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:96Please respect copyright.ＰＥＮＡＮＡmfQaOWCNGH
----------http range request-------96Please respect copyright.ＰＥＮＡＮＡTuKRUfA2g4
GET /zen.iso HTTP/1.196Please respect copyright.ＰＥＮＡＮＡjbevUYyS2h
Range: bytes=100-102396Please respect copyright.ＰＥＮＡＮＡIHQJBQGIHe
If-Match: "534231200_1732277116"96Please respect copyright.ＰＥＮＡＮＡPxtJrgMjco
......96Please respect copyright.ＰＥＮＡＮＡUzs5NdU7OZ
-----------------------------------96Please respect copyright.ＰＥＮＡＮＡgwXzGEORKx
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:96Please respect copyright.ＰＥＮＡＮＡlNFeo7ORXn
----------------html-----------------96Please respect copyright.ＰＥＮＡＮＡzRHb2PQMAU
<a href="/zen.iso">zen.iso</a>96Please respect copyright.ＰＥＮＡＮＡ3UtRsqf14L
-------------------------------------96Please respect copyright.ＰＥＮＡＮＡVh23LH2qL2
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:96Please respect copyright.ＰＥＮＡＮＡrY8LRPft9i
----------get request------------96Please respect copyright.ＰＥＮＡＮＡhXf1z80Ugb
GET /zen.iso HTTP/1.196Please respect copyright.ＰＥＮＡＮＡt1ytxYl14G
......96Please respect copyright.ＰＥＮＡＮＡfw21IUwilL
---------------------------------
96Please respect copyright.ＰＥＮＡＮＡchp5mQp8Ib
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:96Please respect copyright.ＰＥＮＡＮＡFEvLMwzTr5
----------------server response--------------96Please respect copyright.ＰＥＮＡＮＡ4QYBvnktav
HTTP/1.1 200 OK96Please respect copyright.ＰＥＮＡＮＡUYnAd9kA0V
Date: Sat, 23 Nov 2024 06:29:19 GMT96Please respect copyright.ＰＥＮＡＮＡpPxX09KMcs
accept-ranges: bytes96Please respect copyright.ＰＥＮＡＮＡfLBVgmQXmm
content-range: bytes 0-534231199/53423120096Please respect copyright.ＰＥＮＡＮＡtvJClvdFar
ETag: "534231200_1732277116"96Please respect copyright.ＰＥＮＡＮＡ5mh6UeA0rm
Content-Disposition: attachment; filename="zen.iso"96Please respect copyright.ＰＥＮＡＮＡcJwSIBFreh
Content-Length: 53423120096Please respect copyright.ＰＥＮＡＮＡxReeaBgfFv
Content-Type: application/octet-stream
(http response body is the whole file content)96Please respect copyright.ＰＥＮＡＮＡ6fe0xIgOgK
-------------------------------------------
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:96Please respect copyright.ＰＥＮＡＮＡK7TQtW2ToZ
---------------http server send big file content by multiple times--------------96Please respect copyright.ＰＥＮＡＮＡPu5tK8tQqN
const int buf_size=20000000; // buf size is about 20M96Please respect copyright.ＰＥＮＡＮＡnKrBF5iwUM
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers96Please respect copyright.ＰＥＮＡＮＡzvtlfbevHZ
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 big96Please respect copyright.ＰＥＮＡＮＡA7hZt4jvhm
while(there_is_remain_file_content_to_send)96Please respect copyright.ＰＥＮＡＮＡ6hhTY5myuU
{ 96Please respect copyright.ＰＥＮＡＮＡYFQjopHmJO
read_remain_file_content_to_send_buf(file, send_buf);96Please respect copyright.ＰＥＮＡＮＡojemkQReoi
send_result = send(client_socket, send_buf);96Please respect copyright.ＰＥＮＡＮＡsNGit2IFFE
if(send_result == fail) return; else continue;96Please respect copyright.ＰＥＮＡＮＡhsiZSnhHLt
}
// a way to make etag for a file using its file size and last modifed time96Please respect copyright.ＰＥＮＡＮＡDcpbz8vTzc
const char *file_etag(const char *path) 96Please respect copyright.ＰＥＮＡＮＡI9qoAAIWnQ
{96Please respect copyright.ＰＥＮＡＮＡP6AUGq8NSh
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;96Please respect copyright.ＰＥＮＡＮＡz3uEJVraEa
}96Please respect copyright.ＰＥＮＡＮＡBJM2JQIp8Z
-------------------------------------------------96Please respect copyright.ＰＥＮＡＮＡN20tG5usCT
"accept-ranges: bytes" tells client web browser that the http server support range.96Please respect copyright.ＰＥＮＡＮＡVntDEt3Y7A
"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).96Please respect copyright.ＰＥＮＡＮＡeWJ0n6BbAd
'ETag: "534231200_1732277116"' tells client web browser the file version.96Please respect copyright.ＰＥＮＡＮＡbwiFjQX6Oo
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.96Please respect copyright.ＰＥＮＡＮＡ6HW2rszIyO
