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:174Please respect copyright.ＰＥＮＡＮＡFXDKOdf8O3
----------http range request-------174Please respect copyright.ＰＥＮＡＮＡi2ZFfoaZ3g
GET /zen.iso HTTP/1.1174Please respect copyright.ＰＥＮＡＮＡ9IU84PyNqI
Range: bytes=100-1023174Please respect copyright.ＰＥＮＡＮＡ95xDtn4PgF
If-Match: "534231200_1732277116"174Please respect copyright.ＰＥＮＡＮＡGu53xBVeVp
......174Please respect copyright.ＰＥＮＡＮＡoCHYIMfIKd
-----------------------------------174Please respect copyright.ＰＥＮＡＮＡvYOBu2oYz1
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:174Please respect copyright.ＰＥＮＡＮＡZvwgEDPh4P
----------------html-----------------174Please respect copyright.ＰＥＮＡＮＡmRRi5LCyza
<a href="/zen.iso">zen.iso</a>174Please respect copyright.ＰＥＮＡＮＡ2hruFa00gO
-------------------------------------174Please respect copyright.ＰＥＮＡＮＡJviJlmlJnw
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:174Please respect copyright.ＰＥＮＡＮＡwApQvVCbI8
----------get request------------174Please respect copyright.ＰＥＮＡＮＡeCt1zwb24D
GET /zen.iso HTTP/1.1174Please respect copyright.ＰＥＮＡＮＡGatwsWTFRj
......174Please respect copyright.ＰＥＮＡＮＡnwE43XL1zO
---------------------------------
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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:174Please respect copyright.ＰＥＮＡＮＡt52OKGYdXO
----------------server response--------------174Please respect copyright.ＰＥＮＡＮＡZtNelHONTY
HTTP/1.1 200 OK174Please respect copyright.ＰＥＮＡＮＡUzMgGXUkO1
Date: Sat, 23 Nov 2024 06:29:19 GMT174Please respect copyright.ＰＥＮＡＮＡoI1fRvmo6m
accept-ranges: bytes174Please respect copyright.ＰＥＮＡＮＡH6HTCIGvel
content-range: bytes 0-534231199/534231200174Please respect copyright.ＰＥＮＡＮＡB26HQLNCbF
ETag: "534231200_1732277116"174Please respect copyright.ＰＥＮＡＮＡgP9pFGEhAg
Content-Disposition: attachment; filename="zen.iso"174Please respect copyright.ＰＥＮＡＮＡXLvOROSVHH
Content-Length: 534231200174Please respect copyright.ＰＥＮＡＮＡKYg8txtSki
Content-Type: application/octet-stream
(http response body is the whole file content)174Please respect copyright.ＰＥＮＡＮＡcFPx41wdMf
-------------------------------------------
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:174Please respect copyright.ＰＥＮＡＮＡFD6tlIS9sC
---------------http server send big file content by multiple times--------------174Please respect copyright.ＰＥＮＡＮＡgAiRvLPWgm
const int buf_size=20000000; // buf size is about 20M174Please respect copyright.ＰＥＮＡＮＡEAVYdYvRWm
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers174Please respect copyright.ＰＥＮＡＮＡoPUFHdmgAJ
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 big174Please respect copyright.ＰＥＮＡＮＡBYjeHLFv3K
while(there_is_remain_file_content_to_send)174Please respect copyright.ＰＥＮＡＮＡ1JtBCEQK9H
{ 174Please respect copyright.ＰＥＮＡＮＡXIihX0niIS
read_remain_file_content_to_send_buf(file, send_buf);174Please respect copyright.ＰＥＮＡＮＡg8wUrAD4Wt
send_result = send(client_socket, send_buf);174Please respect copyright.ＰＥＮＡＮＡZVgqQNICXi
if(send_result == fail) return; else continue;174Please respect copyright.ＰＥＮＡＮＡleoLdAZyJY
}
// a way to make etag for a file using its file size and last modifed time174Please respect copyright.ＰＥＮＡＮＡoDOtjK4f5y
const char *file_etag(const char *path) 174Please respect copyright.ＰＥＮＡＮＡ8zX1WY835p
{174Please respect copyright.ＰＥＮＡＮＡCciYfDcSvR
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;174Please respect copyright.ＰＥＮＡＮＡunDT26vJMO
}174Please respect copyright.ＰＥＮＡＮＡ11Q2IThj6U
-------------------------------------------------174Please respect copyright.ＰＥＮＡＮＡxwuHy8fHYB
"accept-ranges: bytes" tells client web browser that the http server support range.174Please respect copyright.ＰＥＮＡＮＡb9cFYlnPY2
"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).174Please respect copyright.ＰＥＮＡＮＡfY9A1dAQF6
'ETag: "534231200_1732277116"' tells client web browser the file version.174Please respect copyright.ＰＥＮＡＮＡJlBgdzN6Ml
