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:142Please respect copyright.ＰＥＮＡＮＡLBsntX33jA
----------http range request-------142Please respect copyright.ＰＥＮＡＮＡDsLRnxwBjQ
GET /zen.iso HTTP/1.1142Please respect copyright.ＰＥＮＡＮＡqPNPyH53oF
Range: bytes=100-1023142Please respect copyright.ＰＥＮＡＮＡ5v45XjA2aM
If-Match: "534231200_1732277116"142Please respect copyright.ＰＥＮＡＮＡ6nkPwoAnwP
......142Please respect copyright.ＰＥＮＡＮＡgHSkq5CZK3
-----------------------------------142Please respect copyright.ＰＥＮＡＮＡSJ7TyFU6ET
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:142Please respect copyright.ＰＥＮＡＮＡNlKDcpNcQh
----------------html-----------------142Please respect copyright.ＰＥＮＡＮＡTeXqTKhF06
<a href="/zen.iso">zen.iso</a>142Please respect copyright.ＰＥＮＡＮＡuxTmqvRPHN
-------------------------------------142Please respect copyright.ＰＥＮＡＮＡdXejoYABlx
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:142Please respect copyright.ＰＥＮＡＮＡkcdoybGyMK
----------get request------------142Please respect copyright.ＰＥＮＡＮＡRUmY6eMjKK
GET /zen.iso HTTP/1.1142Please respect copyright.ＰＥＮＡＮＡ0VX8NSXSPl
......142Please respect copyright.ＰＥＮＡＮＡD8YGAvkMiZ
---------------------------------
142Please respect copyright.ＰＥＮＡＮＡuWe5xVmmGN
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:142Please respect copyright.ＰＥＮＡＮＡGbwu3cv1Cb
----------------server response--------------142Please respect copyright.ＰＥＮＡＮＡdqBUlfdaLO
HTTP/1.1 200 OK142Please respect copyright.ＰＥＮＡＮＡRkBdsZ9UgS
Date: Sat, 23 Nov 2024 06:29:19 GMT142Please respect copyright.ＰＥＮＡＮＡVzqA0lyP7V
accept-ranges: bytes142Please respect copyright.ＰＥＮＡＮＡ7ZJtBEibvc
content-range: bytes 0-534231199/534231200142Please respect copyright.ＰＥＮＡＮＡPx9KoVwtwm
ETag: "534231200_1732277116"142Please respect copyright.ＰＥＮＡＮＡHUMaNhtsMv
Content-Disposition: attachment; filename="zen.iso"142Please respect copyright.ＰＥＮＡＮＡeG1RE5jrL0
Content-Length: 534231200142Please respect copyright.ＰＥＮＡＮＡjOba03CmZs
Content-Type: application/octet-stream
(http response body is the whole file content)142Please respect copyright.ＰＥＮＡＮＡ53r2gpT5Va
-------------------------------------------
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:142Please respect copyright.ＰＥＮＡＮＡNtrNK4X2Kz
---------------http server send big file content by multiple times--------------142Please respect copyright.ＰＥＮＡＮＡiReXmSDXEq
const int buf_size=20000000; // buf size is about 20M142Please respect copyright.ＰＥＮＡＮＡuiiKfRJK5w
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers142Please respect copyright.ＰＥＮＡＮＡD142nCr3CG
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 big142Please respect copyright.ＰＥＮＡＮＡWLZOaAnFT7
while(there_is_remain_file_content_to_send)142Please respect copyright.ＰＥＮＡＮＡ9jFnAd9noz
{ 142Please respect copyright.ＰＥＮＡＮＡYYbtLOo3Bs
read_remain_file_content_to_send_buf(file, send_buf);142Please respect copyright.ＰＥＮＡＮＡwmivi5rYWU
send_result = send(client_socket, send_buf);142Please respect copyright.ＰＥＮＡＮＡNHg6fr8gFB
if(send_result == fail) return; else continue;142Please respect copyright.ＰＥＮＡＮＡSd2RJmCl0c
}
// a way to make etag for a file using its file size and last modifed time142Please respect copyright.ＰＥＮＡＮＡD7gK5lD9li
const char *file_etag(const char *path) 142Please respect copyright.ＰＥＮＡＮＡ2pkV1pFY4E
{142Please respect copyright.ＰＥＮＡＮＡaWemkkb2pa
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;142Please respect copyright.ＰＥＮＡＮＡwMiFAdhwZL
}142Please respect copyright.ＰＥＮＡＮＡKvPlOMXDVX
-------------------------------------------------142Please respect copyright.ＰＥＮＡＮＡqPOECFqNAv
"accept-ranges: bytes" tells client web browser that the http server support range.142Please respect copyright.ＰＥＮＡＮＡOjMWFUZ2wo
"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).142Please respect copyright.ＰＥＮＡＮＡUqSYUCOWE2
'ETag: "534231200_1732277116"' tells client web browser the file version.142Please respect copyright.ＰＥＮＡＮＡx46U7PBTnq
