本文描述了Http服务器使用http range支持浏览器的下载管理器下载大文件时可暂停和继续的实现方法，后面用英文写，但我的英文不太好，如有错误之处，希望大家帮我纠正，我的联系方式在后面。
Section A. Summary
I'm making a http server in C, in order to support big file download, I have to support Http Range in my server, as the usual way for web browsers to download big files is to use its download manager, where we can pause and resume the download progress. For the pause and resume functions of download manager to work, the http server must support Http Range requests.
Besides file download, http range is also needed for <video> element to play videos on web page, when the user click a random position of the video time line, the browser use http range requests to get the corresponding file content of the video file.
Http range is imported into http protocol from http 1.1. A http range request is like this:121Please respect copyright.ＰＥＮＡＮＡALW7If2Qzx
----------http range request-------121Please respect copyright.ＰＥＮＡＮＡtvlCn6KUKa
GET /zen.iso HTTP/1.1121Please respect copyright.ＰＥＮＡＮＡjNNB9e2qiP
Range: bytes=100-1023121Please respect copyright.ＰＥＮＡＮＡwGyhzL3WGw
If-Match: "534231200_1732277116"121Please respect copyright.ＰＥＮＡＮＡHW1rb4cYbJ
......121Please respect copyright.ＰＥＮＡＮＡpZG0LlT0tt
-----------------------------------121Please respect copyright.ＰＥＮＡＮＡ4dqJE2flWY
It means to get the content from byte 100 to byte 1023, totally 924 bytes, and it tells the http server the etag value of already downloaded part is "534231200_1732277116", the etag value is used for the web browser to determine if the requested content has been modified, it's necessory for some browsers like chrome and edge to resume the download of a file. If http server do not provide etag for range content, the resume function of web browser download manager will not work.
Section B. Detailed Steps
The following is the steps for serving file download, support pause and resume functions of web browser download manager.
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Step 1. make a link for file to download, for example:121Please respect copyright.ＰＥＮＡＮＡCctmcZu8ff
----------------html-----------------121Please respect copyright.ＰＥＮＡＮＡUkxrEsEkLu
<a href="/zen.iso">zen.iso</a>121Please respect copyright.ＰＥＮＡＮＡz2FcHXADea
-------------------------------------121Please respect copyright.ＰＥＮＡＮＡMsAqcaLsIN
When user click the link, web browser download manager will take care of the file download immediately. We cannot use javascript fetch(url) to start downloading, as in this way javascript code will take care of the download instead of download manager.
When the link is clicked, the http request sent to server is like this:121Please respect copyright.ＰＥＮＡＮＡ5zHkEqoK1F
----------get request------------121Please respect copyright.ＰＥＮＡＮＡU6z7g616w7
GET /zen.iso HTTP/1.1121Please respect copyright.ＰＥＮＡＮＡBsE5dJmjUJ
......121Please respect copyright.ＰＥＮＡＮＡi414sGV0im
---------------------------------
121Please respect copyright.ＰＥＮＡＮＡmSAEJ0dcCD
Step 2. when http server receives above request, it sends the whole file content as response body back as usual, and indicates support of http range in the response headers, like this:121Please respect copyright.ＰＥＮＡＮＡosSElyhsha
----------------server response--------------121Please respect copyright.ＰＥＮＡＮＡAtnGEtH2Mv
HTTP/1.1 200 OK121Please respect copyright.ＰＥＮＡＮＡ4DS6iyOI2G
Date: Sat, 23 Nov 2024 06:29:19 GMT121Please respect copyright.ＰＥＮＡＮＡubPEbiuBeE
accept-ranges: bytes121Please respect copyright.ＰＥＮＡＮＡYQYKPFZ5bH
content-range: bytes 0-534231199/534231200121Please respect copyright.ＰＥＮＡＮＡB4aGYKvk3b
ETag: "534231200_1732277116"121Please respect copyright.ＰＥＮＡＮＡXKxp6EtHXJ
Content-Disposition: attachment; filename="zen.iso"121Please respect copyright.ＰＥＮＡＮＡTrrjN5vPxn
Content-Length: 534231200121Please respect copyright.ＰＥＮＡＮＡRFVNzbKt2k
Content-Type: application/octet-stream
(http response body is the whole file content)121Please respect copyright.ＰＥＮＡＮＡLSegkMdYUK
-------------------------------------------
Http server usually use a small buf to send the whole file content by multiple times if the file size is bigger than it, like this:121Please respect copyright.ＰＥＮＡＮＡ3tNY2l2Thd
