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:33Please respect copyright.ＰＥＮＡＮＡ6yWgkAx4zU
----------http range request-------33Please respect copyright.ＰＥＮＡＮＡetuLpl7CMc
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡmwXXGUXDGi
Range: bytes=100-102333Please respect copyright.ＰＥＮＡＮＡiWMz4RV6I4
If-Match: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡPdXWish5VF
......33Please respect copyright.ＰＥＮＡＮＡCbvs49ebMl
-----------------------------------33Please respect copyright.ＰＥＮＡＮＡhI6NAFx0Ug
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:33Please respect copyright.ＰＥＮＡＮＡEard8l3wqW
----------------html-----------------33Please respect copyright.ＰＥＮＡＮＡReLiB2JHcH
<a href="/zen.iso">zen.iso</a>33Please respect copyright.ＰＥＮＡＮＡTVJoGglwBv
-------------------------------------33Please respect copyright.ＰＥＮＡＮＡAu1JLZ86lW
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:33Please respect copyright.ＰＥＮＡＮＡRwp45UvIa9
----------get request------------33Please respect copyright.ＰＥＮＡＮＡZ3TR7n3Zhf
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡKrjpE8WkjV
......33Please respect copyright.ＰＥＮＡＮＡ5CLjV0df0d
---------------------------------
33Please respect copyright.ＰＥＮＡＮＡXtzb0xHiox
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:33Please respect copyright.ＰＥＮＡＮＡGXF0MeYRii
----------------server response--------------33Please respect copyright.ＰＥＮＡＮＡGXoAWsWMXU
HTTP/1.1 200 OK33Please respect copyright.ＰＥＮＡＮＡQPB2fEjcfe
Date: Sat, 23 Nov 2024 06:29:19 GMT33Please respect copyright.ＰＥＮＡＮＡnbC2I42Enj
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡJUWxC6WfGo
content-range: bytes 0-534231199/53423120033Please respect copyright.ＰＥＮＡＮＡur51W2N1cc
ETag: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡBr1u8jgwR2
Content-Disposition: attachment; filename="zen.iso"33Please respect copyright.ＰＥＮＡＮＡXUZp46PTr7
Content-Length: 53423120033Please respect copyright.ＰＥＮＡＮＡ5XI0xvyrKZ
Content-Type: application/octet-stream
(http response body is the whole file content)33Please respect copyright.ＰＥＮＡＮＡaKi5XzVie1
-------------------------------------------
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:33Please respect copyright.ＰＥＮＡＮＡXeuWQ3C12I
---------------http server send big file content by multiple times--------------33Please respect copyright.ＰＥＮＡＮＡQhXAMQo2e2
const int buf_size=20000000; // buf size is about 20M33Please respect copyright.ＰＥＮＡＮＡkwruGgk5V7
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers33Please respect copyright.ＰＥＮＡＮＡ7ZmiYYJq29
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 big33Please respect copyright.ＰＥＮＡＮＡXSKqYmFCcN
while(there_is_remain_file_content_to_send)33Please respect copyright.ＰＥＮＡＮＡJMym1Bh7KJ
{ 33Please respect copyright.ＰＥＮＡＮＡVNkXhsHTck
read_remain_file_content_to_send_buf(file, send_buf);33Please respect copyright.ＰＥＮＡＮＡlyBeGSkJLZ
send_result = send(client_socket, send_buf);33Please respect copyright.ＰＥＮＡＮＡ0NEtgniRnC
if(send_result == fail) return; else continue;33Please respect copyright.ＰＥＮＡＮＡa3z4UwIXYF
}
// a way to make etag for a file using its file size and last modifed time33Please respect copyright.ＰＥＮＡＮＡKwDGMCGRwG
const char *file_etag(const char *path) 33Please respect copyright.ＰＥＮＡＮＡOJQluQD577
{33Please respect copyright.ＰＥＮＡＮＡZro5Q7zECY
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;33Please respect copyright.ＰＥＮＡＮＡgQzMY3R1yY
}33Please respect copyright.ＰＥＮＡＮＡryuZ9XhDZN
-------------------------------------------------33Please respect copyright.ＰＥＮＡＮＡvINzrkD3tY
"accept-ranges: bytes" tells client web browser that the http server support range.33Please respect copyright.ＰＥＮＡＮＡMrJJu4grUC
"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).33Please respect copyright.ＰＥＮＡＮＡiQzyNECAKy
'ETag: "534231200_1732277116"' tells client web browser the file version.33Please respect copyright.ＰＥＮＡＮＡAgSsPSZ7Cd
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.33Please respect copyright.ＰＥＮＡＮＡlYQpqAMBqg
"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.33Please respect copyright.ＰＥＮＡＮＡ2ljIPVl5z5
