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:54Please respect copyright.ＰＥＮＡＮＡ9FALiJnsNW
----------http range request-------54Please respect copyright.ＰＥＮＡＮＡzI86KHCA7n
GET /zen.iso HTTP/1.154Please respect copyright.ＰＥＮＡＮＡb8MfjxdXrv
Range: bytes=100-102354Please respect copyright.ＰＥＮＡＮＡzHnTQdsvq0
If-Match: "534231200_1732277116"54Please respect copyright.ＰＥＮＡＮＡ19X62ztbt4
......54Please respect copyright.ＰＥＮＡＮＡPQgInz08E8
-----------------------------------54Please respect copyright.ＰＥＮＡＮＡS9AvMl4Nvg
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:54Please respect copyright.ＰＥＮＡＮＡDbDxIOdfKX
----------------html-----------------54Please respect copyright.ＰＥＮＡＮＡdkvN1OeXWH
<a href="/zen.iso">zen.iso</a>54Please respect copyright.ＰＥＮＡＮＡTUPNApaKja
-------------------------------------54Please respect copyright.ＰＥＮＡＮＡB5waRqKh2k
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:54Please respect copyright.ＰＥＮＡＮＡxDsI3v3WJI
----------get request------------54Please respect copyright.ＰＥＮＡＮＡgvwIJtSs5m
GET /zen.iso HTTP/1.154Please respect copyright.ＰＥＮＡＮＡOCzlwwZvld
......54Please respect copyright.ＰＥＮＡＮＡKZ5bBODmqa
---------------------------------
54Please respect copyright.ＰＥＮＡＮＡ70Vh0JUN52
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:54Please respect copyright.ＰＥＮＡＮＡ8qP1bzqnyd
----------------server response--------------54Please respect copyright.ＰＥＮＡＮＡZsnPX2IFI0
HTTP/1.1 200 OK54Please respect copyright.ＰＥＮＡＮＡhPQN65N9Gq
Date: Sat, 23 Nov 2024 06:29:19 GMT54Please respect copyright.ＰＥＮＡＮＡdD4F7zadaI
accept-ranges: bytes54Please respect copyright.ＰＥＮＡＮＡxu1VFFu6WB
content-range: bytes 0-534231199/53423120054Please respect copyright.ＰＥＮＡＮＡtfHj0AChix
ETag: "534231200_1732277116"54Please respect copyright.ＰＥＮＡＮＡt72Anc3zNC
Content-Disposition: attachment; filename="zen.iso"54Please respect copyright.ＰＥＮＡＮＡaJTRJy6LSA
Content-Length: 53423120054Please respect copyright.ＰＥＮＡＮＡTtdCY9ykOe
Content-Type: application/octet-stream
(http response body is the whole file content)54Please respect copyright.ＰＥＮＡＮＡfuySJpjppO
-------------------------------------------
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:54Please respect copyright.ＰＥＮＡＮＡvy70kZL459
---------------http server send big file content by multiple times--------------54Please respect copyright.ＰＥＮＡＮＡXKsnYoax6f
const int buf_size=20000000; // buf size is about 20M54Please respect copyright.ＰＥＮＡＮＡgbkNV79rwT
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers54Please respect copyright.ＰＥＮＡＮＡt7TV2ewdDl
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 big54Please respect copyright.ＰＥＮＡＮＡOB8bUHow9g
while(there_is_remain_file_content_to_send)54Please respect copyright.ＰＥＮＡＮＡ2PlQxQefI8
{ 54Please respect copyright.ＰＥＮＡＮＡSpbQOsTcl2
read_remain_file_content_to_send_buf(file, send_buf);54Please respect copyright.ＰＥＮＡＮＡ6MhcF4mT1h
send_result = send(client_socket, send_buf);54Please respect copyright.ＰＥＮＡＮＡM2viAdtDrI
if(send_result == fail) return; else continue;54Please respect copyright.ＰＥＮＡＮＡBzuC0VJR6X
}
// a way to make etag for a file using its file size and last modifed time54Please respect copyright.ＰＥＮＡＮＡTlwewfyegM
const char *file_etag(const char *path) 54Please respect copyright.ＰＥＮＡＮＡql1fpfohk0
{54Please respect copyright.ＰＥＮＡＮＡ8t4JBYBYsa
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;54Please respect copyright.ＰＥＮＡＮＡNdY8WySpYf
}54Please respect copyright.ＰＥＮＡＮＡ4hX0wLay9P
-------------------------------------------------54Please respect copyright.ＰＥＮＡＮＡ1QDpgS5l5x
"accept-ranges: bytes" tells client web browser that the http server support range.54Please respect copyright.ＰＥＮＡＮＡhnk7DxDsAl
"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).54Please respect copyright.ＰＥＮＡＮＡJ5fm7OJgel
'ETag: "534231200_1732277116"' tells client web browser the file version.54Please respect copyright.ＰＥＮＡＮＡ9EbxDTffki
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.54Please respect copyright.ＰＥＮＡＮＡKaDwICcavB
