, 0))
{
    if (n < 0)
    {
        perror("recv()");
        exit(errno);
    }
    m = fwrite(buffer, sizeof(char), n, fp);

    if(m < n)
    {
        cout << "File write failed." << endl;
        break;
    }

    break;
    memset(buffer, '/0', Rbuffer_size);

Again the memset() is unnecessary: remove.

}

Answer 3

In your client code the buffer is never actually memsetted. That could be the problem.

The first iteration assumes that 'buffer' is already set to '\\0' also this code is redundant if (n < 0) { perror("recv()"); exit(errno); } if (n < 0) { perror("recv()"); exit(errno); } if (n < 0) { perror("recv()"); exit(errno); } Since the while(...) already checks that and in the end of the iteration you break out of the loop before you memset and by doing that making the entire loop redundant. Please do the memsets to rule out those possible culprits and you may apply the advices too, but that's up to you.

EDIT: And by making the loop redundant you might not receive all the data.

EDIT #2: Also you disregard the return value of 'send'. From the msdn page:

If no error occurs, send returns the total number of bytes sent, which can be less than the number requested to be sent in the len parameter. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

So it might be that data gets discarded since there's no guarantee that it sends all data straight away.

EDIT #3 (Sorry for many edits) Also you don't close your file handle in the server code