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:140Please respect copyright.ＰＥＮＡＮＡqjn4Lk0ku9
----------http range request-------140Please respect copyright.ＰＥＮＡＮＡUGSb31Zhqa
GET /zen.iso HTTP/1.1140Please respect copyright.ＰＥＮＡＮＡi75WETOfmr
Range: bytes=100-1023140Please respect copyright.ＰＥＮＡＮＡWASU7s3E3X
If-Match: "534231200_1732277116"140Please respect copyright.ＰＥＮＡＮＡ8piqArgCtO
......140Please respect copyright.ＰＥＮＡＮＡulA5pPXQqe
-----------------------------------140Please respect copyright.ＰＥＮＡＮＡQaJcyqYQWL
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:140Please respect copyright.ＰＥＮＡＮＡJE9q5WqqWn
----------------html-----------------140Please respect copyright.ＰＥＮＡＮＡMS8DvzvMtl
<a href="/zen.iso">zen.iso</a>140Please respect copyright.ＰＥＮＡＮＡTR2npDDv7x
-------------------------------------140Please respect copyright.ＰＥＮＡＮＡ3WfAXqSQGJ
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:140Please respect copyright.ＰＥＮＡＮＡisNVvJ9XDW
----------get request------------140Please respect copyright.ＰＥＮＡＮＡphxUB4StiA
GET /zen.iso HTTP/1.1140Please respect copyright.ＰＥＮＡＮＡ1BL0WXzTrW
......140Please respect copyright.ＰＥＮＡＮＡxZ31nv90De
---------------------------------
140Please respect copyright.ＰＥＮＡＮＡNGmFaKlhEw
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:140Please respect copyright.ＰＥＮＡＮＡKSUURPuBnH
----------------server response--------------140Please respect copyright.ＰＥＮＡＮＡxWYNBocE1J
HTTP/1.1 200 OK140Please respect copyright.ＰＥＮＡＮＡArnO6kJ9Ui
Date: Sat, 23 Nov 2024 06:29:19 GMT140Please respect copyright.ＰＥＮＡＮＡAFbYCvsXXj
accept-ranges: bytes140Please respect copyright.ＰＥＮＡＮＡZ7s4xPLwTW
content-range: bytes 0-534231199/534231200140Please respect copyright.ＰＥＮＡＮＡXQ3OIaiivm
ETag: "534231200_1732277116"140Please respect copyright.ＰＥＮＡＮＡ03H7fL8xwg
Content-Disposition: attachment; filename="zen.iso"140Please respect copyright.ＰＥＮＡＮＡ3tqd5zKADz
Content-Length: 534231200140Please respect copyright.ＰＥＮＡＮＡlvi6iNybHH
Content-Type: application/octet-stream
(http response body is the whole file content)140Please respect copyright.ＰＥＮＡＮＡcZYlsNVmru
-------------------------------------------
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:140Please respect copyright.ＰＥＮＡＮＡRght6jq5yL
---------------http server send big file content by multiple times--------------140Please respect copyright.ＰＥＮＡＮＡOSDpshuMoB
const int buf_size=20000000; // buf size is about 20M140Please respect copyright.ＰＥＮＡＮＡciMjO21DKa
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers140Please respect copyright.ＰＥＮＡＮＡApGu9zhugZ
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 big140Please respect copyright.ＰＥＮＡＮＡRX3ru6FVmy
while(there_is_remain_file_content_to_send)140Please respect copyright.ＰＥＮＡＮＡnt7RVA09Ls
{ 140Please respect copyright.ＰＥＮＡＮＡq5JMsmejHc
read_remain_file_content_to_send_buf(file, send_buf);140Please respect copyright.ＰＥＮＡＮＡVoRs9wdOVO
send_result = send(client_socket, send_buf);140Please respect copyright.ＰＥＮＡＮＡGsaMjNMRno
if(send_result == fail) return; else continue;140Please respect copyright.ＰＥＮＡＮＡS4aDR9GGOd
}
// a way to make etag for a file using its file size and last modifed time140Please respect copyright.ＰＥＮＡＮＡ0vb9uucUvM
const char *file_etag(const char *path) 140Please respect copyright.ＰＥＮＡＮＡcCb5YbAnJu
{140Please respect copyright.ＰＥＮＡＮＡiJFLeWFZm5
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;140Please respect copyright.ＰＥＮＡＮＡ6MIKk4sGT8
}140Please respect copyright.ＰＥＮＡＮＡs5QcWZjsM3
-------------------------------------------------140Please respect copyright.ＰＥＮＡＮＡoPRn0cjGmW
"accept-ranges: bytes" tells client web browser that the http server support range.140Please respect copyright.ＰＥＮＡＮＡfUCEtwJrzR
"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).140Please respect copyright.ＰＥＮＡＮＡQE6yE5wRYE
