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:79Please respect copyright.ＰＥＮＡＮＡFMFyoYHhXe
----------http range request-------79Please respect copyright.ＰＥＮＡＮＡfzGMWTBFWN
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡ1IVQE6cn9D
Range: bytes=100-102379Please respect copyright.ＰＥＮＡＮＡokUWYJxlwG
If-Match: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡBMhUSOjFts
......79Please respect copyright.ＰＥＮＡＮＡUEgpnHMNOd
-----------------------------------79Please respect copyright.ＰＥＮＡＮＡxHJdriaUqF
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:79Please respect copyright.ＰＥＮＡＮＡxlcvuYfWNs
----------------html-----------------79Please respect copyright.ＰＥＮＡＮＡYXrpXXR9Yr
<a href="/zen.iso">zen.iso</a>79Please respect copyright.ＰＥＮＡＮＡzRs8XvGv8v
-------------------------------------79Please respect copyright.ＰＥＮＡＮＡF2gJZ8LC7J
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:79Please respect copyright.ＰＥＮＡＮＡsAxR7CEtmc
----------get request------------79Please respect copyright.ＰＥＮＡＮＡVcFQmVilmA
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡqWMZJwokyC
......79Please respect copyright.ＰＥＮＡＮＡmcUfqBtKtf
---------------------------------
79Please respect copyright.ＰＥＮＡＮＡE80slLnFH6
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:79Please respect copyright.ＰＥＮＡＮＡBRK2uH9mAT
----------------server response--------------79Please respect copyright.ＰＥＮＡＮＡkrrhRuA7LG
HTTP/1.1 200 OK79Please respect copyright.ＰＥＮＡＮＡ75KNSAvYns
Date: Sat, 23 Nov 2024 06:29:19 GMT79Please respect copyright.ＰＥＮＡＮＡxZgVgf5oba
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡVSI7oaMoCN
content-range: bytes 0-534231199/53423120079Please respect copyright.ＰＥＮＡＮＡUWT8rwLpq7
ETag: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡSmxFBWNSi1
Content-Disposition: attachment; filename="zen.iso"79Please respect copyright.ＰＥＮＡＮＡQc4p5XyvGO
Content-Length: 53423120079Please respect copyright.ＰＥＮＡＮＡx4iruxqoeP
Content-Type: application/octet-stream
(http response body is the whole file content)79Please respect copyright.ＰＥＮＡＮＡN9lvt1Hc3f
-------------------------------------------
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:79Please respect copyright.ＰＥＮＡＮＡHBWUVuL8jL
---------------http server send big file content by multiple times--------------79Please respect copyright.ＰＥＮＡＮＡtAaB2Lmjb0
const int buf_size=20000000; // buf size is about 20M79Please respect copyright.ＰＥＮＡＮＡXNJEkXZnT9
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers79Please respect copyright.ＰＥＮＡＮＡZSQja9YUao
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 big79Please respect copyright.ＰＥＮＡＮＡlSd0SoWPFP
while(there_is_remain_file_content_to_send)79Please respect copyright.ＰＥＮＡＮＡQCImoSgujn
{ 79Please respect copyright.ＰＥＮＡＮＡoieZjjfPWC
read_remain_file_content_to_send_buf(file, send_buf);79Please respect copyright.ＰＥＮＡＮＡ6bKGysLVMP
send_result = send(client_socket, send_buf);79Please respect copyright.ＰＥＮＡＮＡ1GiEnDHl1C
if(send_result == fail) return; else continue;79Please respect copyright.ＰＥＮＡＮＡg5gruDDa6T
}
// a way to make etag for a file using its file size and last modifed time79Please respect copyright.ＰＥＮＡＮＡzCpkbvg3N3
const char *file_etag(const char *path) 79Please respect copyright.ＰＥＮＡＮＡAFRAxV6fd6
{79Please respect copyright.ＰＥＮＡＮＡwgdj62CZCd
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;79Please respect copyright.ＰＥＮＡＮＡrg4s8VdX2z
}79Please respect copyright.ＰＥＮＡＮＡWuy0oDJ17v
-------------------------------------------------79Please respect copyright.ＰＥＮＡＮＡhbxAF7Y7T4
"accept-ranges: bytes" tells client web browser that the http server support range.79Please respect copyright.ＰＥＮＡＮＡuIbz2bVpq1
"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).79Please respect copyright.ＰＥＮＡＮＡtuU5cdRG1w
'ETag: "534231200_1732277116"' tells client web browser the file version.79Please respect copyright.ＰＥＮＡＮＡCCqtxsElJC
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.79Please respect copyright.ＰＥＮＡＮＡBKpFaKzf8D
