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:198Please respect copyright.ＰＥＮＡＮＡhycKfNFqBZ
----------http range request-------198Please respect copyright.ＰＥＮＡＮＡQ0Qu9HLmar
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡTVsKrsK1g3
Range: bytes=100-1023198Please respect copyright.ＰＥＮＡＮＡXNhtYlejKw
If-Match: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡ4iyT7KDNNB
......198Please respect copyright.ＰＥＮＡＮＡ1K7NQExwtD
-----------------------------------198Please respect copyright.ＰＥＮＡＮＡCdLoZJyfmU
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:198Please respect copyright.ＰＥＮＡＮＡaFJhw4SZWz
----------------html-----------------198Please respect copyright.ＰＥＮＡＮＡIELDgTGCvt
<a href="/zen.iso">zen.iso</a>198Please respect copyright.ＰＥＮＡＮＡLDvVaGVYNF
-------------------------------------198Please respect copyright.ＰＥＮＡＮＡbDlzPoPiXi
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:198Please respect copyright.ＰＥＮＡＮＡtyKsEg7Y0l
----------get request------------198Please respect copyright.ＰＥＮＡＮＡFU6gaxDJbz
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡ1LAWYnTnWo
......198Please respect copyright.ＰＥＮＡＮＡ83wTPFINEs
---------------------------------
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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:198Please respect copyright.ＰＥＮＡＮＡFE7ofGKKvg
----------------server response--------------198Please respect copyright.ＰＥＮＡＮＡoSm5b52AVL
HTTP/1.1 200 OK198Please respect copyright.ＰＥＮＡＮＡjTnLiazW3X
Date: Sat, 23 Nov 2024 06:29:19 GMT198Please respect copyright.ＰＥＮＡＮＡaV0ejBgIE7
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡreWuiFALyv
content-range: bytes 0-534231199/534231200198Please respect copyright.ＰＥＮＡＮＡyg2qOEgfp1
ETag: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡQzDLPRnbdf
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡNrlyxW8acg
Content-Length: 534231200198Please respect copyright.ＰＥＮＡＮＡBMnq1HzBak
Content-Type: application/octet-stream
(http response body is the whole file content)198Please respect copyright.ＰＥＮＡＮＡo0cfoROHis
-------------------------------------------
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:198Please respect copyright.ＰＥＮＡＮＡJWLTJUOnQa
---------------http server send big file content by multiple times--------------198Please respect copyright.ＰＥＮＡＮＡVGNIenC1yu
const int buf_size=20000000; // buf size is about 20M198Please respect copyright.ＰＥＮＡＮＡ5AMoCFUuc6
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers198Please respect copyright.ＰＥＮＡＮＡVWJqi00iXh
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 big198Please respect copyright.ＰＥＮＡＮＡUNtEsdKQWn
while(there_is_remain_file_content_to_send)198Please respect copyright.ＰＥＮＡＮＡfxff5iOk95
{ 198Please respect copyright.ＰＥＮＡＮＡhWBLNNFHoZ
read_remain_file_content_to_send_buf(file, send_buf);198Please respect copyright.ＰＥＮＡＮＡhGjXsjvkry
send_result = send(client_socket, send_buf);198Please respect copyright.ＰＥＮＡＮＡuH7nbtuZ1C
if(send_result == fail) return; else continue;198Please respect copyright.ＰＥＮＡＮＡMHMkcUTMSh
}
// a way to make etag for a file using its file size and last modifed time198Please respect copyright.ＰＥＮＡＮＡoKmrm1ufqw
const char *file_etag(const char *path) 198Please respect copyright.ＰＥＮＡＮＡHjK59ON1lL
{198Please respect copyright.ＰＥＮＡＮＡlLKzNQHeWz
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;198Please respect copyright.ＰＥＮＡＮＡ2lWh3mzdSa
}198Please respect copyright.ＰＥＮＡＮＡvPKRE2SzIT
-------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡn0bfyihUux
"accept-ranges: bytes" tells client web browser that the http server support range.198Please respect copyright.ＰＥＮＡＮＡeDXahiJnZL
"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).198Please respect copyright.ＰＥＮＡＮＡOvNV0MzR1V
'ETag: "534231200_1732277116"' tells client web browser the file version.198Please respect copyright.ＰＥＮＡＮＡ7izWW070oG
