How to send a file with http with c++

Question

I want to write a server side code. It should work with popular browsers and wget. My server check that file exists or not, if exists then browser can download it. But I have some problems. Honestly, I read lots of question-answer (for example: Send binary file in HTTP response using C sockets) but I didn't find out. My browser (Chrome) can get text. But I cannot send any binary data or images etc. I am changing header according to downloading files. But I cannot send a downloadable files yet.

I have some questions.

void *clientWorker(void * acceptSocket) {
     int newSocket = (int) acceptSocket;
     char okStatus[] = "HTTP/1.1 200 OK\r\n"
                       "Content-Type: text/html\r\n"
                       "Connection: close\r\n"
                       "Content-Length: 20\r\n"
                       "\r\n"
                       "s";
     writeLn(newSocket, okStatus);
     const char * fileName = "/home/tyra/Desktop/example.txt";
     sendF(newSocket, fileName);
}

1- If I wouldn't write "s" or something else inokStatus, my message cannot send. I don't understand anything of this.

This is writeLn function :

void writeLn(int acceptSocket, const char * buffer) {
    int n = write(acceptSocket, buffer, strlen(buffer) - 1);
    if (n < 0) {
        error("Error while writing");
    }
}

This is sendF function :

string buffer;
string line;
ifstream myfile(fileName);
struct stat filestatus;
stat(fileName, &filestatus);
int fsize = filestatus.st_size;
if (myfile.is_open()) {
    while (myfile.good()) {
        getline(myfile, line);
        buffer.append(line);
    }
    cout << buffer << endl;
}
writeLn(acceptSocket, buffer.c_str());
cout << fsize << " bytes\n";

A little messy. I haven't used file size yet. If I send a file, then I rearrange these things.

2- I can send text and browser demonstrates it but browser didn't understand new lines. If text file contains (123\n456\n789), browser demonstrates (123456789). I think I should change Content-Type header, but I didn't find out.

I don't want that browser demonstrates text files. Browser should download it. How can I send downloadable files?

Sorry, I explain everything pretty complicated.

I think one of the problems is that okStatus[] has no zero-termination char at the end. Try adding it to the end, instead of 's' — Tony The Lion
– Tony The Lion, Commented May 25, 2011 at 10:51
@Tony The Tiger Thanks for helping. I tried and it didn't work. — Baskaya
– Baskaya, Commented May 25, 2011 at 10:54
@Tony The Tiger Literal strings are automatically null-terminated — Andy Johnson
– Andy Johnson, Commented May 25, 2011 at 11:02
I don't know if its relevant, but the Connection: close part of okStatus is only followed only by a \n. It should be followed by \r\n. — Andy Johnson
– Andy Johnson, Commented May 25, 2011 at 11:04

sergio · Accepted Answer · 2011-05-25 11:47:30Z

2

As to your first question, you should find out the exact size of the file and specify it in your "Content-Length: xxxx\r\n" header. And, of course, you should ensure that the data is sent completely out.

Indeed, in your writeF function you use a std::string as a buffer:

string buffer;

this is not appropriate for binary data. You should allocate a raw char array of the right size:

int fsize = file.tellg();
char* buffer = new char[fsize];
file.seekg (0, ios::beg);
file.read (buffer, size);
file.close();

As to the second point, when your data is not HTML, specify as Content-Type: text/plain; otherwise, your carriage return should be represented by <br> instead of "\r\n".

In case of binary downloads, to have the data download as a file (and not shown in the browser), you should specify

Content-Type: application/octet-stream

edited May 25, 2011 at 11:47

answered May 25, 2011 at 11:09

sergio

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5 Comments

Baskaya Over a year ago

Hello sergio. If I send binary data, chrome or firefox automatically download it?

sergio Over a year ago

Hello Thorn, see my edit. You need the Content-Type: application/octet-stream

Baskaya Over a year ago

Can I send rar file or something else with the same method (getline()) by changing only Content-Type?

sergio Over a year ago

Thorn, sure you can send rar files. Anyway, the getline approach is not the best. See my edit for code snippet specific to binary data.

Baskaya Over a year ago

If I want to send a downloadable file, I should send it in binary form, am I right? In this way, we can obtain a file which is identical with on the server? (not losing '\n')

Mayank · Accepted Answer · 2011-05-25 10:57:47Z

1

The issue is strlen here. strlen terminates when it gets a '\0' character. In binary file you will have a number of '\0' characters.

While reading the file you should find out the file size. This size should be used in int n = write(acceptSocket, buffer, strlen(buffer) - 1); in place of strlen

Change the writeLn(acceptSocket, buffer.c_str()); to writeLn(acceptSocket, buffer.c_str(), buffer.size()); and try...

For the case of 123\n456\n789 you need to send <PRE>123\n456\n789</PRE> as browser will parse this text as html and not like the OS parses and shows the output. The other way you can do is replace all \n with <BR> ...

edited May 25, 2011 at 10:57

answered May 25, 2011 at 10:52

Mayank

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7 Comments

Baskaya Over a year ago

Hello. Thanks for the answer. My first question is solved by your helping. I changed it and it works. About my second question, I do not want to demonstrate the text, browser should download it. I think I should change Content-Type and I send it right way (not getline), I do not know.

RedX Over a year ago

@Thom If the browser is able to parse mime "text/html" it's very likely that he'll do that as it is more convenient. But there is a way around: use the headers to indicate you want a download with Content-Disposition: attachment;filename="test.jpg". See apptools.com/phptools/force-download.php for more details. It's php but the main http concepts still apply.

Mayank Over a year ago

I'm not sure if I got it. I assume that you want your text file to be downloaded not displayed on the browser. In that case as @RedX suggested you need to use Content-Disposition: attachment;filename="test.txt".

Mayank Over a year ago

@Thorn: You can have a look at this article Content-Type. Its a nice read

Baskaya Over a year ago

@RedX Thank you for advice. It works. Your link is very helpful. @Mayank Exactly:)

|

Andy Johnson · Accepted Answer · 2011-05-25 11:09:37Z

0

Regarding question 1 - if you don't want to send any content back then remove the s from the end of okStatus and specify Content-Length: 0\r\n in the header

answered May 25, 2011 at 11:09

Andy Johnson

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