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:31Please respect copyright.ＰＥＮＡＮＡTRXLDCENGv
----------http range request-------31Please respect copyright.ＰＥＮＡＮＡjutrXhvyqg
GET /zen.iso HTTP/1.131Please respect copyright.ＰＥＮＡＮＡeIivTdFwNU
Range: bytes=100-102331Please respect copyright.ＰＥＮＡＮＡ3w1Z1F09sm
If-Match: "534231200_1732277116"31Please respect copyright.ＰＥＮＡＮＡMJ37rzgUwx
......31Please respect copyright.ＰＥＮＡＮＡpTPaoo3e2F
-----------------------------------31Please respect copyright.ＰＥＮＡＮＡW6giVSdCeE
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:31Please respect copyright.ＰＥＮＡＮＡBhc05LKxaJ
----------------html-----------------31Please respect copyright.ＰＥＮＡＮＡh99cAn8j56
<a href="/zen.iso">zen.iso</a>31Please respect copyright.ＰＥＮＡＮＡ2y8EghUzWT
-------------------------------------31Please respect copyright.ＰＥＮＡＮＡhjPRqdsFqq
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:31Please respect copyright.ＰＥＮＡＮＡ8kIzY7qOLq
----------get request------------31Please respect copyright.ＰＥＮＡＮＡAG1cPxQjmI
GET /zen.iso HTTP/1.131Please respect copyright.ＰＥＮＡＮＡhULmZXdhtv
......31Please respect copyright.ＰＥＮＡＮＡQLTgkMDCZQ
---------------------------------
31Please respect copyright.ＰＥＮＡＮＡbdnmIEmomA
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:31Please respect copyright.ＰＥＮＡＮＡvyx4PNfVgH
----------------server response--------------31Please respect copyright.ＰＥＮＡＮＡJdPvsceQNh
HTTP/1.1 200 OK31Please respect copyright.ＰＥＮＡＮＡKQmhFiQvtj
Date: Sat, 23 Nov 2024 06:29:19 GMT31Please respect copyright.ＰＥＮＡＮＡGBX6SmbCKD
accept-ranges: bytes31Please respect copyright.ＰＥＮＡＮＡ6gMr89w788
content-range: bytes 0-534231199/53423120031Please respect copyright.ＰＥＮＡＮＡicw5KKIY1p
ETag: "534231200_1732277116"31Please respect copyright.ＰＥＮＡＮＡasHRtvpdaO
Content-Disposition: attachment; filename="zen.iso"31Please respect copyright.ＰＥＮＡＮＡ4oMkQPlXSV
Content-Length: 53423120031Please respect copyright.ＰＥＮＡＮＡD4uemV8EjW
Content-Type: application/octet-stream
(http response body is the whole file content)31Please respect copyright.ＰＥＮＡＮＡodDgE7OORK
-------------------------------------------
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:31Please respect copyright.ＰＥＮＡＮＡjoXofnPkJy
---------------http server send big file content by multiple times--------------31Please respect copyright.ＰＥＮＡＮＡlYHmRkWW3S
const int buf_size=20000000; // buf size is about 20M31Please respect copyright.ＰＥＮＡＮＡ4e4BSPsL3z
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers31Please respect copyright.ＰＥＮＡＮＡrPAAHsxwCI
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 big31Please respect copyright.ＰＥＮＡＮＡFR53CVdtAV
while(there_is_remain_file_content_to_send)31Please respect copyright.ＰＥＮＡＮＡLUZfLRnywy
{ 31Please respect copyright.ＰＥＮＡＮＡRmvrGqCvCX
read_remain_file_content_to_send_buf(file, send_buf);31Please respect copyright.ＰＥＮＡＮＡeGHPuT842r
send_result = send(client_socket, send_buf);31Please respect copyright.ＰＥＮＡＮＡzDiosiNro2
if(send_result == fail) return; else continue;31Please respect copyright.ＰＥＮＡＮＡLYPQ3IgS5v
}
// a way to make etag for a file using its file size and last modifed time31Please respect copyright.ＰＥＮＡＮＡXydKPUh9UX
const char *file_etag(const char *path) 31Please respect copyright.ＰＥＮＡＮＡlXkqWzffXa
{31Please respect copyright.ＰＥＮＡＮＡOexvxHpoui
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;31Please respect copyright.ＰＥＮＡＮＡ5vKMINzhd2
}31Please respect copyright.ＰＥＮＡＮＡnEU3pblKP7
-------------------------------------------------31Please respect copyright.ＰＥＮＡＮＡjSthr3ujxf
"accept-ranges: bytes" tells client web browser that the http server support range.31Please respect copyright.ＰＥＮＡＮＡpZoyBke04r
"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).31Please respect copyright.ＰＥＮＡＮＡI6g3mJlgrG
'ETag: "534231200_1732277116"' tells client web browser the file version.31Please respect copyright.ＰＥＮＡＮＡnL0iZIn62a
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.31Please respect copyright.ＰＥＮＡＮＡ4nkTA9SzUg