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.198Please respect copyright.ＰＥＮＡＮＡvi0TdaiUvC
"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.198Please respect copyright.ＰＥＮＡＮＡGYXeEMgcWK
"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.198Please respect copyright.ＰＥＮＡＮＡdfNyIk3hFt
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.)198Please respect copyright.ＰＥＮＡＮＡ01aFgKLJ6A
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:198Please respect copyright.ＰＥＮＡＮＡzdiT4ZNBj2
---------resume range request---------198Please respect copyright.ＰＥＮＡＮＡ1afS9aqYjO
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡuA4qxmbD8b
Range: bytes=243220480-198Please respect copyright.ＰＥＮＡＮＡijih1uMNAP
If-Match: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡw6xZBEtOT0
......198Please respect copyright.ＰＥＮＡＮＡSTtAVcERu7
-------------------------------------198Please respect copyright.ＰＥＮＡＮＡ37QtuZY50Z
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.198Please respect copyright.ＰＥＮＡＮＡUxbmz0U6qX
'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:198Please respect copyright.ＰＥＮＡＮＡBSK2FJ5egW
-------response content from resume point-------198Please respect copyright.ＰＥＮＡＮＡhEgp6IEEwG
HTTP/1.1 206 Partial Content198Please respect copyright.ＰＥＮＡＮＡpur7pGcqKX
Date: Sat, 23 Nov 2024 06:56:17 GMT198Please respect copyright.ＰＥＮＡＮＡmad7P4zCac
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡvv3Shs7TP8
content-range: bytes 243220480-534231199/534231200198Please respect copyright.ＰＥＮＡＮＡJuZ19aWZlS
Content-Length: 291010720198Please respect copyright.ＰＥＮＡＮＡ1O9fsnvTHv
Content-Type: application/octet-stream198Please respect copyright.ＰＥＮＡＮＡsX0ZxC0yop
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡKNJlYVeTee
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)198Please respect copyright.ＰＥＮＡＮＡ7Yu6wJFeQe
------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡ5ZxTcJI16X
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:198Please respect copyright.ＰＥＮＡＮＡEgQaIhlloh
-------response content from resume point-------198Please respect copyright.ＰＥＮＡＮＡ85VajyzzXc
HTTP/1.1 200 OK198Please respect copyright.ＰＥＮＡＮＡhzAqCMzcx0
Date: Sat, 23 Nov 2024 06:29:19 GMT198Please respect copyright.ＰＥＮＡＮＡ0DHmmK4IGB
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡa5Av9HntY4
content-range: bytes 0-634231201/634231202198Please respect copyright.ＰＥＮＡＮＡwWtjfVo5pt
ETag: "634231202_2732277118"198Please respect copyright.ＰＥＮＡＮＡiP9925e3ky
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡPDVaAfBxaN
Content-Length: 634231202198Please respect copyright.ＰＥＮＡＮＡjBKWUgWcB8
Content-Type: application/octet-stream
(http response body is the whole file content)198Please respect copyright.ＰＥＮＡＮＡOyuMOrUKvw
------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡ2LACr6FGFI
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:198Please respect copyright.ＰＥＮＡＮＡaxjVZQbuVc
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198Please respect copyright.ＰＥＮＡＮＡCRHiYzBguY
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198Please respect copyright.ＰＥＮＡＮＡ32oV2CAtvH
https://gemini.google.com/
感谢互联网上无私奉献的参考资198Please respect copyright.ＰＥＮＡＮＡ6Z64AXKFc9
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests198Please respect copyright.ＰＥＮＡＮＡqesuuloRGa
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range198Please respect copyright.ＰＥＮＡＮＡaDSar7pBlk
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag198Please respect copyright.ＰＥＮＡＮＡ9AOPvD1cpx
https://www.rfc-editor.org/rfc/rfc7233198Please respect copyright.ＰＥＮＡＮＡMPA3LvDe1v
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]198Please respect copyright.ＰＥＮＡＮＡhDdaHkRf0z
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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