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:144Please respect copyright.ＰＥＮＡＮＡhAtQN38h6R
----------http range request-------144Please respect copyright.ＰＥＮＡＮＡ2yO32T1DAw
GET /zen.iso HTTP/1.1144Please respect copyright.ＰＥＮＡＮＡmRUWOeoBGf
Range: bytes=100-1023144Please respect copyright.ＰＥＮＡＮＡiO4NhDz3if
If-Match: "534231200_1732277116"144Please respect copyright.ＰＥＮＡＮＡKpoIxhH7Q9
......144Please respect copyright.ＰＥＮＡＮＡdQu3jEBapO
-----------------------------------144Please respect copyright.ＰＥＮＡＮＡaYa7oAFBzO
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:144Please respect copyright.ＰＥＮＡＮＡmd5CuHSyvt
----------------html-----------------144Please respect copyright.ＰＥＮＡＮＡXQnhMI27jN
<a href="/zen.iso">zen.iso</a>144Please respect copyright.ＰＥＮＡＮＡf2m99hPGyh
-------------------------------------144Please respect copyright.ＰＥＮＡＮＡVD5jXjkaww
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:144Please respect copyright.ＰＥＮＡＮＡRPtrOWZbLm
----------get request------------144Please respect copyright.ＰＥＮＡＮＡnGgu7MLhNM
GET /zen.iso HTTP/1.1144Please respect copyright.ＰＥＮＡＮＡYE303VlIPB
......144Please respect copyright.ＰＥＮＡＮＡRog2dQWmDd
---------------------------------
144Please respect copyright.ＰＥＮＡＮＡcaZklvr2WL
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:144Please respect copyright.ＰＥＮＡＮＡUfemb1RkZE
----------------server response--------------144Please respect copyright.ＰＥＮＡＮＡFowaIzs43V
HTTP/1.1 200 OK144Please respect copyright.ＰＥＮＡＮＡvelKl1ZDSB
Date: Sat, 23 Nov 2024 06:29:19 GMT144Please respect copyright.ＰＥＮＡＮＡCfKgVriuze
accept-ranges: bytes144Please respect copyright.ＰＥＮＡＮＡxvNFgET8S1
content-range: bytes 0-534231199/534231200144Please respect copyright.ＰＥＮＡＮＡVdrGcq9XdJ
ETag: "534231200_1732277116"144Please respect copyright.ＰＥＮＡＮＡjslMd2h09L
Content-Disposition: attachment; filename="zen.iso"144Please respect copyright.ＰＥＮＡＮＡGZS0qyAnSH
Content-Length: 534231200144Please respect copyright.ＰＥＮＡＮＡZvpB1rPMUk
Content-Type: application/octet-stream
(http response body is the whole file content)144Please respect copyright.ＰＥＮＡＮＡasaB42fxSa
-------------------------------------------
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:144Please respect copyright.ＰＥＮＡＮＡTaX59EcoLW
---------------http server send big file content by multiple times--------------144Please respect copyright.ＰＥＮＡＮＡ0VAofuPQR1
const int buf_size=20000000; // buf size is about 20M144Please respect copyright.ＰＥＮＡＮＡ5b28GpPWGW
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers144Please respect copyright.ＰＥＮＡＮＡ4nO5NMtUcr
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 big144Please respect copyright.ＰＥＮＡＮＡHceHhoqwig
while(there_is_remain_file_content_to_send)144Please respect copyright.ＰＥＮＡＮＡTZNqeKR0yS
{ 144Please respect copyright.ＰＥＮＡＮＡZTY1HnMp6o
read_remain_file_content_to_send_buf(file, send_buf);144Please respect copyright.ＰＥＮＡＮＡBinxV18FBk
send_result = send(client_socket, send_buf);144Please respect copyright.ＰＥＮＡＮＡDjMb2J3PDb
if(send_result == fail) return; else continue;144Please respect copyright.ＰＥＮＡＮＡZcVr2eRECB
}
// a way to make etag for a file using its file size and last modifed time144Please respect copyright.ＰＥＮＡＮＡwL29m4ikyO
const char *file_etag(const char *path) 144Please respect copyright.ＰＥＮＡＮＡ7olVCX36Bc
{144Please respect copyright.ＰＥＮＡＮＡIFPPufQSEG
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;144Please respect copyright.ＰＥＮＡＮＡ7GASgQ1exN
}144Please respect copyright.ＰＥＮＡＮＡrJKwgV9U3N
-------------------------------------------------144Please respect copyright.ＰＥＮＡＮＡGQor4AbE7U
"accept-ranges: bytes" tells client web browser that the http server support range.144Please respect copyright.ＰＥＮＡＮＡ6PBxi7pjBP
"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).144Please respect copyright.ＰＥＮＡＮＡwWaPjW20ib
'ETag: "534231200_1732277116"' tells client web browser the file version.144Please respect copyright.ＰＥＮＡＮＡrZu2DaB8kr