"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.79Please respect copyright.ＰＥＮＡＮＡVyXKLTlFPF
"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.79Please respect copyright.ＰＥＮＡＮＡvllqpKgwf8
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.)79Please respect copyright.ＰＥＮＡＮＡ65I3JM6XW1
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:79Please respect copyright.ＰＥＮＡＮＡgFNz0m0wE9
---------resume range request---------79Please respect copyright.ＰＥＮＡＮＡSCkW0RFMal
GET /zen.iso HTTP/1.179Please respect copyright.ＰＥＮＡＮＡswgRrOoeUT
Range: bytes=243220480-79Please respect copyright.ＰＥＮＡＮＡ88QMGnq7kb
If-Match: "534231200_1732277116"79Please respect copyright.ＰＥＮＡＮＡ2u0ORhxws2
......79Please respect copyright.ＰＥＮＡＮＡym1eP662VY
-------------------------------------79Please respect copyright.ＰＥＮＡＮＡBRVzYRZMF6
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.79Please respect copyright.ＰＥＮＡＮＡg7Z5UCGW4V
'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:79Please respect copyright.ＰＥＮＡＮＡTr6LwNiVYI
-------response content from resume point-------79Please respect copyright.ＰＥＮＡＮＡm3WUljR6YA
HTTP/1.1 206 Partial Content79Please respect copyright.ＰＥＮＡＮＡZhQDodQSIT
Date: Sat, 23 Nov 2024 06:56:17 GMT79Please respect copyright.ＰＥＮＡＮＡDgByTL3AIS
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡdY40nIYWQU
content-range: bytes 243220480-534231199/53423120079Please respect copyright.ＰＥＮＡＮＡzj2nowIU3L
Content-Length: 29101072079Please respect copyright.ＰＥＮＡＮＡJ6m1kNZuKE
Content-Type: application/octet-stream79Please respect copyright.ＰＥＮＡＮＡ2kmjiWSkq8
Content-Disposition: attachment; filename="zen.iso"79Please respect copyright.ＰＥＮＡＮＡyNjlNQmYSG
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)79Please respect copyright.ＰＥＮＡＮＡ8UnDfjcmQe
------------------------------------------------79Please respect copyright.ＰＥＮＡＮＡfP2GM9fCXJ
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:79Please respect copyright.ＰＥＮＡＮＡ3Gyn2ffX2s
-------response content from resume point-------79Please respect copyright.ＰＥＮＡＮＡChGj5OXB0T
HTTP/1.1 200 OK79Please respect copyright.ＰＥＮＡＮＡWKx8hFAn13
Date: Sat, 23 Nov 2024 06:29:19 GMT79Please respect copyright.ＰＥＮＡＮＡA6cjfD3Q45
accept-ranges: bytes79Please respect copyright.ＰＥＮＡＮＡaSjp7oDKw4
content-range: bytes 0-634231201/63423120279Please respect copyright.ＰＥＮＡＮＡtC30JIap93
ETag: "634231202_2732277118"79Please respect copyright.ＰＥＮＡＮＡshrr8WaZ3q
Content-Disposition: attachment; filename="zen.iso"79Please respect copyright.ＰＥＮＡＮＡfDnpMl7dHS
Content-Length: 63423120279Please respect copyright.ＰＥＮＡＮＡWSdEQsOef8
Content-Type: application/octet-stream
(http response body is the whole file content)79Please respect copyright.ＰＥＮＡＮＡtOx5cqMfgQ
------------------------------------------------79Please respect copyright.ＰＥＮＡＮＡm9NCO8FLve
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
79Please respect copyright.ＰＥＮＡＮＡhS1HsslAMI
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:79Please respect copyright.ＰＥＮＡＮＡNuyJswFzTs
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
79Please respect copyright.ＰＥＮＡＮＡCYKFggTYMB
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79Please respect copyright.ＰＥＮＡＮＡTsuYEnhnvT
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79Please respect copyright.ＰＥＮＡＮＡxvtdZaucNL
https://gemini.google.com/
感谢互联网上无私奉献的参考资79Please respect copyright.ＰＥＮＡＮＡ1dTTOUpfMq
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests79Please respect copyright.ＰＥＮＡＮＡB8dRoYdmWb
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range79Please respect copyright.ＰＥＮＡＮＡqAqubb6FB7
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag79Please respect copyright.ＰＥＮＡＮＡ8tid8bQaHJ
https://www.rfc-editor.org/rfc/rfc723379Please respect copyright.ＰＥＮＡＮＡW7PqntRTsP
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]79Please respect copyright.ＰＥＮＡＮＡD1CmS80yVV
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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