C/C++ Downloading / Uploading Files via socket

Question

I am currently trying to work out Downloading / Uploading Files via socket (SOCK_STREAM). The following two functions are what I use to send and receive the data. I am currently running into the following Issue(s): The result File sometimes is not the same size as the source File This Problem is more sever, the larger the file.

I am pretty sure that I am missing something obvious, in my loops maybe or determining the end of the data stream. I spent the past week trying a bunch of different approaches / solutions and this is the closest to a working version i got...

I am thankfull for any advice and review, if i need to provide further information please tell me

Function Sending Data from Server to Client:

void send_file(char *filename, int sock)
{
    char data[1024] = {0};
    FILE *fp = fopen(filename, "rb");

    while (fread(data, sizeof(char), sizeof(data), fp) == 1024) {
        if (send(sock, data, sizeof(data), 0) == -1) {
            printf("%s%s[-] Error Transmitting File\n\n", KRED, BGHT);
            break;
        }
        bzero(data, sizeof(data));
    }
    bzero(data, sizeof(data));
    strcpy(data, "!EOF!");
    send(sock, data, sizeof(data), 0);
    bzero(data, sizeof(data));
    printf("%s%s[+] Upload Successful\n\n", KGRN, BGHT);
    fclose(fp);
}

Function of Client Receiving Data from Server:

void write_file(int sock, char *filepath)
{
    FILE *fp;
    int n;
    char *lastSlash = strrchr(filepath, '\\');
    char *filename = lastSlash ? lastSlash +1 : filepath;
    char data[1024] = {0};
    fp = fopen(filename, "wb");

    while (1) {
        n = recv(sock, data, sizeof(data), 0);
        if (strncmp("!EOF!", data, 5) == 0) {
            break;
        }
        if (n <= 0) {
            break;
            return;
        }
        fwrite(data, sizeof(char), sizeof(data), fp);
        bzero(data, sizeof(data));
    }
    fclose(fp);
    return;
}

Answer 1

i finally figured it out with the help of the following Post:

Sending files over TCP sockets C++ | Windows

I rewrote the example code to fit my needs. Works like a charm so far.

Thanks to all for giving me some more insight on the topic and helping me figure our the necessary checks on the way!

Here the new Code with a brief explanation:

First thing that needs to be recognized, ist that functions like send() recv() fread() fwrite() are likely to not fill their buffer entirely before passing it on. Meaning if you have a buffer specified with size 100, they might only fill it up to 89 95 or whatever. As a result files are likely to be corrupted. To solve this every call of the send() recv() fread() fwrite() functions needs to be checked.

First you need those two functions both on server and client side. These make sure the entire buffer is being sent / received. Its basically just looping over send() / recv() until the buffer is filled up.

int RecvBuffer(int sock, char* buffer, int bufferSize, int chunkSize) { 
    int i = 0;
    while (i < bufferSize) {
        const int l = recv(sock, &buffer[i], __min(chunkSize, bufferSize - i), 0);
        if (l < 0) { return l; } 
        i += l;
    }
    return i;
}

int SendBuffer(int sock, char* buffer, int bufferSize, int chunkSize) { 
    int i = 0;
    while (i < bufferSize) {
        const int l = send(sock, &buffer[i], __min(chunkSize, bufferSize - i), 0);
        if (l < 0) { return l; } 
        i += l;
    }
    return i;
}

Next we need to apply the same check in the functions that are being called to Download / Upload a file. Here I loop over the above functions that fill our Buffer and over the fread() fwrite() until all bytes (fileSize) have been sent. The chunkSize parameter defines the size of each package being sent. I used 65.536 (64kb) so far without any issues.

int RecvFile(int sock, char *filePath, int chunkSize, int fileSize) { 

    char *lastSlash = strrchr(filePath, '\\');
    char *filename = lastSlash ? lastSlash +1 : filePath;
    FILE *fp = fopen(filename, "wb");

    char buffer[chunkSize];
    int i = fileSize;

    while (i != 0) {
        const int r = RecvBuffer(sock, buffer, (int)__min(i, (int)chunkSize), chunkSize);
        if ((r < 0) || !fwrite(buffer, sizeof(char), r, fp)) { break; }
        i -= r;
    }
    bzero(buffer, sizeof(buffer));

    fclose(fp);

    printf("\n%s%s[+] Download Successful\n\n", KGRN, BGHT);

    return -3;
}

int SendFile(int sock, char *fileName, int chunkSize, int fileSize) { 

    FILE *fp = fopen(fileName, "rb");

    char buffer[chunkSize];
    int i = fileSize;

    while (i != 0) {
        const int ssize = __min(i, (int)chunkSize);
        if (!fread(buffer, sizeof(char), ssize, fp)) { break; }
        const int l = SendBuffer(sock, buffer, (int)ssize, (int)chunkSize);
        if (l < 0) { break; }
        i -= l;
    }
    bzero(buffer, sizeof(buffer));

    fclose(fp);

    printf("\n%s%s[+] Upload Successful\n\n", KGRN, BGHT);

    return -3;
}