Send a File with socket in C for Linux

Question

I'm writing a small and simple server (in C language for Linux stations).

A client requests a file to my server, my server asks this file to another server which sends it to my server.

My server should NOT receive ALL the file before sending it to the client BUT must send the bytes of the file so as they arrive.

This is an exercise in school so I can not dissociate myself from this requirement.

I have implemented the function explained below. The problem is that the client receives a non-deterministic number of bytes and NEVER the entire file.

int Recv_and_send_file (int socketa, int socketb, char *buffer, size_t file_size){

    size_t n;

    ssize_t nread;

    ssize_t nwritten;

    char c;

    for (n=1; n<file_size; n++)
    {        
            nread=recv(socketa, &c, 1, 0);

            if (nread == 1)
            {
                nwritten = send(socketb,&c,1,0);
            }
            else if (nread == 0)
            {
                *buffer = 0;
                return (-1); /* Errore */
            }
            else
                return (-1); /* Errore */
            }
    }
    *buffer = 0;
    return (n); 
}

Someone could kindly tell me where I'm wrong?

Is it an stupid idea to change the values SO_SNDBUF and SO_RCVBUF on both the server and the client?

jxh · Accepted Answer · 2013-06-25 20:20:11Z

1

Assuming the file_size is the total number of bytes you want to send, then your for loop will only send file_size - 1 bytes. In other words, you are off by one. Start from 0 instead to fix this:

    for (n=0; n<file_size; n++)
    { //..

You capture the return value of send(), but you do not check to see if it was successful or not.

You are treating a 0 return value from recv() the same as an error. Since you do not show what you do after returning -1 from your function, I don't know if this may be contributing to your problem or not.

Certain errors on send() and recv() are "soft", in that you are allowed to retry the operation for those particular errors. One such error is EINTR, but check the documentation on your system to see if there are others.

answered Jun 25, 2013 at 20:20

jxh

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Comments

Kristian Evensen · Accepted Answer · 2013-06-25 20:35:16Z

1

In order to optimize performance and simplify your code, you can use splice()+pipes. Sendfile enables you to "forward" data between file descriptors, without the copy to user space.

edited Jun 25, 2013 at 20:35

answered Jun 25, 2013 at 20:25

Kristian Evensen

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user2510406 Over a year ago

Thanks for your answer but it's required that I have to send in explicit way bytes per bytes

ja_mesa · Accepted Answer · 2013-06-25 20:42:12Z

1

Are you sure you have copied the correct code? That part as it is would not compile, there is a } in the last else which don't match with a corresponding {.

Also, how you get to know the file size? if it's send thru the socket as an integer, bear in mind the possible byte order of the source and destination machines.

Anyway, you are reading one byte at a time, you should improve it this way:

EDIT: use buffer and not the extra buff[2048];

int Recv_and_send_file (int socketa, int socketb, char *buffer, size_t file_size){

    ssize_t nread;
    ssize_t nwritten;
    ssize_t bLeft=file_size;

    while (bLeft > 0)
    {
        nread=recv(socketa, buffer, bleft, 0);
        if (nread > 0)
        {
            nwritten = send(socketb, buffer, nread, 0);
            bLeft -= nread;
            buffer+=nread;
        }
        else if (nread == 0)
        {
            // I think this could raise a memory exception, read below
            *buffer = 0;
            return (-1); /* Errore */
        }
        else
        {
            return (-1); /* Errore */
        }
    }
    // If buffer is allocated with file_size bytes this one will raise a memory exception
    // *buffer = 0;
    return (file_size-bLeft);
}

edited Jun 25, 2013 at 20:42

answered Jun 25, 2013 at 20:27

ja_mesa

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2 Comments

user2510406 Over a year ago

The second server, before sending the file, send to my server the file's size in order to allow at my server to do a malloc and create buffer that appear as input in the function. Thank you for the answer I try this code as soon as possible!

ja_mesa Over a year ago

I did not know that, buffer is already allocated with enough room, so the code would be different dont need the extra buff[2048]. I'll edit the response to make it better and have in buffer the contents of the file.

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