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.174Please respect copyright.ＰＥＮＡＮＡ2kfncX73YB
"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.174Please respect copyright.ＰＥＮＡＮＡVMEkLzyz1X
"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.174Please respect copyright.ＰＥＮＡＮＡdMiKQDLXdB
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.)174Please respect copyright.ＰＥＮＡＮＡkQwkYpI2wW
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:174Please respect copyright.ＰＥＮＡＮＡGZ4OOmGrhq
---------resume range request---------174Please respect copyright.ＰＥＮＡＮＡfJeAii3727
GET /zen.iso HTTP/1.1174Please respect copyright.ＰＥＮＡＮＡmHHPpbmlup
Range: bytes=243220480-174Please respect copyright.ＰＥＮＡＮＡRZXBScIPGz
If-Match: "534231200_1732277116"174Please respect copyright.ＰＥＮＡＮＡfJBVZJEEFn
......174Please respect copyright.ＰＥＮＡＮＡqWXzTG7shN
-------------------------------------174Please respect copyright.ＰＥＮＡＮＡkzDHtXt8qV
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.174Please respect copyright.ＰＥＮＡＮＡ5ffRAMf4O0
'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:174Please respect copyright.ＰＥＮＡＮＡTEw4S2j0ls
-------response content from resume point-------174Please respect copyright.ＰＥＮＡＮＡ4ECpG42FNe
HTTP/1.1 206 Partial Content174Please respect copyright.ＰＥＮＡＮＡwAFi8XElQu
Date: Sat, 23 Nov 2024 06:56:17 GMT174Please respect copyright.ＰＥＮＡＮＡ791fOAPLls
accept-ranges: bytes174Please respect copyright.ＰＥＮＡＮＡMFJcJLSFdi
content-range: bytes 243220480-534231199/534231200174Please respect copyright.ＰＥＮＡＮＡBgFKSty1Fx
Content-Length: 291010720174Please respect copyright.ＰＥＮＡＮＡbfQAt7SN0m
Content-Type: application/octet-stream174Please respect copyright.ＰＥＮＡＮＡbeoZtFg43L
Content-Disposition: attachment; filename="zen.iso"174Please respect copyright.ＰＥＮＡＮＡQ77DECnxKj
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)174Please respect copyright.ＰＥＮＡＮＡmvMnEcUNVR
------------------------------------------------174Please respect copyright.ＰＥＮＡＮＡaBFdr5FQbH
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:174Please respect copyright.ＰＥＮＡＮＡ6QGgaZ5yGM
-------response content from resume point-------174Please respect copyright.ＰＥＮＡＮＡiik2m2UYWb
HTTP/1.1 200 OK174Please respect copyright.ＰＥＮＡＮＡiIgt0vQ9IL
Date: Sat, 23 Nov 2024 06:29:19 GMT174Please respect copyright.ＰＥＮＡＮＡr5W9D6f0w7
accept-ranges: bytes174Please respect copyright.ＰＥＮＡＮＡVzwXQsEslZ
content-range: bytes 0-634231201/634231202174Please respect copyright.ＰＥＮＡＮＡ7mktGXMChL
ETag: "634231202_2732277118"174Please respect copyright.ＰＥＮＡＮＡiWVIkEQwnE
Content-Disposition: attachment; filename="zen.iso"174Please respect copyright.ＰＥＮＡＮＡzETzmKLNbK
Content-Length: 634231202174Please respect copyright.ＰＥＮＡＮＡTcXfV6iXvA
Content-Type: application/octet-stream
(http response body is the whole file content)174Please respect copyright.ＰＥＮＡＮＡQWktlUsNik
------------------------------------------------174Please respect copyright.ＰＥＮＡＮＡjMnK7SBBdZ
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:174Please respect copyright.ＰＥＮＡＮＡzg3eYBAor7
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
174Please respect copyright.ＰＥＮＡＮＡm2oCn0nk1o
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174Please respect copyright.ＰＥＮＡＮＡbkYuroEUjB
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174Please respect copyright.ＰＥＮＡＮＡKZ80V3DUnS
https://gemini.google.com/
感谢互联网上无私奉献的参考资174Please respect copyright.ＰＥＮＡＮＡDjsyNSJj2S
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests174Please respect copyright.ＰＥＮＡＮＡG6o1pVk7QY
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range174Please respect copyright.ＰＥＮＡＮＡ3GlX4HsWXs
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag174Please respect copyright.ＰＥＮＡＮＡKajmc5xiJ7
https://www.rfc-editor.org/rfc/rfc7233174Please respect copyright.ＰＥＮＡＮＡTzFZsQG4Ep
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]174Please respect copyright.ＰＥＮＡＮＡMPWqIaKjRs
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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