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.142Please respect copyright.ＰＥＮＡＮＡHj8txJRiE3
"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.142Please respect copyright.ＰＥＮＡＮＡCtjtpg8lR4
"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.142Please respect copyright.ＰＥＮＡＮＡ5qxqOJvbgL
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.)142Please respect copyright.ＰＥＮＡＮＡy47XB0ntxr
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:142Please respect copyright.ＰＥＮＡＮＡQvwEjnpwGH
---------resume range request---------142Please respect copyright.ＰＥＮＡＮＡ0Dwdh91HF3
GET /zen.iso HTTP/1.1142Please respect copyright.ＰＥＮＡＮＡHgZR788FhM
Range: bytes=243220480-142Please respect copyright.ＰＥＮＡＮＡ0W66RvVW8a
If-Match: "534231200_1732277116"142Please respect copyright.ＰＥＮＡＮＡ3BEuUut1VB
......142Please respect copyright.ＰＥＮＡＮＡIpps30uDVE
-------------------------------------142Please respect copyright.ＰＥＮＡＮＡsAkPLZbrvW
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.142Please respect copyright.ＰＥＮＡＮＡCKSlnf11zi
'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:142Please respect copyright.ＰＥＮＡＮＡV1LorYymlN
-------response content from resume point-------142Please respect copyright.ＰＥＮＡＮＡtZdFMrQkMy
HTTP/1.1 206 Partial Content142Please respect copyright.ＰＥＮＡＮＡCjoRTf1Ilo
Date: Sat, 23 Nov 2024 06:56:17 GMT142Please respect copyright.ＰＥＮＡＮＡa4qfzIMYqD
accept-ranges: bytes142Please respect copyright.ＰＥＮＡＮＡnkZcsHcwPK
content-range: bytes 243220480-534231199/534231200142Please respect copyright.ＰＥＮＡＮＡ2tdRlQLXYf
Content-Length: 291010720142Please respect copyright.ＰＥＮＡＮＡHUSlD254d3
Content-Type: application/octet-stream142Please respect copyright.ＰＥＮＡＮＡXXlGKMspI4
Content-Disposition: attachment; filename="zen.iso"142Please respect copyright.ＰＥＮＡＮＡR4DnBuvre0
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)142Please respect copyright.ＰＥＮＡＮＡvWSHBoMHSH
------------------------------------------------142Please respect copyright.ＰＥＮＡＮＡgQvfhuZ7ZM
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:142Please respect copyright.ＰＥＮＡＮＡyE6Oi7whTd
-------response content from resume point-------142Please respect copyright.ＰＥＮＡＮＡC8bnp6Gnpo
HTTP/1.1 200 OK142Please respect copyright.ＰＥＮＡＮＡjFVrn8v4nX
Date: Sat, 23 Nov 2024 06:29:19 GMT142Please respect copyright.ＰＥＮＡＮＡryv9ZsfFpE
accept-ranges: bytes142Please respect copyright.ＰＥＮＡＮＡ6ErtEeb5tk
content-range: bytes 0-634231201/634231202142Please respect copyright.ＰＥＮＡＮＡ06KtrkFxBT
ETag: "634231202_2732277118"142Please respect copyright.ＰＥＮＡＮＡsvmydbydWq
Content-Disposition: attachment; filename="zen.iso"142Please respect copyright.ＰＥＮＡＮＡbGdF9NGSjQ
Content-Length: 634231202142Please respect copyright.ＰＥＮＡＮＡ5wZ2JFjIoI
Content-Type: application/octet-stream
(http response body is the whole file content)142Please respect copyright.ＰＥＮＡＮＡ7R8EYricy3
------------------------------------------------142Please respect copyright.ＰＥＮＡＮＡDZXlyQpwIZ
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:142Please respect copyright.ＰＥＮＡＮＡncHqLkjatG
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
142Please respect copyright.ＰＥＮＡＮＡnbT2MsXdR7
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142Please respect copyright.ＰＥＮＡＮＡukqy24dPjb
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142Please respect copyright.ＰＥＮＡＮＡW2IUjJGRou
https://gemini.google.com/
感谢互联网上无私奉献的参考资142Please respect copyright.ＰＥＮＡＮＡ4zi0vM0aYY
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests142Please respect copyright.ＰＥＮＡＮＡfM0nFytRXH
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range142Please respect copyright.ＰＥＮＡＮＡicTdGLVHLn
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag142Please respect copyright.ＰＥＮＡＮＡyW3rwvfHsB
https://www.rfc-editor.org/rfc/rfc7233142Please respect copyright.ＰＥＮＡＮＡEjSsHdRJmV
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]142Please respect copyright.ＰＥＮＡＮＡAX3bd4NmWM
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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