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:68Please respect copyright.ＰＥＮＡＮＡlzVOLNm1Kk
----------http range request-------68Please respect copyright.ＰＥＮＡＮＡ6KGwvmFatW
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡLl9VvFd6yL
Range: bytes=100-102368Please respect copyright.ＰＥＮＡＮＡoyrmaBfluP
If-Match: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡdNodMj1XMY
......68Please respect copyright.ＰＥＮＡＮＡS3DbSgHs8a
-----------------------------------68Please respect copyright.ＰＥＮＡＮＡOsdb9IN1yo
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.
68Please respect copyright.ＰＥＮＡＮＡGU6KQ3an9H
Step 1. make a link for file to download, for example:68Please respect copyright.ＰＥＮＡＮＡFj4WNwxmjs
----------------html-----------------68Please respect copyright.ＰＥＮＡＮＡAvEDEw4tZj
<a href="/zen.iso">zen.iso</a>68Please respect copyright.ＰＥＮＡＮＡvx9oY4Gwru
-------------------------------------68Please respect copyright.ＰＥＮＡＮＡz9wjtIgIIH
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:68Please respect copyright.ＰＥＮＡＮＡD8230unPP1
----------get request------------68Please respect copyright.ＰＥＮＡＮＡlw3D9GVl7P
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡgJ778Ftvjy
......68Please respect copyright.ＰＥＮＡＮＡi9if2gl1Jl
---------------------------------
68Please respect copyright.ＰＥＮＡＮＡFEhUizC3XD
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:68Please respect copyright.ＰＥＮＡＮＡKraAI8uCSc
----------------server response--------------68Please respect copyright.ＰＥＮＡＮＡ5zMvZkbBP8
HTTP/1.1 200 OK68Please respect copyright.ＰＥＮＡＮＡ4XPg3f9OgX
Date: Sat, 23 Nov 2024 06:29:19 GMT68Please respect copyright.ＰＥＮＡＮＡ0oC3gXRwhL
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡ5qOrL8NygI
content-range: bytes 0-534231199/53423120068Please respect copyright.ＰＥＮＡＮＡ9qTXYK9mf9
ETag: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡw2MQ4fiaV2
Content-Disposition: attachment; filename="zen.iso"68Please respect copyright.ＰＥＮＡＮＡNUAwLlNGrq
Content-Length: 53423120068Please respect copyright.ＰＥＮＡＮＡdUPM8k9j0w
Content-Type: application/octet-stream
(http response body is the whole file content)68Please respect copyright.ＰＥＮＡＮＡyNvpVwIojK
-------------------------------------------
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:68Please respect copyright.ＰＥＮＡＮＡcDZP047zQu
---------------http server send big file content by multiple times--------------68Please respect copyright.ＰＥＮＡＮＡoDsbfIjHbt
const int buf_size=20000000; // buf size is about 20M68Please respect copyright.ＰＥＮＡＮＡHMmxctN22Q
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers68Please respect copyright.ＰＥＮＡＮＡtPscV0rUqz
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 big68Please respect copyright.ＰＥＮＡＮＡLrzGrBn0fD
while(there_is_remain_file_content_to_send)68Please respect copyright.ＰＥＮＡＮＡkeb2o8aqFb
{ 68Please respect copyright.ＰＥＮＡＮＡ0k0e2WmTgH
read_remain_file_content_to_send_buf(file, send_buf);68Please respect copyright.ＰＥＮＡＮＡjkq0RZe7bZ
send_result = send(client_socket, send_buf);68Please respect copyright.ＰＥＮＡＮＡPj5245KbFp
if(send_result == fail) return; else continue;68Please respect copyright.ＰＥＮＡＮＡjxOP9fBpRv
}
// a way to make etag for a file using its file size and last modifed time68Please respect copyright.ＰＥＮＡＮＡuSrRNf9K55
const char *file_etag(const char *path) 68Please respect copyright.ＰＥＮＡＮＡ7oMvlbhIXX
{68Please respect copyright.ＰＥＮＡＮＡdM4WCzl5wL
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;68Please respect copyright.ＰＥＮＡＮＡqmHWXPmXP2
}68Please respect copyright.ＰＥＮＡＮＡKVZz5pGMxi
-------------------------------------------------68Please respect copyright.ＰＥＮＡＮＡUKCOAbt2MX
"accept-ranges: bytes" tells client web browser that the http server support range.68Please respect copyright.ＰＥＮＡＮＡbA5h9CjDfN
"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).68Please respect copyright.ＰＥＮＡＮＡtvEoNPuOqp
'ETag: "534231200_1732277116"' tells client web browser the file version.68Please respect copyright.ＰＥＮＡＮＡ6clsPKrFUV
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.68Please respect copyright.ＰＥＮＡＮＡK9YiGIejXh
"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.68Please respect copyright.ＰＥＮＡＮＡfAXKBDfJSc
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
68Please respect copyright.ＰＥＮＡＮＡI0r0NZboJm
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.68Please respect copyright.ＰＥＮＡＮＡXSiIAOS5wc
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.)68Please respect copyright.ＰＥＮＡＮＡquTC385ada
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.
