recv() returns strange buffer C++

Question

I know C++ for quite long, but started using it for my purposes some what a year and a half ago. I started learning network programming on C++ and the first networking project is "File Transfering between hosts over TCP/IP" which sounds kind easy but I am stuck with sending data.

I am trying to send small buffer less than 4KB, so buffer[4096] works fine for me, but I am planning to expand this. WSAStartup(), socket(), bind(), listen(), accept() functions work fine and values for them are initialised for both Server and Client, but I am dealing with other problems, maybe recv(), send() etc. I still couldn't find the source of the problem.

Also it would be a ton helpful if somebody give me an example of transfering files over TCP/IP, but not in one packet, I want the file to be chunked and sent in parts or as it's called "ring model", but I couldn't find a working model;

P.S. This is first time I am asking here, pls give feedback about how well all of this is written, so that I could write more informative for community help, thanks)

Server

char* buffer = new char[4096];
ZeroMemory(buffer, sizeof(buffer));
ofstream file("a.txt", ios::binary);
int err = recv(conn, buffer, sizeof(buffer), 0);
file << buffer;
file.close();
if (err == 0)
{
    printf("Client diconnected...\n");
}

printf("Quitting...\n");
delete[] buffer;

Client

ifstream file("a.txt", ios::binary);
file.seekg(0, ios::end);
int size = file.tellg();
file.seekg(0, ios::beg);
char* buffer = new char[size];
file.read(buffer, size);
file.close();

int err = send(client, buffer, size, 0);
if (err == 0)
{
    printf("Disconnecting...\n");
}

printf("Quitting...\n");
delete[] buffer;

"a.txt" file on Client side is 45 bytes in here are 45 * 'a'

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

And this is what I get on Server side, file size is 14 bytes

aaaaaaaa ’pÈ/

You may find Boost Asio to be highly relevant for your use case. — Eljay
You also want to pay attention to the fact that recv(sock, buffer, 4096, 0) doesn't receive 4096 bytes. — user253751
You got the answer below. (sizeof problems). You also asked for feedback on your code. It's C++ but as it used to be written literally 25 years ago. C++ has moved on a lot. Since you learned it a long time ago you might be interested in buying a book that covers the newer aspects of C++. It's a far superior lanugage than it used to be (and this is coming from someone who always resists any change). — john

MSalters MSalters · Accepted Answer · 2020-08-18T14:50:39

In C++, sizeof(buffer) is the size of the pointer type.

You may want to read up on more modern (as in after 1998) C++. We have std::vector nowadays, and that has a convenient size method. It would return 4096 for your buffer. Also, vector handles new[] and delete[] for you.

The fact that you get 8 "a"'s suggests that you built for x64. The remaining bytes are garbage; you should check how many bytes recv actually wrote to buffer. You cannot assume that you got all the bytes you asked for (whether that's 8 or 4096).

recv() returns strange buffer C++

4 Answers