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:153Please respect copyright.ＰＥＮＡＮＡWs89Iblqx4
----------http range request-------153Please respect copyright.ＰＥＮＡＮＡk1vlJUk7TR
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡXyKL7CV9Kx
Range: bytes=100-1023153Please respect copyright.ＰＥＮＡＮＡTTZkumYwYC
If-Match: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡEdOcMlwkMJ
......153Please respect copyright.ＰＥＮＡＮＡcu9QOSXkNj
-----------------------------------153Please respect copyright.ＰＥＮＡＮＡ1vJMnXuEaR
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:153Please respect copyright.ＰＥＮＡＮＡ13jDSPga4X
----------------html-----------------153Please respect copyright.ＰＥＮＡＮＡLeZfIEL41T
<a href="/zen.iso">zen.iso</a>153Please respect copyright.ＰＥＮＡＮＡK4YxuJfrXJ
-------------------------------------153Please respect copyright.ＰＥＮＡＮＡgqOqkNOJy4
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:153Please respect copyright.ＰＥＮＡＮＡGsdRNPFsFU
----------get request------------153Please respect copyright.ＰＥＮＡＮＡrZOZGEGxxv
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡNxGbUNOoom
......153Please respect copyright.ＰＥＮＡＮＡMDtMTQYqqC
---------------------------------
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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:153Please respect copyright.ＰＥＮＡＮＡy6gBADAMRf
----------------server response--------------153Please respect copyright.ＰＥＮＡＮＡjmTmY46RTa
HTTP/1.1 200 OK153Please respect copyright.ＰＥＮＡＮＡZQNtDP9qa8
Date: Sat, 23 Nov 2024 06:29:19 GMT153Please respect copyright.ＰＥＮＡＮＡppZaqqvldz
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡOgTzIBfENF
content-range: bytes 0-534231199/534231200153Please respect copyright.ＰＥＮＡＮＡbnSnCc42Jo
ETag: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡTTShxcjVbf
Content-Disposition: attachment; filename="zen.iso"153Please respect copyright.ＰＥＮＡＮＡPURWDjHUdi
Content-Length: 534231200153Please respect copyright.ＰＥＮＡＮＡApespkCR9r
Content-Type: application/octet-stream
(http response body is the whole file content)153Please respect copyright.ＰＥＮＡＮＡtNlXHPVT9C
-------------------------------------------
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:153Please respect copyright.ＰＥＮＡＮＡGiLQW6bojk
---------------http server send big file content by multiple times--------------153Please respect copyright.ＰＥＮＡＮＡBBa0H2Sddf
const int buf_size=20000000; // buf size is about 20M153Please respect copyright.ＰＥＮＡＮＡ5DnKWihqQi
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers153Please respect copyright.ＰＥＮＡＮＡsSCNRHzNC2
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 big153Please respect copyright.ＰＥＮＡＮＡ6lVWrmHw1V
while(there_is_remain_file_content_to_send)153Please respect copyright.ＰＥＮＡＮＡVzUAiNV6Ie
{ 153Please respect copyright.ＰＥＮＡＮＡ4cQzuWvyKY
read_remain_file_content_to_send_buf(file, send_buf);153Please respect copyright.ＰＥＮＡＮＡIibUJBdhmv
send_result = send(client_socket, send_buf);153Please respect copyright.ＰＥＮＡＮＡKlfvLv0GtG
if(send_result == fail) return; else continue;153Please respect copyright.ＰＥＮＡＮＡocs7DlWbjD
}
// a way to make etag for a file using its file size and last modifed time153Please respect copyright.ＰＥＮＡＮＡqFLIuSA4I0
const char *file_etag(const char *path) 153Please respect copyright.ＰＥＮＡＮＡTQw0Ybq6Ec
{153Please respect copyright.ＰＥＮＡＮＡ7bytEY7aZN
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;153Please respect copyright.ＰＥＮＡＮＡCLPR53MHxe
}153Please respect copyright.ＰＥＮＡＮＡV6on6pd9Os
-------------------------------------------------153Please respect copyright.ＰＥＮＡＮＡM10pgg64on
"accept-ranges: bytes" tells client web browser that the http server support range.153Please respect copyright.ＰＥＮＡＮＡTdJLRWHUYI
"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).153Please respect copyright.ＰＥＮＡＮＡzFVxsx3mge
'ETag: "534231200_1732277116"' tells client web browser the file version.153Please respect copyright.ＰＥＮＡＮＡARPTeGr0BD
