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:195Please respect copyright.ＰＥＮＡＮＡ5qqjWPXt8x
----------http range request-------195Please respect copyright.ＰＥＮＡＮＡCcnW8s10OY
GET /zen.iso HTTP/1.1195Please respect copyright.ＰＥＮＡＮＡSNS6xSRk6y
Range: bytes=100-1023195Please respect copyright.ＰＥＮＡＮＡfABCkoWhLG
If-Match: "534231200_1732277116"195Please respect copyright.ＰＥＮＡＮＡ4HLeWUWXF7
......195Please respect copyright.ＰＥＮＡＮＡGjXnFK4m5J
-----------------------------------195Please respect copyright.ＰＥＮＡＮＡzZWGUPNwmU
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:195Please respect copyright.ＰＥＮＡＮＡJFHBxCntnm
----------------html-----------------195Please respect copyright.ＰＥＮＡＮＡEZFWIZ4U3G
<a href="/zen.iso">zen.iso</a>195Please respect copyright.ＰＥＮＡＮＡ2jqTwiSiNu
-------------------------------------195Please respect copyright.ＰＥＮＡＮＡmiAeEwPyDg
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:195Please respect copyright.ＰＥＮＡＮＡnqYVmF7jC9
----------get request------------195Please respect copyright.ＰＥＮＡＮＡ3ieUSlotI2
GET /zen.iso HTTP/1.1195Please respect copyright.ＰＥＮＡＮＡYuMwEyxMpD
......195Please respect copyright.ＰＥＮＡＮＡRCDx9233lx
---------------------------------
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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:195Please respect copyright.ＰＥＮＡＮＡbWiEAKILwk
----------------server response--------------195Please respect copyright.ＰＥＮＡＮＡc83hHqrJWj
HTTP/1.1 200 OK195Please respect copyright.ＰＥＮＡＮＡ8wtIZzCC5h
Date: Sat, 23 Nov 2024 06:29:19 GMT195Please respect copyright.ＰＥＮＡＮＡpQ2AD0GWZ2
accept-ranges: bytes195Please respect copyright.ＰＥＮＡＮＡWfhtNqhcMN
content-range: bytes 0-534231199/534231200195Please respect copyright.ＰＥＮＡＮＡrGPFaDGEk2
ETag: "534231200_1732277116"195Please respect copyright.ＰＥＮＡＮＡCxtii5F3CC
Content-Disposition: attachment; filename="zen.iso"195Please respect copyright.ＰＥＮＡＮＡhWUE8xq0eo
Content-Length: 534231200195Please respect copyright.ＰＥＮＡＮＡvRVPvO1uHI
Content-Type: application/octet-stream
(http response body is the whole file content)195Please respect copyright.ＰＥＮＡＮＡ33SR31PFqt
-------------------------------------------
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:195Please respect copyright.ＰＥＮＡＮＡQ2I3RMYk1A
---------------http server send big file content by multiple times--------------195Please respect copyright.ＰＥＮＡＮＡQP1JRBghOI
const int buf_size=20000000; // buf size is about 20M195Please respect copyright.ＰＥＮＡＮＡV1ed4b56dk
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers195Please respect copyright.ＰＥＮＡＮＡIJuLOKRuwR
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 big195Please respect copyright.ＰＥＮＡＮＡ1bpDP0o76u
while(there_is_remain_file_content_to_send)195Please respect copyright.ＰＥＮＡＮＡC8OCayGaMg
{ 195Please respect copyright.ＰＥＮＡＮＡwm8ZfeoVc0
read_remain_file_content_to_send_buf(file, send_buf);195Please respect copyright.ＰＥＮＡＮＡZEV78AMLhN
send_result = send(client_socket, send_buf);195Please respect copyright.ＰＥＮＡＮＡdXqrVeI8ll
if(send_result == fail) return; else continue;195Please respect copyright.ＰＥＮＡＮＡ4BaiEIaDYN
}
// a way to make etag for a file using its file size and last modifed time195Please respect copyright.ＰＥＮＡＮＡPGZeUa6Nmv
const char *file_etag(const char *path) 195Please respect copyright.ＰＥＮＡＮＡ9Mummmkztt
{195Please respect copyright.ＰＥＮＡＮＡ6QxLYIEwQY
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;195Please respect copyright.ＰＥＮＡＮＡHaQZrnKvBN
}195Please respect copyright.ＰＥＮＡＮＡr4F1eH24A1
-------------------------------------------------195Please respect copyright.ＰＥＮＡＮＡOc5V1NhXVm
"accept-ranges: bytes" tells client web browser that the http server support range.195Please respect copyright.ＰＥＮＡＮＡOD15v5IdW1
"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).195Please respect copyright.ＰＥＮＡＮＡ63bLj8kCl2
'ETag: "534231200_1732277116"' tells client web browser the file version.195Please respect copyright.ＰＥＮＡＮＡR8ykjtiEuM
