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:125Please respect copyright.ＰＥＮＡＮＡP2wVfg1XJ3
----------http range request-------125Please respect copyright.ＰＥＮＡＮＡGkEtJZfpsa
GET /zen.iso HTTP/1.1125Please respect copyright.ＰＥＮＡＮＡFleZZODeF1
Range: bytes=100-1023125Please respect copyright.ＰＥＮＡＮＡYwOHZy4VqF
If-Match: "534231200_1732277116"125Please respect copyright.ＰＥＮＡＮＡOh8Riy65c1
......125Please respect copyright.ＰＥＮＡＮＡ2qCzzTqsam
-----------------------------------125Please respect copyright.ＰＥＮＡＮＡlrDOY2V8SL
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:125Please respect copyright.ＰＥＮＡＮＡYHBHm70MaB
----------------html-----------------125Please respect copyright.ＰＥＮＡＮＡxVagzAIT1r
<a href="/zen.iso">zen.iso</a>125Please respect copyright.ＰＥＮＡＮＡRvbsZYdJiP
-------------------------------------125Please respect copyright.ＰＥＮＡＮＡyy4wr21SI0
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:125Please respect copyright.ＰＥＮＡＮＡ1CjiDF9kK0
----------get request------------125Please respect copyright.ＰＥＮＡＮＡzvLxypTKga
GET /zen.iso HTTP/1.1125Please respect copyright.ＰＥＮＡＮＡTlTFof01zo
......125Please respect copyright.ＰＥＮＡＮＡgkBg6rhIUS
---------------------------------
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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:125Please respect copyright.ＰＥＮＡＮＡplIvcaGdsH
----------------server response--------------125Please respect copyright.ＰＥＮＡＮＡXic3WEz7gY
HTTP/1.1 200 OK125Please respect copyright.ＰＥＮＡＮＡ0dwcKStWwJ
Date: Sat, 23 Nov 2024 06:29:19 GMT125Please respect copyright.ＰＥＮＡＮＡZYvOdmTX0X
accept-ranges: bytes125Please respect copyright.ＰＥＮＡＮＡTB0RgsD3ni
content-range: bytes 0-534231199/534231200125Please respect copyright.ＰＥＮＡＮＡagy0UeR6xv
ETag: "534231200_1732277116"125Please respect copyright.ＰＥＮＡＮＡzhdZGGc6yZ
Content-Disposition: attachment; filename="zen.iso"125Please respect copyright.ＰＥＮＡＮＡPF1rY9itM0
Content-Length: 534231200125Please respect copyright.ＰＥＮＡＮＡAbUL22Cqna
Content-Type: application/octet-stream
(http response body is the whole file content)125Please respect copyright.ＰＥＮＡＮＡuZV2vouBZZ
-------------------------------------------
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:125Please respect copyright.ＰＥＮＡＮＡhXONj9X3WE
---------------http server send big file content by multiple times--------------125Please respect copyright.ＰＥＮＡＮＡDCWYTUkVsX
const int buf_size=20000000; // buf size is about 20M125Please respect copyright.ＰＥＮＡＮＡkYPfzMXw9n
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers125Please respect copyright.ＰＥＮＡＮＡRe9VyNaYsm
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 big125Please respect copyright.ＰＥＮＡＮＡRZGfDDgW9x
while(there_is_remain_file_content_to_send)125Please respect copyright.ＰＥＮＡＮＡULnIagxGBy
{ 125Please respect copyright.ＰＥＮＡＮＡcAkfyBaG5s
read_remain_file_content_to_send_buf(file, send_buf);125Please respect copyright.ＰＥＮＡＮＡJ6XEkG8DKB
send_result = send(client_socket, send_buf);125Please respect copyright.ＰＥＮＡＮＡ7ufYG1WoLu
if(send_result == fail) return; else continue;125Please respect copyright.ＰＥＮＡＮＡQj2SvvMZMd
}
// a way to make etag for a file using its file size and last modifed time125Please respect copyright.ＰＥＮＡＮＡKPqPvrM1LA
const char *file_etag(const char *path) 125Please respect copyright.ＰＥＮＡＮＡ40ATuqLHQc
{125Please respect copyright.ＰＥＮＡＮＡ9T6KvAzWSq
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;125Please respect copyright.ＰＥＮＡＮＡXvI7UIxwiP
}125Please respect copyright.ＰＥＮＡＮＡUE7Ep557YG
-------------------------------------------------125Please respect copyright.ＰＥＮＡＮＡMsOzYrev64
"accept-ranges: bytes" tells client web browser that the http server support range.125Please respect copyright.ＰＥＮＡＮＡeFfn2WyM1h
"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).125Please respect copyright.ＰＥＮＡＮＡtzP50ncc3h
'ETag: "534231200_1732277116"' tells client web browser the file version.125Please respect copyright.ＰＥＮＡＮＡYnaP1HxTbg
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.125Please respect copyright.ＰＥＮＡＮＡ8scC5WxadA
