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:29Please respect copyright.ＰＥＮＡＮＡaN89sP9NqQ
----------http range request-------29Please respect copyright.ＰＥＮＡＮＡvLcAtKjSkB
GET /zen.iso HTTP/1.129Please respect copyright.ＰＥＮＡＮＡWnNmgA8QDi
Range: bytes=100-102329Please respect copyright.ＰＥＮＡＮＡMOcOZu8qTT
If-Match: "534231200_1732277116"29Please respect copyright.ＰＥＮＡＮＡvYrwpI1fQn
......29Please respect copyright.ＰＥＮＡＮＡhfBM5a9q8X
-----------------------------------29Please respect copyright.ＰＥＮＡＮＡW3QWz2g9ax
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:29Please respect copyright.ＰＥＮＡＮＡe0vM2t2NyI
----------------html-----------------29Please respect copyright.ＰＥＮＡＮＡQBuFRkeVrc
<a href="/zen.iso">zen.iso</a>29Please respect copyright.ＰＥＮＡＮＡmvjca52fwS
-------------------------------------29Please respect copyright.ＰＥＮＡＮＡxV5OsMjKW7
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:29Please respect copyright.ＰＥＮＡＮＡbdjcJiQdBN
----------get request------------29Please respect copyright.ＰＥＮＡＮＡFUOgLngtAS
GET /zen.iso HTTP/1.129Please respect copyright.ＰＥＮＡＮＡ2EVzUVa4D8
......29Please respect copyright.ＰＥＮＡＮＡFnqfkUMUrC
---------------------------------
29Please respect copyright.ＰＥＮＡＮＡ6nrKlZmejE
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:29Please respect copyright.ＰＥＮＡＮＡwyLfmhfYtC
----------------server response--------------29Please respect copyright.ＰＥＮＡＮＡx8fUZeNKqh
HTTP/1.1 200 OK29Please respect copyright.ＰＥＮＡＮＡf5GhwU5y0c
Date: Sat, 23 Nov 2024 06:29:19 GMT29Please respect copyright.ＰＥＮＡＮＡsFMxsnIkKQ
accept-ranges: bytes29Please respect copyright.ＰＥＮＡＮＡt4XGbwsHKk
content-range: bytes 0-534231199/53423120029Please respect copyright.ＰＥＮＡＮＡUSP1KL6ejp
ETag: "534231200_1732277116"29Please respect copyright.ＰＥＮＡＮＡAR0JKd3EWA
Content-Disposition: attachment; filename="zen.iso"29Please respect copyright.ＰＥＮＡＮＡbbRENAZ0EI
Content-Length: 53423120029Please respect copyright.ＰＥＮＡＮＡQtK9AM0dL1
Content-Type: application/octet-stream
(http response body is the whole file content)29Please respect copyright.ＰＥＮＡＮＡdCnBkeEITr
-------------------------------------------
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:29Please respect copyright.ＰＥＮＡＮＡ5i2lg0dERn
---------------http server send big file content by multiple times--------------29Please respect copyright.ＰＥＮＡＮＡbSK1lJygsZ
const int buf_size=20000000; // buf size is about 20M29Please respect copyright.ＰＥＮＡＮＡ9d526Fs72N
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers29Please respect copyright.ＰＥＮＡＮＡZz1UWIR29H
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 big29Please respect copyright.ＰＥＮＡＮＡ3ngKAJzl5G
while(there_is_remain_file_content_to_send)29Please respect copyright.ＰＥＮＡＮＡEutlpfAkIa
{ 29Please respect copyright.ＰＥＮＡＮＡhKuscR4jQu
read_remain_file_content_to_send_buf(file, send_buf);29Please respect copyright.ＰＥＮＡＮＡmDunGLkqyF
send_result = send(client_socket, send_buf);29Please respect copyright.ＰＥＮＡＮＡ6BxPxTnL1m
if(send_result == fail) return; else continue;29Please respect copyright.ＰＥＮＡＮＡNHHBE1nGzu
}
// a way to make etag for a file using its file size and last modifed time29Please respect copyright.ＰＥＮＡＮＡYmqJYtMrAJ
const char *file_etag(const char *path) 29Please respect copyright.ＰＥＮＡＮＡzDuwxP7H8G
{29Please respect copyright.ＰＥＮＡＮＡ0cxboqfZLc
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;29Please respect copyright.ＰＥＮＡＮＡqMQ1QPZktx
}29Please respect copyright.ＰＥＮＡＮＡcG1atrGV1m
-------------------------------------------------29Please respect copyright.ＰＥＮＡＮＡreprsruZa9
"accept-ranges: bytes" tells client web browser that the http server support range.29Please respect copyright.ＰＥＮＡＮＡePXl4yw67y
"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).29Please respect copyright.ＰＥＮＡＮＡesZQ7pA6c9
'ETag: "534231200_1732277116"' tells client web browser the file version.29Please respect copyright.ＰＥＮＡＮＡQwVYHVLnZF
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.29Please respect copyright.ＰＥＮＡＮＡaD0sQwgHyP
