明慈淨行記 - 第68章《Coding Art》第20篇 The Implementation of Http Range for Supporting Big File Download Pause and Re

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:331Please respect copyright.ＰＥＮＡＮＡpaLZfoTb9U
----------http range request-------331Please respect copyright.ＰＥＮＡＮＡwVWQFosazK
GET /zen.iso HTTP/1.1331Please respect copyright.ＰＥＮＡＮＡ9jLC8xnWA7
Range: bytes=100-1023331Please respect copyright.ＰＥＮＡＮＡOedw2KUjby
If-Match: "534231200_1732277116"331Please respect copyright.ＰＥＮＡＮＡp43B4ikb49
......331Please respect copyright.ＰＥＮＡＮＡOu1WrnO8W9
-----------------------------------331Please respect copyright.ＰＥＮＡＮＡlqgA8faKug
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:331Please respect copyright.ＰＥＮＡＮＡbdAzfdxSdB
----------------html-----------------331Please respect copyright.ＰＥＮＡＮＡrA2wfze6vD
<a href="/zen.iso">zen.iso</a>331Please respect copyright.ＰＥＮＡＮＡFifVBl1mX8
-------------------------------------331Please respect copyright.ＰＥＮＡＮＡ2O4b1AweXJ
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:331Please respect copyright.ＰＥＮＡＮＡkfUISG9U6k
----------get request------------331Please respect copyright.ＰＥＮＡＮＡZcHc1qFLn3
GET /zen.iso HTTP/1.1331Please respect copyright.ＰＥＮＡＮＡMEMLkpxLGV
......331Please respect copyright.ＰＥＮＡＮＡmhsxWpUflX
---------------------------------

331Please respect copyright.ＰＥＮＡＮＡt8pmQ9FCcO
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:331Please respect copyright.ＰＥＮＡＮＡzjVKgdfnrT
----------------server response--------------331Please respect copyright.ＰＥＮＡＮＡOrXDFGlB5m
HTTP/1.1 200 OK331Please respect copyright.ＰＥＮＡＮＡnAXVnfg6ji
Date: Sat, 23 Nov 2024 06:29:19 GMT331Please respect copyright.ＰＥＮＡＮＡlec0xf1E6F
accept-ranges: bytes331Please respect copyright.ＰＥＮＡＮＡIg6v3EiIxd
content-range: bytes 0-534231199/534231200331Please respect copyright.ＰＥＮＡＮＡ8KQTSV1AG5
ETag: "534231200_1732277116"331Please respect copyright.ＰＥＮＡＮＡ6qKoOh1S84
Content-Disposition: attachment; filename="zen.iso"331Please respect copyright.ＰＥＮＡＮＡfaUGFOyAkd
Content-Length: 534231200331Please respect copyright.ＰＥＮＡＮＡ0xyHQGQl6f
Content-Type: application/octet-stream

(http response body is the whole file content)331Please respect copyright.ＰＥＮＡＮＡnMxycLtVrc
-------------------------------------------

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:331Please respect copyright.ＰＥＮＡＮＡoGAWrtG4ZZ
---------------http server send big file content by multiple times--------------331Please respect copyright.ＰＥＮＡＮＡwmIPkXpTST
const int buf_size=20000000; // buf size is about 20M331Please respect copyright.ＰＥＮＡＮＡ8NpelxJW3w
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers331Please respect copyright.ＰＥＮＡＮＡ3hXtI8Toqa
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 big331Please respect copyright.ＰＥＮＡＮＡp0Wx0fSGjA
while(there_is_remain_file_content_to_send)331Please respect copyright.ＰＥＮＡＮＡFMLMRzvQNt
{ 331Please respect copyright.ＰＥＮＡＮＡTohyND5i7V
read_remain_file_content_to_send_buf(file, send_buf);331Please respect copyright.ＰＥＮＡＮＡB7qR67sO0R
send_result = send(client_socket, send_buf);331Please respect copyright.ＰＥＮＡＮＡDGQ9x7WtAq
if(send_result == fail) return; else continue;331Please respect copyright.ＰＥＮＡＮＡb22zOwz4Ln
}

