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:59Please respect copyright.ＰＥＮＡＮＡSqptAxyOYG
----------http range request-------59Please respect copyright.ＰＥＮＡＮＡUSZRZxmGEd
GET /zen.iso HTTP/1.159Please respect copyright.ＰＥＮＡＮＡpLfM04mw7y
Range: bytes=100-102359Please respect copyright.ＰＥＮＡＮＡmBHioGQrQp
If-Match: "534231200_1732277116"59Please respect copyright.ＰＥＮＡＮＡtEVKRLY8HD
......59Please respect copyright.ＰＥＮＡＮＡcm2kJamvCk
-----------------------------------59Please respect copyright.ＰＥＮＡＮＡ4SHvG4SH8Q
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:59Please respect copyright.ＰＥＮＡＮＡ4aLOEBkiPq
----------------html-----------------59Please respect copyright.ＰＥＮＡＮＡrOOBh8Ugc3
<a href="/zen.iso">zen.iso</a>59Please respect copyright.ＰＥＮＡＮＡ63hxZEHBRT
-------------------------------------59Please respect copyright.ＰＥＮＡＮＡzeyKCGmEVo
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:59Please respect copyright.ＰＥＮＡＮＡmEo4ykEuOK
----------get request------------59Please respect copyright.ＰＥＮＡＮＡvwiVlb0t3w
GET /zen.iso HTTP/1.159Please respect copyright.ＰＥＮＡＮＡJtv1jjgbPj
......59Please respect copyright.ＰＥＮＡＮＡWe4g8Yho2s
---------------------------------
59Please respect copyright.ＰＥＮＡＮＡA9Eeg1i4uz
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:59Please respect copyright.ＰＥＮＡＮＡw3HMe1RJj9
----------------server response--------------59Please respect copyright.ＰＥＮＡＮＡJ2QZtjjwbb
HTTP/1.1 200 OK59Please respect copyright.ＰＥＮＡＮＡLi6clfcjX4
Date: Sat, 23 Nov 2024 06:29:19 GMT59Please respect copyright.ＰＥＮＡＮＡUoxxEB71Ik
accept-ranges: bytes59Please respect copyright.ＰＥＮＡＮＡkxDv5UEU3i
content-range: bytes 0-534231199/53423120059Please respect copyright.ＰＥＮＡＮＡiC3xs9hPYh
ETag: "534231200_1732277116"59Please respect copyright.ＰＥＮＡＮＡrJCg0tJ0HY
Content-Disposition: attachment; filename="zen.iso"59Please respect copyright.ＰＥＮＡＮＡYvM0F7VTaP
Content-Length: 53423120059Please respect copyright.ＰＥＮＡＮＡ2z2aQVxwsA
Content-Type: application/octet-stream
(http response body is the whole file content)59Please respect copyright.ＰＥＮＡＮＡp5pSSThHWV
-------------------------------------------
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:59Please respect copyright.ＰＥＮＡＮＡ3cCuTR4Zta
---------------http server send big file content by multiple times--------------59Please respect copyright.ＰＥＮＡＮＡvKRZwJnHNB
const int buf_size=20000000; // buf size is about 20M59Please respect copyright.ＰＥＮＡＮＡwwN1WIwoSc
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers59Please respect copyright.ＰＥＮＡＮＡFRDweYhSsO
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 big59Please respect copyright.ＰＥＮＡＮＡJCLanyRwOx
while(there_is_remain_file_content_to_send)59Please respect copyright.ＰＥＮＡＮＡtNwVd4EtFk
{ 59Please respect copyright.ＰＥＮＡＮＡeeiaSEzAhm
read_remain_file_content_to_send_buf(file, send_buf);59Please respect copyright.ＰＥＮＡＮＡdSd57j1ZRg
send_result = send(client_socket, send_buf);59Please respect copyright.ＰＥＮＡＮＡCG3kiRUGsG
if(send_result == fail) return; else continue;59Please respect copyright.ＰＥＮＡＮＡCaYRcGmsRf
}
// a way to make etag for a file using its file size and last modifed time59Please respect copyright.ＰＥＮＡＮＡAIXcyRBFGG
const char *file_etag(const char *path) 59Please respect copyright.ＰＥＮＡＮＡfrrWLopcBh
{59Please respect copyright.ＰＥＮＡＮＡl7n4y7o4n0
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;59Please respect copyright.ＰＥＮＡＮＡBfCwULLyrR
}59Please respect copyright.ＰＥＮＡＮＡ6OsCrwvZMa
-------------------------------------------------59Please respect copyright.ＰＥＮＡＮＡ3iY77n4syS
"accept-ranges: bytes" tells client web browser that the http server support range.59Please respect copyright.ＰＥＮＡＮＡxF1wak6uIY
"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).59Please respect copyright.ＰＥＮＡＮＡgPShdXPlRh
'ETag: "534231200_1732277116"' tells client web browser the file version.59Please respect copyright.ＰＥＮＡＮＡce610wZ6eG