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
33Please respect copyright.ＰＥＮＡＮＡfdDgDLBajn
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.33Please respect copyright.ＰＥＮＡＮＡuC0xLLItBk
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.)33Please respect copyright.ＰＥＮＡＮＡ9uN3IAjazF
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:33Please respect copyright.ＰＥＮＡＮＡ6UVviAyKRa
---------resume range request---------33Please respect copyright.ＰＥＮＡＮＡpRo0seeL81
GET /zen.iso HTTP/1.133Please respect copyright.ＰＥＮＡＮＡcdAT97tvGC
Range: bytes=243220480-33Please respect copyright.ＰＥＮＡＮＡnjAVKCziMr
If-Match: "534231200_1732277116"33Please respect copyright.ＰＥＮＡＮＡkcqmUW9WYy
......33Please respect copyright.ＰＥＮＡＮＡjmjYo0gJDO
-------------------------------------33Please respect copyright.ＰＥＮＡＮＡZ22Bm2yzn7
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.33Please respect copyright.ＰＥＮＡＮＡxvUNYhFiHP
'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:33Please respect copyright.ＰＥＮＡＮＡ9IoEwQCrwY
-------response content from resume point-------33Please respect copyright.ＰＥＮＡＮＡQDlfUF6THp
HTTP/1.1 206 Partial Content33Please respect copyright.ＰＥＮＡＮＡPuX3YR3OEs
Date: Sat, 23 Nov 2024 06:56:17 GMT33Please respect copyright.ＰＥＮＡＮＡzulxgAzgPo
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡEM3mGXlb6v
content-range: bytes 243220480-534231199/53423120033Please respect copyright.ＰＥＮＡＮＡ7VvWvZKldy
Content-Length: 29101072033Please respect copyright.ＰＥＮＡＮＡfFBS2YdlYY
Content-Type: application/octet-stream33Please respect copyright.ＰＥＮＡＮＡiy9hH0XVT4
Content-Disposition: attachment; filename="zen.iso"33Please respect copyright.ＰＥＮＡＮＡVZzNpcMjOh
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)33Please respect copyright.ＰＥＮＡＮＡsIhiHqM4me
------------------------------------------------33Please respect copyright.ＰＥＮＡＮＡjid6bLFhOd
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:33Please respect copyright.ＰＥＮＡＮＡxGNwpeEhCz
-------response content from resume point-------33Please respect copyright.ＰＥＮＡＮＡaTfKdJ0TNN
HTTP/1.1 200 OK33Please respect copyright.ＰＥＮＡＮＡftfbNlJnNe
Date: Sat, 23 Nov 2024 06:29:19 GMT33Please respect copyright.ＰＥＮＡＮＡeun4TwMomp
accept-ranges: bytes33Please respect copyright.ＰＥＮＡＮＡsjbOjSdKVZ
content-range: bytes 0-634231201/63423120233Please respect copyright.ＰＥＮＡＮＡx52CsLzad1
ETag: "634231202_2732277118"33Please respect copyright.ＰＥＮＡＮＡeVDSx4hIfB
Content-Disposition: attachment; filename="zen.iso"33Please respect copyright.ＰＥＮＡＮＡsvDO2D3xur
Content-Length: 63423120233Please respect copyright.ＰＥＮＡＮＡm0Pyh4MrQm
Content-Type: application/octet-stream
(http response body is the whole file content)33Please respect copyright.ＰＥＮＡＮＡgwRgq0DGHs
------------------------------------------------33Please respect copyright.ＰＥＮＡＮＡSusjT4KjVh
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
33Please respect copyright.ＰＥＮＡＮＡlVQuk1CEls
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:33Please respect copyright.ＰＥＮＡＮＡCZZbtcOc5E
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
33Please respect copyright.ＰＥＮＡＮＡ3aCMMEBkOz
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33Please respect copyright.ＰＥＮＡＮＡN0E0QwR4hs
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33Please respect copyright.ＰＥＮＡＮＡsxT40nJxBs
https://gemini.google.com/
感谢互联网上无私奉献的参考资33Please respect copyright.ＰＥＮＡＮＡ0sWkcfx4PG
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests33Please respect copyright.ＰＥＮＡＮＡWhFXFCDrZJ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range33Please respect copyright.ＰＥＮＡＮＡurZUPbQXnE
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag33Please respect copyright.ＰＥＮＡＮＡUx9lDQtb2y
https://www.rfc-editor.org/rfc/rfc723333Please respect copyright.ＰＥＮＡＮＡzfSt1NtLAg
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: si.jinmin@gmail.com33Please respect copyright.ＰＥＮＡＮＡ5Js8wPMSUM
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
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