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:165Please respect copyright.PENANArj0qFIhtly
----------http range request-------165Please respect copyright.PENANA5dfDrCD7gR
GET /zen.iso HTTP/1.1165Please respect copyright.PENANA7m9EaWRGtY
Range: bytes=100-1023165Please respect copyright.PENANAfG7tERcWIv
If-Match: "534231200_1732277116"165Please respect copyright.PENANAvnPZELUQbH
......165Please respect copyright.PENANAQoLfSsQf2P
-----------------------------------165Please respect copyright.PENANAdNO1Ng7H9F
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:165Please respect copyright.PENANAAiuUnjzAju
----------------html-----------------165Please respect copyright.PENANAoGvhrswH41
<a href="/zen.iso">zen.iso</a>165Please respect copyright.PENANATjxfdfmq0w
-------------------------------------165Please respect copyright.PENANAmb7EAhC7xW
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:165Please respect copyright.PENANA1ScHfamteb
----------get request------------165Please respect copyright.PENANAJUCYvDtT1x
GET /zen.iso HTTP/1.1165Please respect copyright.PENANAbY3JwT8G7O
......165Please respect copyright.PENANAKx7i2I85Eg
---------------------------------
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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:165Please respect copyright.PENANA0nEWFpEXGd
----------------server response--------------165Please respect copyright.PENANA3miJlKGMcO
HTTP/1.1 200 OK165Please respect copyright.PENANAwycEFwiT5c
Date: Sat, 23 Nov 2024 06:29:19 GMT165Please respect copyright.PENANAzM4YpsO2PS
accept-ranges: bytes165Please respect copyright.PENANA8x84mo44kt
content-range: bytes 0-534231199/534231200165Please respect copyright.PENANAjhfIURzSbt
ETag: "534231200_1732277116"165Please respect copyright.PENANAe9TWYH134l
Content-Disposition: attachment; filename="zen.iso"165Please respect copyright.PENANAYh7na3kxQb
Content-Length: 534231200165Please respect copyright.PENANAKAeZQoIQ3L
Content-Type: application/octet-stream
(http response body is the whole file content)165Please respect copyright.PENANAEFIIfso9EQ
-------------------------------------------
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:165Please respect copyright.PENANAE5FqZDTKNS
---------------http server send big file content by multiple times--------------165Please respect copyright.PENANAQGPAhgfqqp
const int buf_size=20000000; // buf size is about 20M165Please respect copyright.PENANAwycI18xjPd
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers165Please respect copyright.PENANA1ZUJgwgRaC
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 big165Please respect copyright.PENANA75Nh9f4YlC
while(there_is_remain_file_content_to_send)165Please respect copyright.PENANAEwKjBDbsW5
{ 165Please respect copyright.PENANAVAVxCforwX
read_remain_file_content_to_send_buf(file, send_buf);165Please respect copyright.PENANAkCaNPIk64q
send_result = send(client_socket, send_buf);165Please respect copyright.PENANAvdlHKG7Gc3
if(send_result == fail) return; else continue;165Please respect copyright.PENANAreuRxujtpP
}
// a way to make etag for a file using its file size and last modifed time165Please respect copyright.PENANAN60pBQDdfy
const char *file_etag(const char *path) 165Please respect copyright.PENANAbNZ4p11v2a
{165Please respect copyright.PENANA68YgYQGuxh
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;165Please respect copyright.PENANAEAlmFABVje
}165Please respect copyright.PENANAm1k3ATb7vA
-------------------------------------------------165Please respect copyright.PENANAl4aJ5zCJll
"accept-ranges: bytes" tells client web browser that the http server support range.165Please respect copyright.PENANA6lrul89Xqb
"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).165Please respect copyright.PENANAhFirfurMtz
'ETag: "534231200_1732277116"' tells client web browser the file version.165Please respect copyright.PENANA8GGTcdRxVq
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.165Please respect copyright.PENANAWBHqqXJQZi
"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.165Please respect copyright.PENANAWxUXSjXPYc
"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.165Please respect copyright.PENANAKCiW3mPgPn
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.)165Please respect copyright.PENANA1DFm1FwGT1
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:165Please respect copyright.PENANAcYheLvx4ZM
---------resume range request---------165Please respect copyright.PENANA9sRcVsk1Ci
GET /zen.iso HTTP/1.1165Please respect copyright.PENANA4nJDxuGzjQ
Range: bytes=243220480-165Please respect copyright.PENANAKSoRF2CCVa
If-Match: "534231200_1732277116"165Please respect copyright.PENANA1RO2s5xwHl
......165Please respect copyright.PENANA5psvEWqMWd
-------------------------------------165Please respect copyright.PENANAfkCsOoQeJb
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.165Please respect copyright.PENANA1oq8ZPtEkG
'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:165Please respect copyright.PENANAguntzChZy2
-------response content from resume point-------165Please respect copyright.PENANATH9lrBeecV
HTTP/1.1 206 Partial Content165Please respect copyright.PENANAMexFvmoAHh
Date: Sat, 23 Nov 2024 06:56:17 GMT165Please respect copyright.PENANAEf4mQpGy98
accept-ranges: bytes165Please respect copyright.PENANAL3IMJAfPXg
content-range: bytes 243220480-534231199/534231200165Please respect copyright.PENANAWFpbNRJCWX
Content-Length: 291010720165Please respect copyright.PENANAmosvSDOV4z
Content-Type: application/octet-stream165Please respect copyright.PENANAYSGaQxPHpx
Content-Disposition: attachment; filename="zen.iso"165Please respect copyright.PENANAsTfXwCLetE
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)165Please respect copyright.PENANAf2jQ7E3M5O
------------------------------------------------165Please respect copyright.PENANA8sGkflGT2O
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:165Please respect copyright.PENANAnLvREcPbHf
-------response content from resume point-------165Please respect copyright.PENANAj9ARcGa8xC
HTTP/1.1 200 OK165Please respect copyright.PENANAuVi0GaKC8b
Date: Sat, 23 Nov 2024 06:29:19 GMT165Please respect copyright.PENANANHRBhJ1LeN
accept-ranges: bytes165Please respect copyright.PENANA8v8dtvxGuP
content-range: bytes 0-634231201/634231202165Please respect copyright.PENANAt3JkGyNkQr
ETag: "634231202_2732277118"165Please respect copyright.PENANAN1alAnTneF
Content-Disposition: attachment; filename="zen.iso"165Please respect copyright.PENANAI8gAy3EWM8
Content-Length: 634231202165Please respect copyright.PENANAU6rtq3JAmB
Content-Type: application/octet-stream
(http response body is the whole file content)165Please respect copyright.PENANAepDJtPN950
------------------------------------------------165Please respect copyright.PENANA51h0YDDDMi
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:165Please respect copyright.PENANAkuZOJyXenq
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165Please respect copyright.PENANAiKMxgjuqiF
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165Please respect copyright.PENANARqbq4JMhMY
https://gemini.google.com/
感谢互联网上无私奉献的参考资165Please respect copyright.PENANAkFoGYpc4BH
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests165Please respect copyright.PENANAGjK3H6ILd1
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range165Please respect copyright.PENANAjLfQ9JdpKp
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag165Please respect copyright.PENANAeqj1YMmBJG
https://www.rfc-editor.org/rfc/rfc7233165Please respect copyright.PENANAwHcLwPeAm1
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]165Please respect copyright.PENANAZufHodHwnB
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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