"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.31Please respect copyright.ＰＥＮＡＮＡdzOS1Fd5c0
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
31Please respect copyright.ＰＥＮＡＮＡZ8CyYja1cp
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.31Please respect copyright.ＰＥＮＡＮＡZuBXU9IjyG
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.)31Please respect copyright.ＰＥＮＡＮＡqQTpfrbUVX
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:31Please respect copyright.ＰＥＮＡＮＡp4g9qrKfTC
---------resume range request---------31Please respect copyright.ＰＥＮＡＮＡAtzo6deGZN
GET /zen.iso HTTP/1.131Please respect copyright.ＰＥＮＡＮＡgPOOFKw6wD
Range: bytes=243220480-31Please respect copyright.ＰＥＮＡＮＡ5jVQpOv8Ya
If-Match: "534231200_1732277116"31Please respect copyright.ＰＥＮＡＮＡlpmBi1XJKo
......31Please respect copyright.ＰＥＮＡＮＡz7UjfFe0l6
-------------------------------------31Please respect copyright.ＰＥＮＡＮＡ4Txd9PG1cE
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.31Please respect copyright.ＰＥＮＡＮＡmh1c901fuB
'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:31Please respect copyright.ＰＥＮＡＮＡONITM9G23f
-------response content from resume point-------31Please respect copyright.ＰＥＮＡＮＡZvorf6JY6P
HTTP/1.1 206 Partial Content31Please respect copyright.ＰＥＮＡＮＡeiBTp7TZpH
Date: Sat, 23 Nov 2024 06:56:17 GMT31Please respect copyright.ＰＥＮＡＮＡQYd39l5Gw7
accept-ranges: bytes31Please respect copyright.ＰＥＮＡＮＡlilYvDcTWg
content-range: bytes 243220480-534231199/53423120031Please respect copyright.ＰＥＮＡＮＡ0kHp1snGC0
Content-Length: 29101072031Please respect copyright.ＰＥＮＡＮＡxzeb2LcQy3
Content-Type: application/octet-stream31Please respect copyright.ＰＥＮＡＮＡPe1CUkfvAX
Content-Disposition: attachment; filename="zen.iso"31Please respect copyright.ＰＥＮＡＮＡSwzbfik5nZ
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)31Please respect copyright.ＰＥＮＡＮＡxEsymyclL9
------------------------------------------------31Please respect copyright.ＰＥＮＡＮＡeafE8wAqiv
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:31Please respect copyright.ＰＥＮＡＮＡ7j2olrmFhA
-------response content from resume point-------31Please respect copyright.ＰＥＮＡＮＡC80lV5H688
HTTP/1.1 200 OK31Please respect copyright.ＰＥＮＡＮＡArKABao8yT
Date: Sat, 23 Nov 2024 06:29:19 GMT31Please respect copyright.ＰＥＮＡＮＡvwFxBfFSs6
accept-ranges: bytes31Please respect copyright.ＰＥＮＡＮＡGEpHk9rolr
content-range: bytes 0-634231201/63423120231Please respect copyright.ＰＥＮＡＮＡkcfnFcGTH5
ETag: "634231202_2732277118"31Please respect copyright.ＰＥＮＡＮＡP9B5CjNJ2u
Content-Disposition: attachment; filename="zen.iso"31Please respect copyright.ＰＥＮＡＮＡ1BGR6Ydqe0
Content-Length: 63423120231Please respect copyright.ＰＥＮＡＮＡyPmFmaALwQ
Content-Type: application/octet-stream
(http response body is the whole file content)31Please respect copyright.ＰＥＮＡＮＡfxqy2MDBG7
------------------------------------------------31Please respect copyright.ＰＥＮＡＮＡtM2ucjkMc3
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:31Please respect copyright.ＰＥＮＡＮＡDay77bQjLl
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
31Please respect copyright.ＰＥＮＡＮＡCvCYuPRzfk
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31Please respect copyright.ＰＥＮＡＮＡBQulqZwWbp
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31Please respect copyright.ＰＥＮＡＮＡz6rMYa7CaM
https://gemini.google.com/
感谢互联网上无私奉献的参考资31Please respect copyright.ＰＥＮＡＮＡOpgVtIPzrD
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests31Please respect copyright.ＰＥＮＡＮＡLJa5aOc0bc
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range31Please respect copyright.ＰＥＮＡＮＡYjul4pz8GV
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag31Please respect copyright.ＰＥＮＡＮＡPKDnsMsg1g
https://www.rfc-editor.org/rfc/rfc723331Please respect copyright.ＰＥＮＡＮＡMrdD4USv4A
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.com31Please respect copyright.ＰＥＮＡＮＡZg8oUAdkim
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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