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.59Please respect copyright.ＰＥＮＡＮＡsXolRIN445
"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.59Please respect copyright.ＰＥＮＡＮＡQnn6Qd4pI8
"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.59Please respect copyright.ＰＥＮＡＮＡw42Poz6B4O
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.)59Please respect copyright.ＰＥＮＡＮＡdXks6f95fm
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:59Please respect copyright.ＰＥＮＡＮＡffZ0Bn0nSh
---------resume range request---------59Please respect copyright.ＰＥＮＡＮＡDP3VJVdxty
GET /zen.iso HTTP/1.159Please respect copyright.ＰＥＮＡＮＡ0UWtPZbCeK
Range: bytes=243220480-59Please respect copyright.ＰＥＮＡＮＡBIV9HRkRgU
If-Match: "534231200_1732277116"59Please respect copyright.ＰＥＮＡＮＡprcmskULe5
......59Please respect copyright.ＰＥＮＡＮＡ8PthFXNnOL
-------------------------------------59Please respect copyright.ＰＥＮＡＮＡLAwh2HxUkZ
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.59Please respect copyright.ＰＥＮＡＮＡvnBDDTADzN
'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:59Please respect copyright.ＰＥＮＡＮＡsJYodcc9WV
-------response content from resume point-------59Please respect copyright.ＰＥＮＡＮＡNrl1RhMRIv
HTTP/1.1 206 Partial Content59Please respect copyright.ＰＥＮＡＮＡewOpy2FZYu
Date: Sat, 23 Nov 2024 06:56:17 GMT59Please respect copyright.ＰＥＮＡＮＡs3PR7PiLz4
accept-ranges: bytes59Please respect copyright.ＰＥＮＡＮＡ2hqrxnEysu
content-range: bytes 243220480-534231199/53423120059Please respect copyright.ＰＥＮＡＮＡ5beTio3wRn
Content-Length: 29101072059Please respect copyright.ＰＥＮＡＮＡ3P03wpGTVN
Content-Type: application/octet-stream59Please respect copyright.ＰＥＮＡＮＡ43seWNAxxm
Content-Disposition: attachment; filename="zen.iso"59Please respect copyright.ＰＥＮＡＮＡ7kyQ8miHhn
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)59Please respect copyright.ＰＥＮＡＮＡ2ESeJjgOvw
------------------------------------------------59Please respect copyright.ＰＥＮＡＮＡNcdZ6V35ry
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:59Please respect copyright.ＰＥＮＡＮＡsIIS7kNukJ
-------response content from resume point-------59Please respect copyright.ＰＥＮＡＮＡMkcCLdmTUc
HTTP/1.1 200 OK59Please respect copyright.ＰＥＮＡＮＡaGa8WPb7Xz
Date: Sat, 23 Nov 2024 06:29:19 GMT59Please respect copyright.ＰＥＮＡＮＡA6qGCk2J18
accept-ranges: bytes59Please respect copyright.ＰＥＮＡＮＡIKtRIKs5mX
content-range: bytes 0-634231201/63423120259Please respect copyright.ＰＥＮＡＮＡHf36mpZdTv
ETag: "634231202_2732277118"59Please respect copyright.ＰＥＮＡＮＡANSRVj0BKN
Content-Disposition: attachment; filename="zen.iso"59Please respect copyright.ＰＥＮＡＮＡemGGsiGMuX
Content-Length: 63423120259Please respect copyright.ＰＥＮＡＮＡBWf65Y3u4D
Content-Type: application/octet-stream
(http response body is the whole file content)59Please respect copyright.ＰＥＮＡＮＡzc5MGEE1HL
------------------------------------------------59Please respect copyright.ＰＥＮＡＮＡiyuUpP7yHl
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:59Please respect copyright.ＰＥＮＡＮＡaEV7NZeYRB
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59Please respect copyright.ＰＥＮＡＮＡCzaR0eOe04
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59Please respect copyright.ＰＥＮＡＮＡeOemooxr9n
https://gemini.google.com/
感谢互联网上无私奉献的参考资59Please respect copyright.ＰＥＮＡＮＡCUAA5NMM2Y
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests59Please respect copyright.ＰＥＮＡＮＡTdSzB7AVMQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range59Please respect copyright.ＰＥＮＡＮＡD5mPP4jSJo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag59Please respect copyright.ＰＥＮＡＮＡ9KVigia4Ee
https://www.rfc-editor.org/rfc/rfc723359Please respect copyright.ＰＥＮＡＮＡJNr7PtHrh9
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]59Please respect copyright.ＰＥＮＡＮＡWdrZDIgI5R
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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