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:110Please respect copyright.ＰＥＮＡＮＡEIDmiloMQ0
----------http range request-------110Please respect copyright.ＰＥＮＡＮＡkeZhGCBHuu
GET /zen.iso HTTP/1.1110Please respect copyright.ＰＥＮＡＮＡo1zTbPToJ0
Range: bytes=100-1023110Please respect copyright.ＰＥＮＡＮＡPi7AXob2fF
If-Match: "534231200_1732277116"110Please respect copyright.ＰＥＮＡＮＡYcKJwo4yXU
......110Please respect copyright.ＰＥＮＡＮＡwDCiNvzqGE
-----------------------------------110Please respect copyright.ＰＥＮＡＮＡGXPp0pAXwZ
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:110Please respect copyright.ＰＥＮＡＮＡxdoJivlSqY
----------------html-----------------110Please respect copyright.ＰＥＮＡＮＡGjbriGP0Yj
<a href="/zen.iso">zen.iso</a>110Please respect copyright.ＰＥＮＡＮＡ17FvictUd6
-------------------------------------110Please respect copyright.ＰＥＮＡＮＡQrlVjoEMjv
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:110Please respect copyright.ＰＥＮＡＮＡKTByDuWgpL
----------get request------------110Please respect copyright.ＰＥＮＡＮＡxncWzJPEv1
GET /zen.iso HTTP/1.1110Please respect copyright.ＰＥＮＡＮＡBw9Ap4xCEa
......110Please respect copyright.ＰＥＮＡＮＡnxJdrWQqwk
---------------------------------
110Please respect copyright.ＰＥＮＡＮＡ3dsEKDRiCV
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:110Please respect copyright.ＰＥＮＡＮＡuyT860cmtV
----------------server response--------------110Please respect copyright.ＰＥＮＡＮＡ2l5Ol5CJR2
HTTP/1.1 200 OK110Please respect copyright.ＰＥＮＡＮＡISb4LyObc2
Date: Sat, 23 Nov 2024 06:29:19 GMT110Please respect copyright.ＰＥＮＡＮＡi1QR5TTD9m
accept-ranges: bytes110Please respect copyright.ＰＥＮＡＮＡVt7u1HGWwP
content-range: bytes 0-534231199/534231200110Please respect copyright.ＰＥＮＡＮＡYxaL8eIgtg
ETag: "534231200_1732277116"110Please respect copyright.ＰＥＮＡＮＡhDdTiEwHsE
Content-Disposition: attachment; filename="zen.iso"110Please respect copyright.ＰＥＮＡＮＡxmUq7rW7f8
Content-Length: 534231200110Please respect copyright.ＰＥＮＡＮＡoGvczkCjJd
Content-Type: application/octet-stream
(http response body is the whole file content)110Please respect copyright.ＰＥＮＡＮＡ7adUbNZEZh
-------------------------------------------
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:110Please respect copyright.ＰＥＮＡＮＡdoNaMbP5Sd
---------------http server send big file content by multiple times--------------110Please respect copyright.ＰＥＮＡＮＡBYNGgzCmhI
const int buf_size=20000000; // buf size is about 20M110Please respect copyright.ＰＥＮＡＮＡOPVER5u7B8
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers110Please respect copyright.ＰＥＮＡＮＡ2adD9Z0rWI
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 big110Please respect copyright.ＰＥＮＡＮＡLCP6lG2rqx
while(there_is_remain_file_content_to_send)110Please respect copyright.ＰＥＮＡＮＡx9Zpm5pi48
{ 110Please respect copyright.ＰＥＮＡＮＡN7sQTXOEY4
read_remain_file_content_to_send_buf(file, send_buf);110Please respect copyright.ＰＥＮＡＮＡpZaazoGScr
send_result = send(client_socket, send_buf);110Please respect copyright.ＰＥＮＡＮＡGG0BXecCr9
if(send_result == fail) return; else continue;110Please respect copyright.ＰＥＮＡＮＡTp49ZkStJ1
}
// a way to make etag for a file using its file size and last modifed time110Please respect copyright.ＰＥＮＡＮＡUe4WxEaXS9
const char *file_etag(const char *path) 110Please respect copyright.ＰＥＮＡＮＡSsv8t0V8NJ
{110Please respect copyright.ＰＥＮＡＮＡIQvDDKGcWi
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;110Please respect copyright.ＰＥＮＡＮＡbWSp28mAQI
}110Please respect copyright.ＰＥＮＡＮＡMihxf1k456
-------------------------------------------------110Please respect copyright.ＰＥＮＡＮＡROKCIzLm5p
"accept-ranges: bytes" tells client web browser that the http server support range.110Please respect copyright.ＰＥＮＡＮＡ7NcXuTutzS
"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).110Please respect copyright.ＰＥＮＡＮＡjLpQUCgbpj
'ETag: "534231200_1732277116"' tells client web browser the file version.110Please respect copyright.ＰＥＮＡＮＡic3RDkbhH6
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.110Please respect copyright.ＰＥＮＡＮＡsSubDVjrIm