"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.29Please respect copyright.ＰＥＮＡＮＡswGv6Poujl
"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.29Please respect copyright.ＰＥＮＡＮＡZhSzCg6qy6
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.)29Please respect copyright.ＰＥＮＡＮＡfttLPkxXKY
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:29Please respect copyright.ＰＥＮＡＮＡbkB7gEw9v1
---------resume range request---------29Please respect copyright.ＰＥＮＡＮＡi8QJbIyu8o
GET /zen.iso HTTP/1.129Please respect copyright.ＰＥＮＡＮＡVJKf8kGLoJ
Range: bytes=243220480-29Please respect copyright.ＰＥＮＡＮＡRnzo6B9bpU
If-Match: "534231200_1732277116"29Please respect copyright.ＰＥＮＡＮＡa6QwyFEzt9
......29Please respect copyright.ＰＥＮＡＮＡrxntzzVa1O
-------------------------------------29Please respect copyright.ＰＥＮＡＮＡmHdE3KQIAf
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.29Please respect copyright.ＰＥＮＡＮＡBmvEEVGv2v
'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:29Please respect copyright.ＰＥＮＡＮＡgzn9ZW6Mg8
-------response content from resume point-------29Please respect copyright.ＰＥＮＡＮＡtBIiQXyE4X
HTTP/1.1 206 Partial Content29Please respect copyright.ＰＥＮＡＮＡ0dl3xXeodq
Date: Sat, 23 Nov 2024 06:56:17 GMT29Please respect copyright.ＰＥＮＡＮＡR84gflAlzU
accept-ranges: bytes29Please respect copyright.ＰＥＮＡＮＡijwPqDMCBw
content-range: bytes 243220480-534231199/53423120029Please respect copyright.ＰＥＮＡＮＡtuqKSyGJRb
Content-Length: 29101072029Please respect copyright.ＰＥＮＡＮＡbM2a9l5JjU
Content-Type: application/octet-stream29Please respect copyright.ＰＥＮＡＮＡ2nh0EXe6f0
Content-Disposition: attachment; filename="zen.iso"29Please respect copyright.ＰＥＮＡＮＡC0yqHygVkA
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)29Please respect copyright.ＰＥＮＡＮＡFzMt2mCsTg
------------------------------------------------29Please respect copyright.ＰＥＮＡＮＡC5eoJYzuyu
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:29Please respect copyright.ＰＥＮＡＮＡtDy6GOnA4g
-------response content from resume point-------29Please respect copyright.ＰＥＮＡＮＡjZvYNb6c7W
HTTP/1.1 200 OK29Please respect copyright.ＰＥＮＡＮＡcA4cHFshsf
Date: Sat, 23 Nov 2024 06:29:19 GMT29Please respect copyright.ＰＥＮＡＮＡ1mqgdbIwFw
accept-ranges: bytes29Please respect copyright.ＰＥＮＡＮＡVyx7k0O7He
content-range: bytes 0-634231201/63423120229Please respect copyright.ＰＥＮＡＮＡzY3L6Cs8WG
ETag: "634231202_2732277118"29Please respect copyright.ＰＥＮＡＮＡSV7q2XLpCK
Content-Disposition: attachment; filename="zen.iso"29Please respect copyright.ＰＥＮＡＮＡ1iH1tdkz6v
Content-Length: 63423120229Please respect copyright.ＰＥＮＡＮＡGJHW3mkKdw
Content-Type: application/octet-stream
(http response body is the whole file content)29Please respect copyright.ＰＥＮＡＮＡxYIgbctK5R
------------------------------------------------29Please respect copyright.ＰＥＮＡＮＡCYpPMux6hF
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:29Please respect copyright.ＰＥＮＡＮＡYiqjdI4ajH
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
29Please respect copyright.ＰＥＮＡＮＡU41rNzyvKs
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29Please respect copyright.ＰＥＮＡＮＡWcmr0Bx0pT
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29Please respect copyright.ＰＥＮＡＮＡoo2darrSLp
https://gemini.google.com/
感谢互联网上无私奉献的参考资29Please respect copyright.ＰＥＮＡＮＡZlIFxJJSaK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests29Please respect copyright.ＰＥＮＡＮＡEzuFDttA3N
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range29Please respect copyright.ＰＥＮＡＮＡRiYj2VCJhN
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag29Please respect copyright.ＰＥＮＡＮＡAHzBOGPc9y
https://www.rfc-editor.org/rfc/rfc723329Please respect copyright.ＰＥＮＡＮＡ6OjyfSdKnr
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/
29Please respect copyright.ＰＥＮＡＮＡEM3tvgWuro
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.com29Please respect copyright.ＰＥＮＡＮＡZOGhqj8P17
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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