"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.125Please respect copyright.ＰＥＮＡＮＡAOIKhifVDY
"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.125Please respect copyright.ＰＥＮＡＮＡClQb9uteWj
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.)125Please respect copyright.ＰＥＮＡＮＡLlR82PzxpQ
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:125Please respect copyright.ＰＥＮＡＮＡVOorNggSkm
---------resume range request---------125Please respect copyright.ＰＥＮＡＮＡCfO2DXzlHx
GET /zen.iso HTTP/1.1125Please respect copyright.ＰＥＮＡＮＡIAoUy0PGQs
Range: bytes=243220480-125Please respect copyright.ＰＥＮＡＮＡWDFr6WIrr5
If-Match: "534231200_1732277116"125Please respect copyright.ＰＥＮＡＮＡLnE6ockVKu
......125Please respect copyright.ＰＥＮＡＮＡMQiKfGwEiZ
-------------------------------------125Please respect copyright.ＰＥＮＡＮＡcegilQ5fDl
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.125Please respect copyright.ＰＥＮＡＮＡLCLBUSiL76
'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:125Please respect copyright.ＰＥＮＡＮＡZLoMkPkbsP
-------response content from resume point-------125Please respect copyright.ＰＥＮＡＮＡTcVqrislNF
HTTP/1.1 206 Partial Content125Please respect copyright.ＰＥＮＡＮＡFFdcgtq0FD
Date: Sat, 23 Nov 2024 06:56:17 GMT125Please respect copyright.ＰＥＮＡＮＡwPaxBCaURI
accept-ranges: bytes125Please respect copyright.ＰＥＮＡＮＡrw3JplRfbH
content-range: bytes 243220480-534231199/534231200125Please respect copyright.ＰＥＮＡＮＡsnBJCl268L
Content-Length: 291010720125Please respect copyright.ＰＥＮＡＮＡzPCAEnMafr
Content-Type: application/octet-stream125Please respect copyright.ＰＥＮＡＮＡta2etY1fRr
Content-Disposition: attachment; filename="zen.iso"125Please respect copyright.ＰＥＮＡＮＡwxfU2RV1e2
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)125Please respect copyright.ＰＥＮＡＮＡqzcWFH9KwE
------------------------------------------------125Please respect copyright.ＰＥＮＡＮＡjWZ5zxBhcw
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:125Please respect copyright.ＰＥＮＡＮＡfLWrTuHmme
-------response content from resume point-------125Please respect copyright.ＰＥＮＡＮＡc4iP685m0R
HTTP/1.1 200 OK125Please respect copyright.ＰＥＮＡＮＡe5HLejiJEI
Date: Sat, 23 Nov 2024 06:29:19 GMT125Please respect copyright.ＰＥＮＡＮＡOG8wYMFjI6
accept-ranges: bytes125Please respect copyright.ＰＥＮＡＮＡcKHJw7WICb
content-range: bytes 0-634231201/634231202125Please respect copyright.ＰＥＮＡＮＡ2Hs0Vnl9v4
ETag: "634231202_2732277118"125Please respect copyright.ＰＥＮＡＮＡwCDpXxzmUq
Content-Disposition: attachment; filename="zen.iso"125Please respect copyright.ＰＥＮＡＮＡXrFLHUsCgQ
Content-Length: 634231202125Please respect copyright.ＰＥＮＡＮＡtLghxQSF5n
Content-Type: application/octet-stream
(http response body is the whole file content)125Please respect copyright.ＰＥＮＡＮＡuanx2nXt27
------------------------------------------------125Please respect copyright.ＰＥＮＡＮＡb8xh5dpX8c
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:125Please respect copyright.ＰＥＮＡＮＡQHnfSin3O0
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
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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125Please respect copyright.ＰＥＮＡＮＡX17L8jT94b
https://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125Please respect copyright.ＰＥＮＡＮＡqOnihRXexR
https://gemini.google.com/
感谢互联网上无私奉献的参考资125Please respect copyright.ＰＥＮＡＮＡaKbgL1WUlD
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests125Please respect copyright.ＰＥＮＡＮＡpzFOvhtLft
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range125Please respect copyright.ＰＥＮＡＮＡk89YCbVAya
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag125Please respect copyright.ＰＥＮＡＮＡG8R4k9JEZz
https://www.rfc-editor.org/rfc/rfc7233125Please respect copyright.ＰＥＮＡＮＡCVIc31QoOs
https://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: [email protected]125Please respect copyright.ＰＥＮＡＮＡw3O4LSUMkG
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: [email protected]
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