"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.110Please respect copyright.ＰＥＮＡＮＡyJwK2BFHUU
"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.110Please respect copyright.ＰＥＮＡＮＡV52m6KPqUI
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.)110Please respect copyright.ＰＥＮＡＮＡ53nTeIARtQ
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:110Please respect copyright.ＰＥＮＡＮＡbEwmFGJ6Me
---------resume range request---------110Please respect copyright.ＰＥＮＡＮＡCIy5CzRtcW
GET /zen.iso HTTP/1.1110Please respect copyright.ＰＥＮＡＮＡE0DNaYsm1w
Range: bytes=243220480-110Please respect copyright.ＰＥＮＡＮＡdjHZEa3O90
If-Match: "534231200_1732277116"110Please respect copyright.ＰＥＮＡＮＡgjKcJgoNy3
......110Please respect copyright.ＰＥＮＡＮＡYcTPSany8v
-------------------------------------110Please respect copyright.ＰＥＮＡＮＡmCmw8aZwkU
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.110Please respect copyright.ＰＥＮＡＮＡpCiu5EG2RH
'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:110Please respect copyright.ＰＥＮＡＮＡ0wPkZPPEhR
-------response content from resume point-------110Please respect copyright.ＰＥＮＡＮＡIk3B6sPJej
HTTP/1.1 206 Partial Content110Please respect copyright.ＰＥＮＡＮＡBCYPy4L078
Date: Sat, 23 Nov 2024 06:56:17 GMT110Please respect copyright.ＰＥＮＡＮＡr6fxBJHH7N
accept-ranges: bytes110Please respect copyright.ＰＥＮＡＮＡrljRhnW87c
content-range: bytes 243220480-534231199/534231200110Please respect copyright.ＰＥＮＡＮＡiu00ZueF91
Content-Length: 291010720110Please respect copyright.ＰＥＮＡＮＡs24Dco5SQH
Content-Type: application/octet-stream110Please respect copyright.ＰＥＮＡＮＡOqi0ibEC9d
Content-Disposition: attachment; filename="zen.iso"110Please respect copyright.ＰＥＮＡＮＡQiy6fyhxVh
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)110Please respect copyright.ＰＥＮＡＮＡrPOAM2Ke0J
------------------------------------------------110Please respect copyright.ＰＥＮＡＮＡCQD68ZVzbh
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:110Please respect copyright.ＰＥＮＡＮＡd1fZUqISOy
-------response content from resume point-------110Please respect copyright.ＰＥＮＡＮＡhOTZlAh4ib
HTTP/1.1 200 OK110Please respect copyright.ＰＥＮＡＮＡULcsDkMbGV
Date: Sat, 23 Nov 2024 06:29:19 GMT110Please respect copyright.ＰＥＮＡＮＡzyX2NRutVB
accept-ranges: bytes110Please respect copyright.ＰＥＮＡＮＡ07qv9JTF6U
content-range: bytes 0-634231201/634231202110Please respect copyright.ＰＥＮＡＮＡQSkUmfFKyy
ETag: "634231202_2732277118"110Please respect copyright.ＰＥＮＡＮＡynBWoByIEs
Content-Disposition: attachment; filename="zen.iso"110Please respect copyright.ＰＥＮＡＮＡY3J9AHkK3c
Content-Length: 634231202110Please respect copyright.ＰＥＮＡＮＡnzk4GtYJUg
Content-Type: application/octet-stream
(http response body is the whole file content)110Please respect copyright.ＰＥＮＡＮＡWyXqf8GbeT
------------------------------------------------110Please respect copyright.ＰＥＮＡＮＡEhCrjCUXTC
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
110Please respect copyright.ＰＥＮＡＮＡLcZ74nZeO0
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:110Please respect copyright.ＰＥＮＡＮＡEoyRh8NqxT
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
110Please respect copyright.ＰＥＮＡＮＡTYQ7ruVZg9
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110Please respect copyright.ＰＥＮＡＮＡPF5pCOlXuf
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110Please respect copyright.ＰＥＮＡＮＡ2cs0sTj1vU
https://gemini.google.com/
感谢互联网上无私奉献的参考资110Please respect copyright.ＰＥＮＡＮＡAhyjvXUbFH
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests110Please respect copyright.ＰＥＮＡＮＡGSWL4Pleqn
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range110Please respect copyright.ＰＥＮＡＮＡms0pWqqOCG
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag110Please respect copyright.ＰＥＮＡＮＡwGJ7fOvGz5
https://www.rfc-editor.org/rfc/rfc7233110Please respect copyright.ＰＥＮＡＮＡD0VsocsQK6
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]110Please respect copyright.ＰＥＮＡＮＡ4opLsK1suP
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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