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:200Please respect copyright.ＰＥＮＡＮＡiI0fCuMwyp
----------http range request-------200Please respect copyright.ＰＥＮＡＮＡMQFrsN0Ior
GET /zen.iso HTTP/1.1200Please respect copyright.ＰＥＮＡＮＡxim5erne88
Range: bytes=100-1023200Please respect copyright.ＰＥＮＡＮＡ9esX8wFEGt
If-Match: "534231200_1732277116"200Please respect copyright.ＰＥＮＡＮＡQ3zaZISPjG
......200Please respect copyright.ＰＥＮＡＮＡ24F2tN6pYt
-----------------------------------200Please respect copyright.ＰＥＮＡＮＡBLZ5EWILRl
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:200Please respect copyright.ＰＥＮＡＮＡLvaOF1cFIT
----------------html-----------------200Please respect copyright.ＰＥＮＡＮＡzXO8q8BTVq
<a href="/zen.iso">zen.iso</a>200Please respect copyright.ＰＥＮＡＮＡgmYLuSIQQ7
-------------------------------------200Please respect copyright.ＰＥＮＡＮＡvsCZ49Ek4A
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:200Please respect copyright.ＰＥＮＡＮＡNnmAXcq7nA
----------get request------------200Please respect copyright.ＰＥＮＡＮＡ8yadkkLyeE
GET /zen.iso HTTP/1.1200Please respect copyright.ＰＥＮＡＮＡx3gP9GW4Ak
......200Please respect copyright.ＰＥＮＡＮＡ2TruY3EyD5
---------------------------------
200Please respect copyright.ＰＥＮＡＮＡa9jUEEYuHi
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:200Please respect copyright.ＰＥＮＡＮＡgawixUJYrE
----------------server response--------------200Please respect copyright.ＰＥＮＡＮＡ7xuHnLWKcQ
HTTP/1.1 200 OK200Please respect copyright.ＰＥＮＡＮＡf6pN4YUFWb
Date: Sat, 23 Nov 2024 06:29:19 GMT200Please respect copyright.ＰＥＮＡＮＡwHEewbw1AA
accept-ranges: bytes200Please respect copyright.ＰＥＮＡＮＡtkgYC3oxKe
content-range: bytes 0-534231199/534231200200Please respect copyright.ＰＥＮＡＮＡDr8NQu9oQ4
ETag: "534231200_1732277116"200Please respect copyright.ＰＥＮＡＮＡm8ZCHcbsXj
Content-Disposition: attachment; filename="zen.iso"200Please respect copyright.ＰＥＮＡＮＡDD9Cs57cko
Content-Length: 534231200200Please respect copyright.ＰＥＮＡＮＡvXR3pRkwKA
Content-Type: application/octet-stream
(http response body is the whole file content)200Please respect copyright.ＰＥＮＡＮＡa8j8YDdxSv
-------------------------------------------
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:200Please respect copyright.ＰＥＮＡＮＡTgn6X1Phny
---------------http server send big file content by multiple times--------------200Please respect copyright.ＰＥＮＡＮＡrF07GUxdq9
const int buf_size=20000000; // buf size is about 20M200Please respect copyright.ＰＥＮＡＮＡYOKw0ykkp9
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers200Please respect copyright.ＰＥＮＡＮＡx43qD2QUUN
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 big200Please respect copyright.ＰＥＮＡＮＡahpW8kTKQR
while(there_is_remain_file_content_to_send)200Please respect copyright.ＰＥＮＡＮＡcs2kYqVuF3
{ 200Please respect copyright.ＰＥＮＡＮＡ94WLoGwSYr
read_remain_file_content_to_send_buf(file, send_buf);200Please respect copyright.ＰＥＮＡＮＡjFQ2BH7hQ9
send_result = send(client_socket, send_buf);200Please respect copyright.ＰＥＮＡＮＡxkHHSWyBt5
if(send_result == fail) return; else continue;200Please respect copyright.ＰＥＮＡＮＡWR3lYykWLs
}
// a way to make etag for a file using its file size and last modifed time200Please respect copyright.ＰＥＮＡＮＡX7OgUhqjCc
const char *file_etag(const char *path) 200Please respect copyright.ＰＥＮＡＮＡnOlRBopLFj
{200Please respect copyright.ＰＥＮＡＮＡp309cEGRcf
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;200Please respect copyright.ＰＥＮＡＮＡPCVsQ6cxVN
}200Please respect copyright.ＰＥＮＡＮＡ0yXYmvfatJ
-------------------------------------------------200Please respect copyright.ＰＥＮＡＮＡIYtcKF5RIH
"accept-ranges: bytes" tells client web browser that the http server support range.200Please respect copyright.ＰＥＮＡＮＡpxN27nRH9I
"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).200Please respect copyright.ＰＥＮＡＮＡW4jzURgcCm
'ETag: "534231200_1732277116"' tells client web browser the file version.200Please respect copyright.ＰＥＮＡＮＡXwddZKJZcQ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.200Please respect copyright.ＰＥＮＡＮＡOcNVlStlcH