"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.96Please respect copyright.ＰＥＮＡＮＡQD4Kmzb6NN
"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.96Please respect copyright.ＰＥＮＡＮＡl6yFX1Igig
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.)96Please respect copyright.ＰＥＮＡＮＡZkIH1mj7Uk
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:96Please respect copyright.ＰＥＮＡＮＡ0UAU6qnqMK
---------resume range request---------96Please respect copyright.ＰＥＮＡＮＡgpSuzF2qgz
GET /zen.iso HTTP/1.196Please respect copyright.ＰＥＮＡＮＡvUYEMPmCuc
Range: bytes=243220480-96Please respect copyright.ＰＥＮＡＮＡO9i60zZZIL
If-Match: "534231200_1732277116"96Please respect copyright.ＰＥＮＡＮＡ2cC2FF0Qe0
......96Please respect copyright.ＰＥＮＡＮＡMyFPqfUGuH
-------------------------------------96Please respect copyright.ＰＥＮＡＮＡPWmklZNJ4y
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.96Please respect copyright.ＰＥＮＡＮＡFQED5MlxXV
'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:96Please respect copyright.ＰＥＮＡＮＡSm40cnTwUU
-------response content from resume point-------96Please respect copyright.ＰＥＮＡＮＡy4JcT0Bvkw
HTTP/1.1 206 Partial Content96Please respect copyright.ＰＥＮＡＮＡPGC5oC9wfx
Date: Sat, 23 Nov 2024 06:56:17 GMT96Please respect copyright.ＰＥＮＡＮＡJHD437BYAq
accept-ranges: bytes96Please respect copyright.ＰＥＮＡＮＡUYtV2M3H0y
content-range: bytes 243220480-534231199/53423120096Please respect copyright.ＰＥＮＡＮＡ9NJ02UBNod
Content-Length: 29101072096Please respect copyright.ＰＥＮＡＮＡNBMqekTGtY
Content-Type: application/octet-stream96Please respect copyright.ＰＥＮＡＮＡe5rWodA6mr
Content-Disposition: attachment; filename="zen.iso"96Please respect copyright.ＰＥＮＡＮＡBhEHsSDIQ9
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)96Please respect copyright.ＰＥＮＡＮＡ1BndGWdOih
------------------------------------------------96Please respect copyright.ＰＥＮＡＮＡZJSiWvhx0T
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:96Please respect copyright.ＰＥＮＡＮＡzgpV46Sedm
-------response content from resume point-------96Please respect copyright.ＰＥＮＡＮＡ7uqxEe36WV
HTTP/1.1 200 OK96Please respect copyright.ＰＥＮＡＮＡxXIEJoJiYO
Date: Sat, 23 Nov 2024 06:29:19 GMT96Please respect copyright.ＰＥＮＡＮＡdPsGk6op3X
accept-ranges: bytes96Please respect copyright.ＰＥＮＡＮＡLfv6wANHyo
content-range: bytes 0-634231201/63423120296Please respect copyright.ＰＥＮＡＮＡBRSC2CLhbP
ETag: "634231202_2732277118"96Please respect copyright.ＰＥＮＡＮＡpJDgqpnKDw
Content-Disposition: attachment; filename="zen.iso"96Please respect copyright.ＰＥＮＡＮＡpqNnhkl4Dk
Content-Length: 63423120296Please respect copyright.ＰＥＮＡＮＡLiWJT0UiEZ
Content-Type: application/octet-stream
(http response body is the whole file content)96Please respect copyright.ＰＥＮＡＮＡ0XawWnYR4b
------------------------------------------------96Please respect copyright.ＰＥＮＡＮＡAeX0DGAmBb
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
96Please respect copyright.ＰＥＮＡＮＡcKUHF2a9rF
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:96Please respect copyright.ＰＥＮＡＮＡsBxOAXcpuC
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
96Please respect copyright.ＰＥＮＡＮＡBhd2bye8jU
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96Please respect copyright.ＰＥＮＡＮＡHYdFRZNyVY
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96Please respect copyright.ＰＥＮＡＮＡdWiptw8SU0
https://gemini.google.com/
感谢互联网上无私奉献的参考资96Please respect copyright.ＰＥＮＡＮＡB0bayiuYuV
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests96Please respect copyright.ＰＥＮＡＮＡ6g9DAMs4bY
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range96Please respect copyright.ＰＥＮＡＮＡ9Wr32I55hQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag96Please respect copyright.ＰＥＮＡＮＡGCtSSFdtq6
https://www.rfc-editor.org/rfc/rfc723396Please respect copyright.ＰＥＮＡＮＡ5esDGFfset
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]96Please respect copyright.ＰＥＮＡＮＡc5SU1rM7Vz
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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