68Please respect copyright.ＰＥＮＡＮＡlXzccEvfwb
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:68Please respect copyright.ＰＥＮＡＮＡkIDiysp0hA
---------resume range request---------68Please respect copyright.ＰＥＮＡＮＡ3ymtJLl2wo
GET /zen.iso HTTP/1.168Please respect copyright.ＰＥＮＡＮＡyq063SLfLt
Range: bytes=243220480-68Please respect copyright.ＰＥＮＡＮＡBxFjx8IvjS
If-Match: "534231200_1732277116"68Please respect copyright.ＰＥＮＡＮＡap37V3HhdE
......68Please respect copyright.ＰＥＮＡＮＡqOmQ1fgpAd
-------------------------------------68Please respect copyright.ＰＥＮＡＮＡhGhd3SSv54
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.68Please respect copyright.ＰＥＮＡＮＡbuwHCgMnVf
'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:68Please respect copyright.ＰＥＮＡＮＡzAF0Dkeyp7
-------response content from resume point-------68Please respect copyright.ＰＥＮＡＮＡ8YYo1D1DiK
HTTP/1.1 206 Partial Content68Please respect copyright.ＰＥＮＡＮＡWbOYC9cUmH
Date: Sat, 23 Nov 2024 06:56:17 GMT68Please respect copyright.ＰＥＮＡＮＡeEQpPqqa5Z
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡN43UldiHxP
content-range: bytes 243220480-534231199/53423120068Please respect copyright.ＰＥＮＡＮＡ9PKP8WrtpA
Content-Length: 29101072068Please respect copyright.ＰＥＮＡＮＡ8r1kCBKypR
Content-Type: application/octet-stream68Please respect copyright.ＰＥＮＡＮＡAsoKRgAOz4
Content-Disposition: attachment; filename="zen.iso"68Please respect copyright.ＰＥＮＡＮＡgCOZwIRmIf
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)68Please respect copyright.ＰＥＮＡＮＡ8U1c7I3aAd
------------------------------------------------68Please respect copyright.ＰＥＮＡＮＡmMwOJA2BTn
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:68Please respect copyright.ＰＥＮＡＮＡ00bal4RH6B
-------response content from resume point-------68Please respect copyright.ＰＥＮＡＮＡN5PR6AkUcp
HTTP/1.1 200 OK68Please respect copyright.ＰＥＮＡＮＡGhybODedUz
Date: Sat, 23 Nov 2024 06:29:19 GMT68Please respect copyright.ＰＥＮＡＮＡPMDIQqxvrp
accept-ranges: bytes68Please respect copyright.ＰＥＮＡＮＡ1iAS825apl
content-range: bytes 0-634231201/63423120268Please respect copyright.ＰＥＮＡＮＡwtaPMhHQBD
ETag: "634231202_2732277118"68Please respect copyright.ＰＥＮＡＮＡtLV2ZU8HT5
Content-Disposition: attachment; filename="zen.iso"68Please respect copyright.ＰＥＮＡＮＡz6yQDtTMEX
Content-Length: 63423120268Please respect copyright.ＰＥＮＡＮＡv0eraXEgk5
Content-Type: application/octet-stream
(http response body is the whole file content)68Please respect copyright.ＰＥＮＡＮＡljPibWYkVZ
------------------------------------------------68Please respect copyright.ＰＥＮＡＮＡtfI349CzhO
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
68Please respect copyright.ＰＥＮＡＮＡ0yqgAhd1c2
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:68Please respect copyright.ＰＥＮＡＮＡu7g2qujJ59
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
68Please respect copyright.ＰＥＮＡＮＡuTSiJ3XCXd
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68Please respect copyright.ＰＥＮＡＮＡZbegqt0Jnh
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68Please respect copyright.ＰＥＮＡＮＡHAB4yPmUNm
https://gemini.google.com/
感谢互联网上无私奉献的参考资68Please respect copyright.ＰＥＮＡＮＡUVm7G9UoJ9
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests68Please respect copyright.ＰＥＮＡＮＡWyXvtVFW3b
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range68Please respect copyright.ＰＥＮＡＮＡYuwn3hjr2W
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag68Please respect copyright.ＰＥＮＡＮＡWNPTfs6Pt7
https://www.rfc-editor.org/rfc/rfc723368Please respect copyright.ＰＥＮＡＮＡGE1N5KF2tf
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/
68Please respect copyright.ＰＥＮＡＮＡU0YtNmSHat
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: si.jinmin@gmail.com68Please respect copyright.ＰＥＮＡＮＡ0yjbYGBPLJ
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: si.jinmin@gmail.com
如果您對C/C++ programming, Linux, website development, Vue, Git, vscode感興趣，邀請您加入「Linux/C/C++ Website Development」 微信群，請加我的微信（si_jinmin）以便拉您进群。
ns3.144.149.217da2
