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:157Please respect copyright.ＰＥＮＡＮＡm3cUYbEnKX
----------http range request-------157Please respect copyright.ＰＥＮＡＮＡTMv5CGyfyx
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡx2yyicslqc
Range: bytes=100-1023157Please respect copyright.ＰＥＮＡＮＡjV8tbBzY0c
If-Match: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡJo632H3OOn
......157Please respect copyright.ＰＥＮＡＮＡUE32qzqwQD
-----------------------------------157Please respect copyright.ＰＥＮＡＮＡaVvhsRoeqG
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:157Please respect copyright.ＰＥＮＡＮＡLN4XQKhb8u
----------------html-----------------157Please respect copyright.ＰＥＮＡＮＡR4hBWy4ChL
<a href="/zen.iso">zen.iso</a>157Please respect copyright.ＰＥＮＡＮＡJ7aHQixuTJ
-------------------------------------157Please respect copyright.ＰＥＮＡＮＡaHFHBTw4Zc
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:157Please respect copyright.ＰＥＮＡＮＡqteopSUDEt
----------get request------------157Please respect copyright.ＰＥＮＡＮＡQ18hBuBhyG
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡnt8BvZAdxR
......157Please respect copyright.ＰＥＮＡＮＡdBouUP5tjq
---------------------------------
157Please respect copyright.ＰＥＮＡＮＡ9iNpAkeYNq
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:157Please respect copyright.ＰＥＮＡＮＡWJkDQaJDnq
----------------server response--------------157Please respect copyright.ＰＥＮＡＮＡqygomsTLt6
HTTP/1.1 200 OK157Please respect copyright.ＰＥＮＡＮＡeKpcWRrXp6
Date: Sat, 23 Nov 2024 06:29:19 GMT157Please respect copyright.ＰＥＮＡＮＡDrznPQB07V
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡJlmpeJmtou
content-range: bytes 0-534231199/534231200157Please respect copyright.ＰＥＮＡＮＡeFajGSqY48
ETag: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡP5jupjey3x
Content-Disposition: attachment; filename="zen.iso"157Please respect copyright.ＰＥＮＡＮＡw6iM184R7T
Content-Length: 534231200157Please respect copyright.ＰＥＮＡＮＡ8XJwjDrDDm
Content-Type: application/octet-stream
(http response body is the whole file content)157Please respect copyright.ＰＥＮＡＮＡ7owixhl1zt
-------------------------------------------
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:157Please respect copyright.ＰＥＮＡＮＡetDS50MRV2
---------------http server send big file content by multiple times--------------157Please respect copyright.ＰＥＮＡＮＡohmvIXUPhb
const int buf_size=20000000; // buf size is about 20M157Please respect copyright.ＰＥＮＡＮＡ0L9uhomwMP
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers157Please respect copyright.ＰＥＮＡＮＡdSWKMqTJeq
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 big157Please respect copyright.ＰＥＮＡＮＡuTtRnTCIDa
while(there_is_remain_file_content_to_send)157Please respect copyright.ＰＥＮＡＮＡ8JbvymTDFu
{ 157Please respect copyright.ＰＥＮＡＮＡBzIamiB1jB
read_remain_file_content_to_send_buf(file, send_buf);157Please respect copyright.ＰＥＮＡＮＡZVbVB7FiXO
send_result = send(client_socket, send_buf);157Please respect copyright.ＰＥＮＡＮＡQkNvJNhqUd
if(send_result == fail) return; else continue;157Please respect copyright.ＰＥＮＡＮＡJJcBxdxvj6
}
// a way to make etag for a file using its file size and last modifed time157Please respect copyright.ＰＥＮＡＮＡYMjekkPtBd
const char *file_etag(const char *path) 157Please respect copyright.ＰＥＮＡＮＡ44avkjAR82
{157Please respect copyright.ＰＥＮＡＮＡ9wkkvBKz7g
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;157Please respect copyright.ＰＥＮＡＮＡPsa4NK2zw3
}157Please respect copyright.ＰＥＮＡＮＡhrdFr8COxp
-------------------------------------------------157Please respect copyright.ＰＥＮＡＮＡnTYKk4uIWc
"accept-ranges: bytes" tells client web browser that the http server support range.157Please respect copyright.ＰＥＮＡＮＡ3rhLw5R2VD
"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).157Please respect copyright.ＰＥＮＡＮＡ7t0rwh5dXM
'ETag: "534231200_1732277116"' tells client web browser the file version.157Please respect copyright.ＰＥＮＡＮＡzuSEjm9piz
