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
---------------------------------
165Please respect copyright.ＰＥＮＡＮＡgayxDnJ943
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/?
165Please respect copyright.ＰＥＮＡＮＡRRRc9E3155
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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