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:205Please respect copyright.ＰＥＮＡＮＡbGN283EqWo
----------http range request-------205Please respect copyright.ＰＥＮＡＮＡiTWCtzpNYe
GET /zen.iso HTTP/1.1205Please respect copyright.ＰＥＮＡＮＡhvAYF4n2eW
Range: bytes=100-1023205Please respect copyright.ＰＥＮＡＮＡIZCb4bhjKx
If-Match: "534231200_1732277116"205Please respect copyright.ＰＥＮＡＮＡz7owDfcwwU
......205Please respect copyright.ＰＥＮＡＮＡVqnnQzsFPw
-----------------------------------205Please respect copyright.ＰＥＮＡＮＡ7cue7AVnya
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:205Please respect copyright.ＰＥＮＡＮＡiKrTsW7KKV
----------------html-----------------205Please respect copyright.ＰＥＮＡＮＡmvmwUK61jI
<a href="/zen.iso">zen.iso</a>205Please respect copyright.ＰＥＮＡＮＡtPw8oqcD4o
-------------------------------------205Please respect copyright.ＰＥＮＡＮＡ5TerfjMOA8
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:205Please respect copyright.ＰＥＮＡＮＡM4vQqFJ61u
----------get request------------205Please respect copyright.ＰＥＮＡＮＡ3xD78nrY2H
GET /zen.iso HTTP/1.1205Please respect copyright.ＰＥＮＡＮＡbNAdwj2mz8
......205Please respect copyright.ＰＥＮＡＮＡVBP7P6XmpP
---------------------------------
205Please respect copyright.ＰＥＮＡＮＡPRIX36uZ4Q
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:205Please respect copyright.ＰＥＮＡＮＡuZKLBOcz7t
----------------server response--------------205Please respect copyright.ＰＥＮＡＮＡ9FPKDJxxlV
HTTP/1.1 200 OK205Please respect copyright.ＰＥＮＡＮＡycB0K87DqA
Date: Sat, 23 Nov 2024 06:29:19 GMT205Please respect copyright.ＰＥＮＡＮＡe5VUy6d1UP
accept-ranges: bytes205Please respect copyright.ＰＥＮＡＮＡ6YZIkO7I04
content-range: bytes 0-534231199/534231200205Please respect copyright.ＰＥＮＡＮＡouVWmQVC3y
ETag: "534231200_1732277116"205Please respect copyright.ＰＥＮＡＮＡM1LD0WTK8Z
Content-Disposition: attachment; filename="zen.iso"205Please respect copyright.ＰＥＮＡＮＡsYNt5ke93I
Content-Length: 534231200205Please respect copyright.ＰＥＮＡＮＡ33ekRcqnRL
Content-Type: application/octet-stream
(http response body is the whole file content)205Please respect copyright.ＰＥＮＡＮＡeNjnpmfPxl
-------------------------------------------
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:205Please respect copyright.ＰＥＮＡＮＡQZzASjcEcj
---------------http server send big file content by multiple times--------------205Please respect copyright.ＰＥＮＡＮＡ1SZ3y0ABbr
const int buf_size=20000000; // buf size is about 20M205Please respect copyright.ＰＥＮＡＮＡOfljGGSIbZ
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers205Please respect copyright.ＰＥＮＡＮＡgZTqExBe0M
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 big205Please respect copyright.ＰＥＮＡＮＡV57h4IiJLe
while(there_is_remain_file_content_to_send)205Please respect copyright.ＰＥＮＡＮＡoCkLpg6ar8
{ 205Please respect copyright.ＰＥＮＡＮＡViBrzQxAXF
read_remain_file_content_to_send_buf(file, send_buf);205Please respect copyright.ＰＥＮＡＮＡq46q7xgBzn
send_result = send(client_socket, send_buf);205Please respect copyright.ＰＥＮＡＮＡJuhWsTkCJA
if(send_result == fail) return; else continue;205Please respect copyright.ＰＥＮＡＮＡ4rqSkMmLC2
}
// a way to make etag for a file using its file size and last modifed time205Please respect copyright.ＰＥＮＡＮＡoHUSdHjQoH
const char *file_etag(const char *path) 205Please respect copyright.ＰＥＮＡＮＡA42YPKlcT9
{205Please respect copyright.ＰＥＮＡＮＡ2kWez3E9WU
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;205Please respect copyright.ＰＥＮＡＮＡijwp7NBDvZ
}205Please respect copyright.ＰＥＮＡＮＡIy9QlPfU77
-------------------------------------------------205Please respect copyright.ＰＥＮＡＮＡxthVEApFih
"accept-ranges: bytes" tells client web browser that the http server support range.205Please respect copyright.ＰＥＮＡＮＡLZJ5UNashM
"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).205Please respect copyright.ＰＥＮＡＮＡYqpci95PFx
'ETag: "534231200_1732277116"' tells client web browser the file version.205Please respect copyright.ＰＥＮＡＮＡsOnRLonjeJ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.205Please respect copyright.ＰＥＮＡＮＡu6yCWVibTE