// a way to make etag for a file using its file size and last modifed time331Please respect copyright.ＰＥＮＡＮＡ2EEtyXIODb
const char *file_etag(const char *path) 331Please respect copyright.ＰＥＮＡＮＡdNB9C8Myjp
{331Please respect copyright.ＰＥＮＡＮＡ3CSnYJY5fT
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;331Please respect copyright.ＰＥＮＡＮＡojBoH7r9we
}331Please respect copyright.ＰＥＮＡＮＡ8XfSlOGpiC
-------------------------------------------------331Please respect copyright.ＰＥＮＡＮＡxGrK9OoJgY
"accept-ranges: bytes" tells client web browser that the http server support range.331Please respect copyright.ＰＥＮＡＮＡMVpRlVqPf9
"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).331Please respect copyright.ＰＥＮＡＮＡ4NvUAIJCmu
'ETag: "534231200_1732277116"' tells client web browser the file version.331Please respect copyright.ＰＥＮＡＮＡES8ZperHj7
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.331Please respect copyright.ＰＥＮＡＮＡW1heXpKfZz
"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.331Please respect copyright.ＰＥＮＡＮＡCMXDKNFm73
"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.331Please respect copyright.ＰＥＮＡＮＡzfNZPrfeR8
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.)331Please respect copyright.ＰＥＮＡＮＡPNeRZ9vaZO
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:331Please respect copyright.ＰＥＮＡＮＡxNTFNm8rxW
---------resume range request---------331Please respect copyright.ＰＥＮＡＮＡ1GBI0EjRnY
GET /zen.iso HTTP/1.1331Please respect copyright.ＰＥＮＡＮＡNZQgE1bFt3
Range: bytes=243220480-331Please respect copyright.ＰＥＮＡＮＡpi2D2vz5Fb
If-Match: "534231200_1732277116"331Please respect copyright.ＰＥＮＡＮＡGuPHHh0hwf
......331Please respect copyright.ＰＥＮＡＮＡjX3NWDoqGw
-------------------------------------331Please respect copyright.ＰＥＮＡＮＡRna9EmdNBR
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.331Please respect copyright.ＰＥＮＡＮＡwh0wnD0WVH
'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:331Please respect copyright.ＰＥＮＡＮＡPcW0L9PPbA
-------response content from resume point-------331Please respect copyright.ＰＥＮＡＮＡznGwTXRPxV
HTTP/1.1 206 Partial Content331Please respect copyright.ＰＥＮＡＮＡTFPgN2fVni
Date: Sat, 23 Nov 2024 06:56:17 GMT331Please respect copyright.ＰＥＮＡＮＡu917Jy6oUB
accept-ranges: bytes331Please respect copyright.ＰＥＮＡＮＡyKIkG0oT9s
content-range: bytes 243220480-534231199/534231200331Please respect copyright.ＰＥＮＡＮＡpZtRJatTzX
Content-Length: 291010720331Please respect copyright.ＰＥＮＡＮＡ2zN2cLnEmM
Content-Type: application/octet-stream331Please respect copyright.ＰＥＮＡＮＡolqllVnNTe
Content-Disposition: attachment; filename="zen.iso"331Please respect copyright.ＰＥＮＡＮＡxHouAM5lLs
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)331Please respect copyright.ＰＥＮＡＮＡ7mlcQjH8qa
------------------------------------------------331Please respect copyright.ＰＥＮＡＮＡptyPulqbc2
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:331Please respect copyright.ＰＥＮＡＮＡ2O1oO1wIII
-------response content from resume point-------331Please respect copyright.ＰＥＮＡＮＡNfNWRhWaoz
HTTP/1.1 200 OK331Please respect copyright.ＰＥＮＡＮＡLQOWPJvmnL
Date: Sat, 23 Nov 2024 06:29:19 GMT331Please respect copyright.ＰＥＮＡＮＡ5xPwmMULKj
accept-ranges: bytes331Please respect copyright.ＰＥＮＡＮＡ0Whcj2D77F
content-range: bytes 0-634231201/634231202331Please respect copyright.ＰＥＮＡＮＡNe6kQ704d7
ETag: "634231202_2732277118"331Please respect copyright.ＰＥＮＡＮＡ4yesoAtpOH
Content-Disposition: attachment; filename="zen.iso"331Please respect copyright.ＰＥＮＡＮＡ2Yl9jtyZyG
Content-Length: 634231202331Please respect copyright.ＰＥＮＡＮＡdezGvF2AAz
Content-Type: application/octet-stream

(http response body is the whole file content)331Please respect copyright.ＰＥＮＡＮＡI6P27L8dG9
------------------------------------------------331Please respect copyright.ＰＥＮＡＮＡrAPGUUrYFy
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:331Please respect copyright.ＰＥＮＡＮＡtlHWTCeaHC
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331Please respect copyright.ＰＥＮＡＮＡqB84etJXbW
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331Please respect copyright.ＰＥＮＡＮＡctw793c8zQ
https://gemini.google.com/

感谢互联网上无私奉献的参考资331Please respect copyright.ＰＥＮＡＮＡHxhPgmDfyV
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests331Please respect copyright.ＰＥＮＡＮＡYg9DlM74mS
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range331Please respect copyright.ＰＥＮＡＮＡfCySmFekGe
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag331Please respect copyright.ＰＥＮＡＮＡkgTyb4FC5G
https://www.rfc-editor.org/rfc/rfc7233331Please respect copyright.ＰＥＮＡＮＡamZBjz5etI
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]331Please respect copyright.ＰＥＮＡＮＡQ2mhVAlHxG
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]

如果您對C/C++ programming, Linux, website development, Vue, Git, vscode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。

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