"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.200Please respect copyright.ＰＥＮＡＮＡuyrX8Wmquc
"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.200Please respect copyright.ＰＥＮＡＮＡLHZi3MbrXu
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.)200Please respect copyright.ＰＥＮＡＮＡzI2CyxvHix
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:200Please respect copyright.ＰＥＮＡＮＡlAaRzQSBUP
---------resume range request---------200Please respect copyright.ＰＥＮＡＮＡtsm4xriZwk
GET /zen.iso HTTP/1.1200Please respect copyright.ＰＥＮＡＮＡyfVl2prOT0
Range: bytes=243220480-200Please respect copyright.ＰＥＮＡＮＡs0BSYnJDCe
If-Match: "534231200_1732277116"200Please respect copyright.ＰＥＮＡＮＡVlvbFimCFs
......200Please respect copyright.ＰＥＮＡＮＡHBOi2rfF2i
-------------------------------------200Please respect copyright.ＰＥＮＡＮＡAp9ClvAPm7
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.200Please respect copyright.ＰＥＮＡＮＡenBW6RIsA6
'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:200Please respect copyright.ＰＥＮＡＮＡXYBPKF7r4n
-------response content from resume point-------200Please respect copyright.ＰＥＮＡＮＡi7c0Z8ewBh
HTTP/1.1 206 Partial Content200Please respect copyright.ＰＥＮＡＮＡx1aTVZnFvl
Date: Sat, 23 Nov 2024 06:56:17 GMT200Please respect copyright.ＰＥＮＡＮＡQ4JILTrFox
accept-ranges: bytes200Please respect copyright.ＰＥＮＡＮＡzXdeGxhYT6
content-range: bytes 243220480-534231199/534231200200Please respect copyright.ＰＥＮＡＮＡ7MwvHQhT5e
Content-Length: 291010720200Please respect copyright.ＰＥＮＡＮＡHK8zV42HEr
Content-Type: application/octet-stream200Please respect copyright.ＰＥＮＡＮＡVbgdMc4kMB
Content-Disposition: attachment; filename="zen.iso"200Please respect copyright.ＰＥＮＡＮＡasoPCe1zFp
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)200Please respect copyright.ＰＥＮＡＮＡOGjZU4hSDr
------------------------------------------------200Please respect copyright.ＰＥＮＡＮＡnk9uKQNlcR
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:200Please respect copyright.ＰＥＮＡＮＡ4zhFW2Cked
-------response content from resume point-------200Please respect copyright.ＰＥＮＡＮＡkrg1VZVToV
HTTP/1.1 200 OK200Please respect copyright.ＰＥＮＡＮＡHXhRdCd9in
Date: Sat, 23 Nov 2024 06:29:19 GMT200Please respect copyright.ＰＥＮＡＮＡHxLKErWdZ5
accept-ranges: bytes200Please respect copyright.ＰＥＮＡＮＡpOXPCczqnr
content-range: bytes 0-634231201/634231202200Please respect copyright.ＰＥＮＡＮＡAn2uSV89RW
ETag: "634231202_2732277118"200Please respect copyright.ＰＥＮＡＮＡ6HXM0bkSBs
Content-Disposition: attachment; filename="zen.iso"200Please respect copyright.ＰＥＮＡＮＡ1PVCAX2uKl
Content-Length: 634231202200Please respect copyright.ＰＥＮＡＮＡMZdtmqlpev
Content-Type: application/octet-stream
(http response body is the whole file content)200Please respect copyright.ＰＥＮＡＮＡeJAVvwSYbv
------------------------------------------------200Please respect copyright.ＰＥＮＡＮＡqX5457Mzs3
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:200Please respect copyright.ＰＥＮＡＮＡzRkdy2sGEW
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
200Please respect copyright.ＰＥＮＡＮＡV4ceOzRjsK
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200Please respect copyright.ＰＥＮＡＮＡpZejLT1F7B
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200Please respect copyright.ＰＥＮＡＮＡyOcuwCPGAG
https://gemini.google.com/
感谢互联网上无私奉献的参考资200Please respect copyright.ＰＥＮＡＮＡJfK3XKmkY5
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests200Please respect copyright.ＰＥＮＡＮＡHnRBkAYrE2
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range200Please respect copyright.ＰＥＮＡＮＡ0smIw1fIMX
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag200Please respect copyright.ＰＥＮＡＮＡXYTvCPtEnn
https://www.rfc-editor.org/rfc/rfc7233200Please respect copyright.ＰＥＮＡＮＡpV4xg5dOj1
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]200Please respect copyright.ＰＥＮＡＮＡpcdFLlpNMW
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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