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:76Please respect copyright.ＰＥＮＡＮＡKBSLsMdap9
----------http range request-------76Please respect copyright.ＰＥＮＡＮＡ6JyjeWdjLe
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡNyDj6339nj
Range: bytes=100-102376Please respect copyright.ＰＥＮＡＮＡkqjGQiywiF
If-Match: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡMGriISAACx
......76Please respect copyright.ＰＥＮＡＮＡp5tTFDFrQg
-----------------------------------76Please respect copyright.ＰＥＮＡＮＡZpqhdsSTxy
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:76Please respect copyright.ＰＥＮＡＮＡrEPawhNpJT
----------------html-----------------76Please respect copyright.ＰＥＮＡＮＡgjyUmE78nK
<a href="/zen.iso">zen.iso</a>76Please respect copyright.ＰＥＮＡＮＡt7eB6etUL1
-------------------------------------76Please respect copyright.ＰＥＮＡＮＡjJxV1L8FFT
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:76Please respect copyright.ＰＥＮＡＮＡRma46CVBYP
----------get request------------76Please respect copyright.ＰＥＮＡＮＡUxhRNFPfIT
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡ8P1g9jKE5U
......76Please respect copyright.ＰＥＮＡＮＡAhI0WVzKkX
---------------------------------
76Please respect copyright.ＰＥＮＡＮＡKf5mDSdfXn
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:76Please respect copyright.ＰＥＮＡＮＡEh4LK5vJjC
----------------server response--------------76Please respect copyright.ＰＥＮＡＮＡH4VqexfIPV
HTTP/1.1 200 OK76Please respect copyright.ＰＥＮＡＮＡbE8hgkrXna
Date: Sat, 23 Nov 2024 06:29:19 GMT76Please respect copyright.ＰＥＮＡＮＡhlJpqlyS4Q
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡOFmyJex4Dc
content-range: bytes 0-534231199/53423120076Please respect copyright.ＰＥＮＡＮＡFvvG25PWsk
ETag: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡRlSgJ2XmPt
Content-Disposition: attachment; filename="zen.iso"76Please respect copyright.ＰＥＮＡＮＡOmQdhd0MB3
Content-Length: 53423120076Please respect copyright.ＰＥＮＡＮＡXwCpfsMQnD
Content-Type: application/octet-stream
(http response body is the whole file content)76Please respect copyright.ＰＥＮＡＮＡdiBQl39rin
-------------------------------------------
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:76Please respect copyright.ＰＥＮＡＮＡ1xOhF86JDr
---------------http server send big file content by multiple times--------------76Please respect copyright.ＰＥＮＡＮＡm7iLMO3cE1
const int buf_size=20000000; // buf size is about 20M76Please respect copyright.ＰＥＮＡＮＡSurlXoPvIT
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers76Please respect copyright.ＰＥＮＡＮＡ26zHtVlZPC
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 big76Please respect copyright.ＰＥＮＡＮＡTblD0c4XCO
while(there_is_remain_file_content_to_send)76Please respect copyright.ＰＥＮＡＮＡgIEz1wUUj9
{ 76Please respect copyright.ＰＥＮＡＮＡ7xvxoWS9Ps
read_remain_file_content_to_send_buf(file, send_buf);76Please respect copyright.ＰＥＮＡＮＡxrlZ3e1Kzm
send_result = send(client_socket, send_buf);76Please respect copyright.ＰＥＮＡＮＡwbTuiQK3lk
if(send_result == fail) return; else continue;76Please respect copyright.ＰＥＮＡＮＡzuAWreJfi2
}
// a way to make etag for a file using its file size and last modifed time76Please respect copyright.ＰＥＮＡＮＡ2uK99VCDaU
const char *file_etag(const char *path) 76Please respect copyright.ＰＥＮＡＮＡb4co7dSPD6
{76Please respect copyright.ＰＥＮＡＮＡH4SEinAY4j
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;76Please respect copyright.ＰＥＮＡＮＡ8AZXAp2chd
}76Please respect copyright.ＰＥＮＡＮＡ03SWMMpKDv
-------------------------------------------------76Please respect copyright.ＰＥＮＡＮＡqbTCdjvoOW
"accept-ranges: bytes" tells client web browser that the http server support range.76Please respect copyright.ＰＥＮＡＮＡEkvII5QdTm
"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).76Please respect copyright.ＰＥＮＡＮＡs5CYFyZ1rV
'ETag: "534231200_1732277116"' tells client web browser the file version.76Please respect copyright.ＰＥＮＡＮＡlUorXBCQlz