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.195Please respect copyright.ＰＥＮＡＮＡC88gvA0lkO
"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.195Please respect copyright.ＰＥＮＡＮＡJdYE68msFU
"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.195Please respect copyright.ＰＥＮＡＮＡRazdTI3W9J
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.)195Please respect copyright.ＰＥＮＡＮＡweoF8x0hrV
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:195Please respect copyright.ＰＥＮＡＮＡypdmLinV3u
---------resume range request---------195Please respect copyright.ＰＥＮＡＮＡAYhW3XzYWs
GET /zen.iso HTTP/1.1195Please respect copyright.ＰＥＮＡＮＡHskcG8tm9h
Range: bytes=243220480-195Please respect copyright.ＰＥＮＡＮＡ8isl9BC7bs
If-Match: "534231200_1732277116"195Please respect copyright.ＰＥＮＡＮＡ4tcIbBocSa
......195Please respect copyright.ＰＥＮＡＮＡzmKFiOMYkG
-------------------------------------195Please respect copyright.ＰＥＮＡＮＡwp4gzvCohf
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.195Please respect copyright.ＰＥＮＡＮＡzRvPdtccS8
'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:195Please respect copyright.ＰＥＮＡＮＡF8zpgpARQf
-------response content from resume point-------195Please respect copyright.ＰＥＮＡＮＡ5UZ5tkvDAS
HTTP/1.1 206 Partial Content195Please respect copyright.ＰＥＮＡＮＡ7gQAkz5lLp
Date: Sat, 23 Nov 2024 06:56:17 GMT195Please respect copyright.ＰＥＮＡＮＡTR53tvRSxi
accept-ranges: bytes195Please respect copyright.ＰＥＮＡＮＡ4qPOk8vlMT
content-range: bytes 243220480-534231199/534231200195Please respect copyright.ＰＥＮＡＮＡNx6adc93e7
Content-Length: 291010720195Please respect copyright.ＰＥＮＡＮＡ2PkalOU775
Content-Type: application/octet-stream195Please respect copyright.ＰＥＮＡＮＡK1W2vGgty8
Content-Disposition: attachment; filename="zen.iso"195Please respect copyright.ＰＥＮＡＮＡvzZnk3x38J
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)195Please respect copyright.ＰＥＮＡＮＡcsvgjX9nsM
------------------------------------------------195Please respect copyright.ＰＥＮＡＮＡYacTMjV4oh
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:195Please respect copyright.ＰＥＮＡＮＡx2BDSHmJ0Z
-------response content from resume point-------195Please respect copyright.ＰＥＮＡＮＡAaToMVhyto
HTTP/1.1 200 OK195Please respect copyright.ＰＥＮＡＮＡLyVvZri7eh
Date: Sat, 23 Nov 2024 06:29:19 GMT195Please respect copyright.ＰＥＮＡＮＡ8bjwuL9c9v
accept-ranges: bytes195Please respect copyright.ＰＥＮＡＮＡs8S6BvkBxj
content-range: bytes 0-634231201/634231202195Please respect copyright.ＰＥＮＡＮＡxha0UEb2Zt
ETag: "634231202_2732277118"195Please respect copyright.ＰＥＮＡＮＡh4UGrhZLAs
Content-Disposition: attachment; filename="zen.iso"195Please respect copyright.ＰＥＮＡＮＡkSf4SIq5PU
Content-Length: 634231202195Please respect copyright.ＰＥＮＡＮＡV3MqN3jONg
Content-Type: application/octet-stream
(http response body is the whole file content)195Please respect copyright.ＰＥＮＡＮＡDanPLpLbKj
------------------------------------------------195Please respect copyright.ＰＥＮＡＮＡLEPPTFqdIh
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:195Please respect copyright.ＰＥＮＡＮＡfcTsofABwB
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
195Please respect copyright.ＰＥＮＡＮＡaTIFaD5mEa
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195Please respect copyright.ＰＥＮＡＮＡGwlQSFmuW4
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195Please respect copyright.ＰＥＮＡＮＡ1QsFsEwcl4
https://gemini.google.com/
感谢互联网上无私奉献的参考资195Please respect copyright.ＰＥＮＡＮＡIeUrPc04ty
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests195Please respect copyright.ＰＥＮＡＮＡUx5GAm0wet
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range195Please respect copyright.ＰＥＮＡＮＡgCzW8xk4hF
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag195Please respect copyright.ＰＥＮＡＮＡFKXHF553rY
https://www.rfc-editor.org/rfc/rfc7233195Please respect copyright.ＰＥＮＡＮＡOVfK1f5rRc
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]195Please respect copyright.ＰＥＮＡＮＡBQsUDU9LUI
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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