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.144Please respect copyright.ＰＥＮＡＮＡk3UoaWvy2Y
"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.144Please respect copyright.ＰＥＮＡＮＡwbLA64CC9w
"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.144Please respect copyright.ＰＥＮＡＮＡnDgLYi4MXp
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.)144Please respect copyright.ＰＥＮＡＮＡMO9FUKxNzS
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:144Please respect copyright.ＰＥＮＡＮＡp9O30gbvAt
---------resume range request---------144Please respect copyright.ＰＥＮＡＮＡkhw63OfmYj
GET /zen.iso HTTP/1.1144Please respect copyright.ＰＥＮＡＮＡc7mVKz8ODv
Range: bytes=243220480-144Please respect copyright.ＰＥＮＡＮＡ1nx1eMyAtm
If-Match: "534231200_1732277116"144Please respect copyright.ＰＥＮＡＮＡlwZ68PkNSJ
......144Please respect copyright.ＰＥＮＡＮＡKZIJEry7zT
-------------------------------------144Please respect copyright.ＰＥＮＡＮＡp0N3RckbWl
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.144Please respect copyright.ＰＥＮＡＮＡiPThpILmok
'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:144Please respect copyright.ＰＥＮＡＮＡft64YI5Nea
-------response content from resume point-------144Please respect copyright.ＰＥＮＡＮＡCMYZLXpGe4
HTTP/1.1 206 Partial Content144Please respect copyright.ＰＥＮＡＮＡm1IqZ4GWzC
Date: Sat, 23 Nov 2024 06:56:17 GMT144Please respect copyright.ＰＥＮＡＮＡEK5feMWBUY
accept-ranges: bytes144Please respect copyright.ＰＥＮＡＮＡr94OJk9HKo
content-range: bytes 243220480-534231199/534231200144Please respect copyright.ＰＥＮＡＮＡFUsZlYteXh
Content-Length: 291010720144Please respect copyright.ＰＥＮＡＮＡtylZSixaqj
Content-Type: application/octet-stream144Please respect copyright.ＰＥＮＡＮＡikEOjY14fD
Content-Disposition: attachment; filename="zen.iso"144Please respect copyright.ＰＥＮＡＮＡLkrbeYf2fl
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)144Please respect copyright.ＰＥＮＡＮＡU7nqPIS0VL
------------------------------------------------144Please respect copyright.ＰＥＮＡＮＡqPGbankkFE
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:144Please respect copyright.ＰＥＮＡＮＡ7d1TrAD8p2
-------response content from resume point-------144Please respect copyright.ＰＥＮＡＮＡ0btQmEKu1x
HTTP/1.1 200 OK144Please respect copyright.ＰＥＮＡＮＡFhX6NjdhYw
Date: Sat, 23 Nov 2024 06:29:19 GMT144Please respect copyright.ＰＥＮＡＮＡb26ajbnk9W
accept-ranges: bytes144Please respect copyright.ＰＥＮＡＮＡbAH9cZ6iCK
content-range: bytes 0-634231201/634231202144Please respect copyright.ＰＥＮＡＮＡlt8hAGp29z
ETag: "634231202_2732277118"144Please respect copyright.ＰＥＮＡＮＡWE9otk88uI
Content-Disposition: attachment; filename="zen.iso"144Please respect copyright.ＰＥＮＡＮＡS673d5dKs9
Content-Length: 634231202144Please respect copyright.ＰＥＮＡＮＡqIXhi1OIzy
Content-Type: application/octet-stream
(http response body is the whole file content)144Please respect copyright.ＰＥＮＡＮＡ6qbKWrvZRR
------------------------------------------------144Please respect copyright.ＰＥＮＡＮＡptlXsg163o
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:144Please respect copyright.ＰＥＮＡＮＡyuS1tcX9f5
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
144Please respect copyright.ＰＥＮＡＮＡOcGq66kbIQ
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144Please respect copyright.ＰＥＮＡＮＡkumbTvDOeg
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144Please respect copyright.ＰＥＮＡＮＡtObqz5cTDp
https://gemini.google.com/
感谢互联网上无私奉献的参考资144Please respect copyright.ＰＥＮＡＮＡMUeoeHz6aV
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests144Please respect copyright.ＰＥＮＡＮＡFX77INPnXK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range144Please respect copyright.ＰＥＮＡＮＡD2QJioLcxq
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag144Please respect copyright.ＰＥＮＡＮＡm7wurS3lnx
https://www.rfc-editor.org/rfc/rfc7233144Please respect copyright.ＰＥＮＡＮＡZNuMNZ2kFw
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]144Please respect copyright.ＰＥＮＡＮＡKVqFGjIBmN
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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