---------------http server send big file content by multiple times--------------121Please respect copyright.ＰＥＮＡＮＡ2w0wlVv7PE
const int buf_size=20000000; // buf size is about 20M121Please respect copyright.ＰＥＮＡＮＡE7pF0YZ9vQ
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers121Please respect copyright.ＰＥＮＡＮＡBEfDiyOpEy
send_http_response_headers(client_socket, http_response_headers);
// then send the whole file content as response body by multiple times if the file is big121Please respect copyright.ＰＥＮＡＮＡrYL1Jwnv9H
while(there_is_remain_file_content_to_send)121Please respect copyright.ＰＥＮＡＮＡELbtXQTPi4
{ 121Please respect copyright.ＰＥＮＡＮＡ2XWIOPRPNO
read_remain_file_content_to_send_buf(file, send_buf);121Please respect copyright.ＰＥＮＡＮＡXlHg7cUaX7
send_result = send(client_socket, send_buf);121Please respect copyright.ＰＥＮＡＮＡyxDAeGxTEK
if(send_result == fail) return; else continue;121Please respect copyright.ＰＥＮＡＮＡwm6mThEqjm
}
// a way to make etag for a file using its file size and last modifed time121Please respect copyright.ＰＥＮＡＮＡK2yGCutCFt
const char *file_etag(const char *path) 121Please respect copyright.ＰＥＮＡＮＡqHbtUzQ7n2
{121Please respect copyright.ＰＥＮＡＮＡ8lLnIwDqHJ
struct stat st; if (stat(path, &st) < 0) return NULL; char* data=malloc(64); snprintf(data, 63, "%ld_%ld", (long)st.st_size, (long)st.st_mtime); return data;121Please respect copyright.ＰＥＮＡＮＡyUwwoHLwQc
}121Please respect copyright.ＰＥＮＡＮＡ7gi5DlytMI
-------------------------------------------------121Please respect copyright.ＰＥＮＡＮＡa9EJ1odaOF
"accept-ranges: bytes" tells client web browser that the http server support range.121Please respect copyright.ＰＥＮＡＮＡKukyURRp9f
"content-range: bytes 0-534231199/534231200" tells client web browser the content range(0-534231199) of the response body and the total file size(534231200).121Please respect copyright.ＰＥＮＡＮＡ4Obhy8Ck1F
'ETag: "534231200_1732277116"' tells client web browser the file version.121Please respect copyright.ＰＥＮＡＮＡJq0MTHU7dJ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.121Please respect copyright.ＰＥＮＡＮＡNmsg6wHdSF
"Content-Length: 534231200" tells client web browser the total size of the response body, which is the size of the whole file in this example.121Please respect copyright.ＰＥＮＡＮＡPoKELHwpTU
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
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step 3. When the web browser receives above response, it knows the server support http range, and knows the total file size by "content-range: bytes 0-534231199/534231200", it also saves the etag for later comparation.121Please respect copyright.ＰＥＮＡＮＡcSaXU4CWOK
If the file is big, the above sends may take several minutes, during this time, the user can click pause in the download manager of web browser(for firefox, you have to press ctrl and right click the file to show the pause command.)121Please respect copyright.ＰＥＮＡＮＡsIJDnnL5cR
When pause is pressed, the web browser disconnects with http server, and the "send_result = send(client_socket, send_buf);" of http server will be fail, then the http server stops sending anymore.
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step 4. Later the user can click resume to continue the download in download manager, and the download manager will send a http range request to tell the server the resume start point, like this:121Please respect copyright.ＰＥＮＡＮＡtLf71BwCnP
---------resume range request---------121Please respect copyright.ＰＥＮＡＮＡ7bH2x5uOgI
GET /zen.iso HTTP/1.1121Please respect copyright.ＰＥＮＡＮＡelK6eSrMko
Range: bytes=243220480-121Please respect copyright.ＰＥＮＡＮＡPO1mTxKuXv
If-Match: "534231200_1732277116"121Please respect copyright.ＰＥＮＡＮＡlPKA3AQBDA
......121Please respect copyright.ＰＥＮＡＮＡtXGFLbHy1Q
-------------------------------------121Please respect copyright.ＰＥＮＡＮＡ2h0WP7CMax
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.121Please respect copyright.ＰＥＮＡＮＡ4t7FZMQSOc
'If-Match: "534231200_1732277116"' tells the http server to check the file etag.