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.76Please respect copyright.ＰＥＮＡＮＡNDHeVAuFlJ
"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.76Please respect copyright.ＰＥＮＡＮＡOLnko9KCBS
"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.76Please respect copyright.ＰＥＮＡＮＡCEd09BTHkU
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.)76Please respect copyright.ＰＥＮＡＮＡ0dj6v0YqxK
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:76Please respect copyright.ＰＥＮＡＮＡpamFPtpjM2
---------resume range request---------76Please respect copyright.ＰＥＮＡＮＡfswFpkvtQN
GET /zen.iso HTTP/1.176Please respect copyright.ＰＥＮＡＮＡpg7UBeF7LY
Range: bytes=243220480-76Please respect copyright.ＰＥＮＡＮＡIRzW0A524T
If-Match: "534231200_1732277116"76Please respect copyright.ＰＥＮＡＮＡuQG9kHwlVZ
......76Please respect copyright.ＰＥＮＡＮＡhPb2wDVckk
-------------------------------------76Please respect copyright.ＰＥＮＡＮＡVCgHpv4TGB
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.76Please respect copyright.ＰＥＮＡＮＡdpd4v0MoWJ
'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:76Please respect copyright.ＰＥＮＡＮＡ7PeJU0Cz5m
-------response content from resume point-------76Please respect copyright.ＰＥＮＡＮＡBe6IKn1fzw
HTTP/1.1 206 Partial Content76Please respect copyright.ＰＥＮＡＮＡbdkzoctiGo
Date: Sat, 23 Nov 2024 06:56:17 GMT76Please respect copyright.ＰＥＮＡＮＡlR9nmJPEhw
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡTEzFI0cAcU
content-range: bytes 243220480-534231199/53423120076Please respect copyright.ＰＥＮＡＮＡMlyq6ce2Op
Content-Length: 29101072076Please respect copyright.ＰＥＮＡＮＡOC0qakpg98
Content-Type: application/octet-stream76Please respect copyright.ＰＥＮＡＮＡtyjePpJEBA
Content-Disposition: attachment; filename="zen.iso"76Please respect copyright.ＰＥＮＡＮＡWbhtXUSUMu
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)76Please respect copyright.ＰＥＮＡＮＡ9tdk5BxcCO
------------------------------------------------76Please respect copyright.ＰＥＮＡＮＡCVbQ4k3JP0
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:76Please respect copyright.ＰＥＮＡＮＡsprjr3f7ks
-------response content from resume point-------76Please respect copyright.ＰＥＮＡＮＡH7CYgmzKun
HTTP/1.1 200 OK76Please respect copyright.ＰＥＮＡＮＡegshbe8RrA
Date: Sat, 23 Nov 2024 06:29:19 GMT76Please respect copyright.ＰＥＮＡＮＡo0iLkaRxGj
accept-ranges: bytes76Please respect copyright.ＰＥＮＡＮＡV7m5DwN1f0
content-range: bytes 0-634231201/63423120276Please respect copyright.ＰＥＮＡＮＡPSxEe3lMMz
ETag: "634231202_2732277118"76Please respect copyright.ＰＥＮＡＮＡBDijsSoE8g
Content-Disposition: attachment; filename="zen.iso"76Please respect copyright.ＰＥＮＡＮＡImOPuYpl6x
Content-Length: 63423120276Please respect copyright.ＰＥＮＡＮＡiXWXFrvrUL
Content-Type: application/octet-stream
(http response body is the whole file content)76Please respect copyright.ＰＥＮＡＮＡ30ZZuRQSgO
------------------------------------------------76Please respect copyright.ＰＥＮＡＮＡPXckDNozcq
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:76Please respect copyright.ＰＥＮＡＮＡPFhfQIBz1U
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76Please respect copyright.ＰＥＮＡＮＡMdx96Iitr6
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76Please respect copyright.ＰＥＮＡＮＡ2LMESnHbBf
https://gemini.google.com/
感谢互联网上无私奉献的参考资76Please respect copyright.ＰＥＮＡＮＡELucQjuhm9
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests76Please respect copyright.ＰＥＮＡＮＡrMSE1oMkOq
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range76Please respect copyright.ＰＥＮＡＮＡZnCpJBcbrJ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag76Please respect copyright.ＰＥＮＡＮＡc7lnd1WWxw
https://www.rfc-editor.org/rfc/rfc723376Please respect copyright.ＰＥＮＡＮＡvEX56nV6cH
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]76Please respect copyright.ＰＥＮＡＮＡw04atFROp1
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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