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.157Please respect copyright.ＰＥＮＡＮＡubkHGsWTnz
"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.157Please respect copyright.ＰＥＮＡＮＡBuNQOehMLB
"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.157Please respect copyright.ＰＥＮＡＮＡnJKPqfDRJC
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.)157Please respect copyright.ＰＥＮＡＮＡmNbmuiaQ9k
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:157Please respect copyright.ＰＥＮＡＮＡvuyIhCLjbG
---------resume range request---------157Please respect copyright.ＰＥＮＡＮＡh0bS2reOxD
GET /zen.iso HTTP/1.1157Please respect copyright.ＰＥＮＡＮＡDamhfGLrlo
Range: bytes=243220480-157Please respect copyright.ＰＥＮＡＮＡIJBTa6Q1zJ
If-Match: "534231200_1732277116"157Please respect copyright.ＰＥＮＡＮＡwdrKGumm59
......157Please respect copyright.ＰＥＮＡＮＡttUp0gIXRJ
-------------------------------------157Please respect copyright.ＰＥＮＡＮＡTwLdrLEXeL
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.157Please respect copyright.ＰＥＮＡＮＡGPDkKWKrX8
'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:157Please respect copyright.ＰＥＮＡＮＡzstiPr3YBZ
-------response content from resume point-------157Please respect copyright.ＰＥＮＡＮＡYBvF7fovW6
HTTP/1.1 206 Partial Content157Please respect copyright.ＰＥＮＡＮＡxxIe0dpJCg
Date: Sat, 23 Nov 2024 06:56:17 GMT157Please respect copyright.ＰＥＮＡＮＡr40b4I8cPL
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡHvtxBtAHvV
content-range: bytes 243220480-534231199/534231200157Please respect copyright.ＰＥＮＡＮＡiPhUUzH7zM
Content-Length: 291010720157Please respect copyright.ＰＥＮＡＮＡUjWta0lT28
Content-Type: application/octet-stream157Please respect copyright.ＰＥＮＡＮＡbFeiUbWkKa
Content-Disposition: attachment; filename="zen.iso"157Please respect copyright.ＰＥＮＡＮＡkGPyDdEy23
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)157Please respect copyright.ＰＥＮＡＮＡWFWFHo9tUW
------------------------------------------------157Please respect copyright.ＰＥＮＡＮＡoXUPnLikZU
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:157Please respect copyright.ＰＥＮＡＮＡmXbVx6YbJD
-------response content from resume point-------157Please respect copyright.ＰＥＮＡＮＡqXhDbSMOcY
HTTP/1.1 200 OK157Please respect copyright.ＰＥＮＡＮＡManvVf06LZ
Date: Sat, 23 Nov 2024 06:29:19 GMT157Please respect copyright.ＰＥＮＡＮＡOxoClFPeGz
accept-ranges: bytes157Please respect copyright.ＰＥＮＡＮＡ8DZCnADCNK
content-range: bytes 0-634231201/634231202157Please respect copyright.ＰＥＮＡＮＡDU7AFHok6n
ETag: "634231202_2732277118"157Please respect copyright.ＰＥＮＡＮＡssbNipxWfd
Content-Disposition: attachment; filename="zen.iso"157Please respect copyright.ＰＥＮＡＮＡPlhVWZ0qEn
Content-Length: 634231202157Please respect copyright.ＰＥＮＡＮＡi2u0ISIsQW
Content-Type: application/octet-stream
(http response body is the whole file content)157Please respect copyright.ＰＥＮＡＮＡUFtXpkLP2V
------------------------------------------------157Please respect copyright.ＰＥＮＡＮＡNbFnbgDKUU
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:157Please respect copyright.ＰＥＮＡＮＡPffnPlg2ps
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
157Please respect copyright.ＰＥＮＡＮＡkzkEUe59w7
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157Please respect copyright.ＰＥＮＡＮＡ7YlNGbaKeR
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157Please respect copyright.ＰＥＮＡＮＡj7WLUmf5W4
https://gemini.google.com/
感谢互联网上无私奉献的参考资157Please respect copyright.ＰＥＮＡＮＡrH7dhBsdEm
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests157Please respect copyright.ＰＥＮＡＮＡ3Oy6PH4iAQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range157Please respect copyright.ＰＥＮＡＮＡJzd2xh5KOZ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag157Please respect copyright.ＰＥＮＡＮＡzGcwslZ4qg
https://www.rfc-editor.org/rfc/rfc7233157Please respect copyright.ＰＥＮＡＮＡXUfnmGZkUd
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]157Please respect copyright.ＰＥＮＡＮＡDUejckg1wU
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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