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.153Please respect copyright.ＰＥＮＡＮＡkuhB1EdcvJ
"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.153Please respect copyright.ＰＥＮＡＮＡTMKN5OaLiL
"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.153Please respect copyright.ＰＥＮＡＮＡQ26iEAeFXS
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.)153Please respect copyright.ＰＥＮＡＮＡxcIxYDiKgF
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:153Please respect copyright.ＰＥＮＡＮＡ5GPNa0nKcf
---------resume range request---------153Please respect copyright.ＰＥＮＡＮＡ7SrWGwVnY1
GET /zen.iso HTTP/1.1153Please respect copyright.ＰＥＮＡＮＡVWWdRNr3Yx
Range: bytes=243220480-153Please respect copyright.ＰＥＮＡＮＡpCLMgH1oFq
If-Match: "534231200_1732277116"153Please respect copyright.ＰＥＮＡＮＡ9qkfBtCbHL
......153Please respect copyright.ＰＥＮＡＮＡ86sHo3vcrB
-------------------------------------153Please respect copyright.ＰＥＮＡＮＡICQuGH3NE4
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.153Please respect copyright.ＰＥＮＡＮＡCkPcVEB6ut
'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:153Please respect copyright.ＰＥＮＡＮＡ0lwKqAwot3
-------response content from resume point-------153Please respect copyright.ＰＥＮＡＮＡ3CMtNn8bQG
HTTP/1.1 206 Partial Content153Please respect copyright.ＰＥＮＡＮＡWs9fGzteqD
Date: Sat, 23 Nov 2024 06:56:17 GMT153Please respect copyright.ＰＥＮＡＮＡ3X6qUq36ZX
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡ3Gy9Qcmkej
content-range: bytes 243220480-534231199/534231200153Please respect copyright.ＰＥＮＡＮＡdOBZhmCvJe
Content-Length: 291010720153Please respect copyright.ＰＥＮＡＮＡnPasx41Qcg
Content-Type: application/octet-stream153Please respect copyright.ＰＥＮＡＮＡPzwMoj5Evw
Content-Disposition: attachment; filename="zen.iso"153Please respect copyright.ＰＥＮＡＮＡs0FMyppjCw
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)153Please respect copyright.ＰＥＮＡＮＡZuZ1HS8Vj4
------------------------------------------------153Please respect copyright.ＰＥＮＡＮＡAmlmeYZJGx
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:153Please respect copyright.ＰＥＮＡＮＡOpMKfghG1P
-------response content from resume point-------153Please respect copyright.ＰＥＮＡＮＡNNqDTZsSnH
HTTP/1.1 200 OK153Please respect copyright.ＰＥＮＡＮＡxHiOQqJfl7
Date: Sat, 23 Nov 2024 06:29:19 GMT153Please respect copyright.ＰＥＮＡＮＡ9lfp5t2GK3
accept-ranges: bytes153Please respect copyright.ＰＥＮＡＮＡPoJXrU6R0v
content-range: bytes 0-634231201/634231202153Please respect copyright.ＰＥＮＡＮＡdxDq4Q6MPo
ETag: "634231202_2732277118"153Please respect copyright.ＰＥＮＡＮＡQCzSWPOCs1
Content-Disposition: attachment; filename="zen.iso"153Please respect copyright.ＰＥＮＡＮＡ3wMo4hZoUy
Content-Length: 634231202153Please respect copyright.ＰＥＮＡＮＡljcfxySP6l
Content-Type: application/octet-stream
(http response body is the whole file content)153Please respect copyright.ＰＥＮＡＮＡC5utICgjuI
------------------------------------------------153Please respect copyright.ＰＥＮＡＮＡ2lZFatGFwP
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:153Please respect copyright.ＰＥＮＡＮＡlGy3FWf93f
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153Please respect copyright.ＰＥＮＡＮＡ9ubgncIxTj
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153Please respect copyright.ＰＥＮＡＮＡJa2jRHhUuN
https://gemini.google.com/
感谢互联网上无私奉献的参考资153Please respect copyright.ＰＥＮＡＮＡZ3gPuQYNsz
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests153Please respect copyright.ＰＥＮＡＮＡmJXsVUgZvN
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range153Please respect copyright.ＰＥＮＡＮＡxo1igZfmwC
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag153Please respect copyright.ＰＥＮＡＮＡJAX6TGrBDg
https://www.rfc-editor.org/rfc/rfc7233153Please respect copyright.ＰＥＮＡＮＡqQpLmeXbnl
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]153Please respect copyright.ＰＥＮＡＮＡ4uxqo7J76M
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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