If the etag remains the same, it means the file is not changed, so the server can send from the resume point 243220480, and its response status should be 206 partial content, like this:121Please respect copyright.ＰＥＮＡＮＡouLvoxtlI4
-------response content from resume point-------121Please respect copyright.ＰＥＮＡＮＡlQFocAnOm8
HTTP/1.1 206 Partial Content121Please respect copyright.ＰＥＮＡＮＡ4yPJrIk7x0
Date: Sat, 23 Nov 2024 06:56:17 GMT121Please respect copyright.ＰＥＮＡＮＡQGUOc6cTKo
accept-ranges: bytes121Please respect copyright.ＰＥＮＡＮＡt4ZdfJ5Ttc
content-range: bytes 243220480-534231199/534231200121Please respect copyright.ＰＥＮＡＮＡjS5R5zl5v1
Content-Length: 291010720121Please respect copyright.ＰＥＮＡＮＡ9BqyxK8bNm
Content-Type: application/octet-stream121Please respect copyright.ＰＥＮＡＮＡtrroCp4dox
Content-Disposition: attachment; filename="zen.iso"121Please respect copyright.ＰＥＮＡＮＡYNOZ3Gor8X
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)121Please respect copyright.ＰＥＮＡＮＡTafXXB1sx6
------------------------------------------------121Please respect copyright.ＰＥＮＡＮＡbY4viyI7jE
When client download manager receives this response, it will keep the already downloaded content and append the new respone body content to it.
If the etag diffs, it means the file has been changed, then the server should send the file content from beginning to end, with a 200 OK response status, like this:121Please respect copyright.ＰＥＮＡＮＡQyKTcZKFN1
-------response content from resume point-------121Please respect copyright.ＰＥＮＡＮＡLRh2HdjxSg
HTTP/1.1 200 OK121Please respect copyright.ＰＥＮＡＮＡQkuLdFJMiZ
Date: Sat, 23 Nov 2024 06:29:19 GMT121Please respect copyright.ＰＥＮＡＮＡp3AuYIQUck
accept-ranges: bytes121Please respect copyright.ＰＥＮＡＮＡBMxD2Ox1Ta
content-range: bytes 0-634231201/634231202121Please respect copyright.ＰＥＮＡＮＡ2yEyOqcsTv
ETag: "634231202_2732277118"121Please respect copyright.ＰＥＮＡＮＡ9BnJfTsNMX
Content-Disposition: attachment; filename="zen.iso"121Please respect copyright.ＰＥＮＡＮＡsjWWImhSJx
Content-Length: 634231202121Please respect copyright.ＰＥＮＡＮＡGOKmAVgsEI
Content-Type: application/octet-stream
(http response body is the whole file content)121Please respect copyright.ＰＥＮＡＮＡL8rLvP1V89
------------------------------------------------121Please respect copyright.ＰＥＮＡＮＡenRwBy4EXw
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
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step 5. when a <video src="/video_file.mp4"> is used to play videos on web page, the browser also used http range requests, as Zeng Xu's article describes:121Please respect copyright.ＰＥＮＡＮＡgIUo1KOpgI
zeng.dev/post/2023-http-range-and-play-mp4-in-browser?
121Please respect copyright.ＰＥＮＡＮＡcWf6mWkfzz
Section C. Thanks
感謝曾旭的幫助，曾旭的文章对http range有非常好的描述。
感谢stackoverflow对使用etag的提示121Please respect copyright.ＰＥＮＡＮＡwqfgLIEDEd
stackoverflow.com/questions/66998172/does-google-chrome-and-similar-browsers-support-range-headers-for-standard-downl
感谢gemini等ai对写代码的辅助121Please respect copyright.ＰＥＮＡＮＡob5euDKJVa
gemini.google.com
感谢互联网上无私奉献的参考资121Please respect copyright.ＰＥＮＡＮＡIRo2uyBSBZ
developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests121Please respect copyright.ＰＥＮＡＮＡ9MEnVuruVk
developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range121Please respect copyright.ＰＥＮＡＮＡX6VzOBqQhz
developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag121Please respect copyright.ＰＥＮＡＮＡh7S1cFtv1Q
rfc-editor.org/rfc/rfc7233121Please respect copyright.ＰＥＮＡＮＡpjrdiWCx8G
mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10
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Section D. Contacts Me
If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: si.jinmin@gmail.com121Please respect copyright.ＰＥＮＡＮＡSBT0JRGVuZ
如果您发现本文有任何错误，或者对本文有好的建议，欢迎与我联系探讨，我的微信: si_jinmin, 我的email: si.jinmin@gmail.com
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