"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.205Please respect copyright.ＰＥＮＡＮＡ2eEPEPGaVl
"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.205Please respect copyright.ＰＥＮＡＮＡyJdgZ4Oybh
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.)205Please respect copyright.ＰＥＮＡＮＡDP6cI6hSwM
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:205Please respect copyright.ＰＥＮＡＮＡSOiRbvH6Zz
---------resume range request---------205Please respect copyright.ＰＥＮＡＮＡpb5fCdmrhj
GET /zen.iso HTTP/1.1205Please respect copyright.ＰＥＮＡＮＡ1lQY3bDZse
Range: bytes=243220480-205Please respect copyright.ＰＥＮＡＮＡNLHczNpKTr
If-Match: "534231200_1732277116"205Please respect copyright.ＰＥＮＡＮＡuKN3x0vaD9
......205Please respect copyright.ＰＥＮＡＮＡrQJ1CScuf7
-------------------------------------205Please respect copyright.ＰＥＮＡＮＡr9AaJzudOy
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.205Please respect copyright.ＰＥＮＡＮＡBk3N7KLT6U
'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:205Please respect copyright.ＰＥＮＡＮＡFsWvpIg3Z6
-------response content from resume point-------205Please respect copyright.ＰＥＮＡＮＡBHB59bFLN2
HTTP/1.1 206 Partial Content205Please respect copyright.ＰＥＮＡＮＡmR6nmebJ5y
Date: Sat, 23 Nov 2024 06:56:17 GMT205Please respect copyright.ＰＥＮＡＮＡJ08NUajXNO
accept-ranges: bytes205Please respect copyright.ＰＥＮＡＮＡjxfMfIUmtZ
content-range: bytes 243220480-534231199/534231200205Please respect copyright.ＰＥＮＡＮＡdkG51QLnG9
Content-Length: 291010720205Please respect copyright.ＰＥＮＡＮＡYMIO8Y0d9r
Content-Type: application/octet-stream205Please respect copyright.ＰＥＮＡＮＡwlBXDTMcXO
Content-Disposition: attachment; filename="zen.iso"205Please respect copyright.ＰＥＮＡＮＡSn50kzUyqY
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)205Please respect copyright.ＰＥＮＡＮＡWhhL1uFzbk
------------------------------------------------205Please respect copyright.ＰＥＮＡＮＡtUpxc4Wqh4
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:205Please respect copyright.ＰＥＮＡＮＡZT7fWkRdR6
-------response content from resume point-------205Please respect copyright.ＰＥＮＡＮＡXJyPrLRSAa
HTTP/1.1 200 OK205Please respect copyright.ＰＥＮＡＮＡz68QwpjumV
Date: Sat, 23 Nov 2024 06:29:19 GMT205Please respect copyright.ＰＥＮＡＮＡcNO6yAByr6
accept-ranges: bytes205Please respect copyright.ＰＥＮＡＮＡXcWNa9elFz
content-range: bytes 0-634231201/634231202205Please respect copyright.ＰＥＮＡＮＡdc3c5NlXgv
ETag: "634231202_2732277118"205Please respect copyright.ＰＥＮＡＮＡ2MkIDGBMAm
Content-Disposition: attachment; filename="zen.iso"205Please respect copyright.ＰＥＮＡＮＡLMk5jXoHPC
Content-Length: 634231202205Please respect copyright.ＰＥＮＡＮＡUf5BjJ1miH
Content-Type: application/octet-stream
(http response body is the whole file content)205Please respect copyright.ＰＥＮＡＮＡlLIqIsuIfl
------------------------------------------------205Please respect copyright.ＰＥＮＡＮＡ8Gs8sxmzeu
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:205Please respect copyright.ＰＥＮＡＮＡ5lKO0kCqZG
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
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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205Please respect copyright.ＰＥＮＡＮＡxseL73Gs1T
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205Please respect copyright.ＰＥＮＡＮＡ9YBK3CZrdt
https://gemini.google.com/
感谢互联网上无私奉献的参考资205Please respect copyright.ＰＥＮＡＮＡGpnTc1VmY9
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests205Please respect copyright.ＰＥＮＡＮＡKcgeiwXhcd
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range205Please respect copyright.ＰＥＮＡＮＡ29Xz3rUC3J
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag205Please respect copyright.ＰＥＮＡＮＡ4c4Ti8rpvu
https://www.rfc-editor.org/rfc/rfc7233205Please respect copyright.ＰＥＮＡＮＡSqggjhWxV7
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]205Please respect copyright.ＰＥＮＡＮＡk3fBJo4kOG
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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