"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.54Please respect copyright.ＰＥＮＡＮＡyCHVuyGFbg
"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.54Please respect copyright.ＰＥＮＡＮＡJ696BXD4vs
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.)54Please respect copyright.ＰＥＮＡＮＡ7Qi3GXtF03
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:54Please respect copyright.ＰＥＮＡＮＡbmnNDj60vH
---------resume range request---------54Please respect copyright.ＰＥＮＡＮＡtVM0i2KrTr
GET /zen.iso HTTP/1.154Please respect copyright.ＰＥＮＡＮＡ3ERmYDagqP
Range: bytes=243220480-54Please respect copyright.ＰＥＮＡＮＡ1iPvG6fJXe
If-Match: "534231200_1732277116"54Please respect copyright.ＰＥＮＡＮＡgIsTgjWdNE
......54Please respect copyright.ＰＥＮＡＮＡiYTGv6fNUj
-------------------------------------54Please respect copyright.ＰＥＮＡＮＡ3IBjR8juhp
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.54Please respect copyright.ＰＥＮＡＮＡCyq6XTeoc4
'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:54Please respect copyright.ＰＥＮＡＮＡf2bw92bYTh
-------response content from resume point-------54Please respect copyright.ＰＥＮＡＮＡfnoIm1AY27
HTTP/1.1 206 Partial Content54Please respect copyright.ＰＥＮＡＮＡ4xCsDzbIOz
Date: Sat, 23 Nov 2024 06:56:17 GMT54Please respect copyright.ＰＥＮＡＮＡ8BwANJVIFY
accept-ranges: bytes54Please respect copyright.ＰＥＮＡＮＡgppvuTR9dx
content-range: bytes 243220480-534231199/53423120054Please respect copyright.ＰＥＮＡＮＡCl56mtABRr
Content-Length: 29101072054Please respect copyright.ＰＥＮＡＮＡIpAyjKp749
Content-Type: application/octet-stream54Please respect copyright.ＰＥＮＡＮＡYTeUXKEbAa
Content-Disposition: attachment; filename="zen.iso"54Please respect copyright.ＰＥＮＡＮＡhThWhzSIbF
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)54Please respect copyright.ＰＥＮＡＮＡLxr2qcQ9We
------------------------------------------------54Please respect copyright.ＰＥＮＡＮＡuCP70CfaBd
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:54Please respect copyright.ＰＥＮＡＮＡJLw4iZZ9S4
-------response content from resume point-------54Please respect copyright.ＰＥＮＡＮＡN7zM9T1TRo
HTTP/1.1 200 OK54Please respect copyright.ＰＥＮＡＮＡMeFGaVI4fB
Date: Sat, 23 Nov 2024 06:29:19 GMT54Please respect copyright.ＰＥＮＡＮＡhwznyCSTr6
accept-ranges: bytes54Please respect copyright.ＰＥＮＡＮＡ1Ub6iSQrZR
content-range: bytes 0-634231201/63423120254Please respect copyright.ＰＥＮＡＮＡcCJsLNyM36
ETag: "634231202_2732277118"54Please respect copyright.ＰＥＮＡＮＡUKQZ7ThSpI
Content-Disposition: attachment; filename="zen.iso"54Please respect copyright.ＰＥＮＡＮＡkcN0e4Vsb7
Content-Length: 63423120254Please respect copyright.ＰＥＮＡＮＡh8KUklmuQE
Content-Type: application/octet-stream
(http response body is the whole file content)54Please respect copyright.ＰＥＮＡＮＡofHdALChot
------------------------------------------------54Please respect copyright.ＰＥＮＡＮＡcGiaYmgPTe
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
54Please respect copyright.ＰＥＮＡＮＡKEtMT4zkqa
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:54Please respect copyright.ＰＥＮＡＮＡ53BhDWbYQz
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
54Please respect copyright.ＰＥＮＡＮＡD666uxfFxB
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54Please respect copyright.ＰＥＮＡＮＡRcdHBc8JTY
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54Please respect copyright.ＰＥＮＡＮＡkT4Tk8xxAz
https://gemini.google.com/
感谢互联网上无私奉献的参考资54Please respect copyright.ＰＥＮＡＮＡBSCKp0BOk3
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests54Please respect copyright.ＰＥＮＡＮＡvPBbRhEvyK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range54Please respect copyright.ＰＥＮＡＮＡvkKRqt1fKL
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag54Please respect copyright.ＰＥＮＡＮＡTqQ3DTgrKS
https://www.rfc-editor.org/rfc/rfc723354Please respect copyright.ＰＥＮＡＮＡdwUfZVHASX
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]54Please respect copyright.ＰＥＮＡＮＡQtoRkwB538
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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