'ETag: "534231200_1732277116"' tells client web browser the file version.140Please respect copyright.ＰＥＮＡＮＡkKBuMZBhER
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.140Please respect copyright.ＰＥＮＡＮＡIloHomVuwa
"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.140Please respect copyright.ＰＥＮＡＮＡ8ZsQozLEVE
"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.140Please respect copyright.ＰＥＮＡＮＡg0KcxqFekw
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.)140Please respect copyright.ＰＥＮＡＮＡlXfkIiwosM
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:140Please respect copyright.ＰＥＮＡＮＡgNQFjJ7H0e
---------resume range request---------140Please respect copyright.ＰＥＮＡＮＡEtE1YaSkc6
GET /zen.iso HTTP/1.1140Please respect copyright.ＰＥＮＡＮＡyFETgvAuac
Range: bytes=243220480-140Please respect copyright.ＰＥＮＡＮＡdUsiLBrWER
If-Match: "534231200_1732277116"140Please respect copyright.ＰＥＮＡＮＡXIhyShROuK
......140Please respect copyright.ＰＥＮＡＮＡZiaj7zF2RW
-------------------------------------140Please respect copyright.ＰＥＮＡＮＡqBAV18MLgG
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.140Please respect copyright.ＰＥＮＡＮＡhBRAuMluJE
'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:140Please respect copyright.ＰＥＮＡＮＡB0TqeKfQKg
-------response content from resume point-------140Please respect copyright.ＰＥＮＡＮＡ5VugzJgHnv
HTTP/1.1 206 Partial Content140Please respect copyright.ＰＥＮＡＮＡpcIKXXhAxd
Date: Sat, 23 Nov 2024 06:56:17 GMT140Please respect copyright.ＰＥＮＡＮＡyar5NYIlkb
accept-ranges: bytes140Please respect copyright.ＰＥＮＡＮＡHdnw1BLgbO
content-range: bytes 243220480-534231199/534231200140Please respect copyright.ＰＥＮＡＮＡIBkOmtxhpu
Content-Length: 291010720140Please respect copyright.ＰＥＮＡＮＡgbHoLGpU4k
Content-Type: application/octet-stream140Please respect copyright.ＰＥＮＡＮＡbIQ4EgKeOq
Content-Disposition: attachment; filename="zen.iso"140Please respect copyright.ＰＥＮＡＮＡg8ZQcQDprH
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)140Please respect copyright.ＰＥＮＡＮＡmx8BEcWnj3
------------------------------------------------140Please respect copyright.ＰＥＮＡＮＡLyf1LXNPH4
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:140Please respect copyright.ＰＥＮＡＮＡDzzOpfBzBf
-------response content from resume point-------140Please respect copyright.ＰＥＮＡＮＡurSSF26COZ
HTTP/1.1 200 OK140Please respect copyright.ＰＥＮＡＮＡL1t65Kl53P
Date: Sat, 23 Nov 2024 06:29:19 GMT140Please respect copyright.ＰＥＮＡＮＡG3m8DvU7sR
accept-ranges: bytes140Please respect copyright.ＰＥＮＡＮＡ1dtdyLH7Pq
content-range: bytes 0-634231201/634231202140Please respect copyright.ＰＥＮＡＮＡQSR1tZhX70
ETag: "634231202_2732277118"140Please respect copyright.ＰＥＮＡＮＡOvXpeg0XEn
Content-Disposition: attachment; filename="zen.iso"140Please respect copyright.ＰＥＮＡＮＡGDdyQtqP3o
Content-Length: 634231202140Please respect copyright.ＰＥＮＡＮＡ3rxOo9SRgQ
Content-Type: application/octet-stream
(http response body is the whole file content)140Please respect copyright.ＰＥＮＡＮＡJf6QTPIJkX
------------------------------------------------140Please respect copyright.ＰＥＮＡＮＡsNyZq11EXC
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:140Please respect copyright.ＰＥＮＡＮＡl6pGdd21hf
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140Please respect copyright.ＰＥＮＡＮＡAGkTCDKiBZ
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140Please respect copyright.ＰＥＮＡＮＡQtTGCCg6lc
https://gemini.google.com/
感谢互联网上无私奉献的参考资140Please respect copyright.ＰＥＮＡＮＡnvF7Qrgni8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests140Please respect copyright.ＰＥＮＡＮＡluAiKGycQ0
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range140Please respect copyright.ＰＥＮＡＮＡvQN27UpBmK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag140Please respect copyright.ＰＥＮＡＮＡw5Pne01kZp
https://www.rfc-editor.org/rfc/rfc7233140Please respect copyright.ＰＥＮＡＮＡ2HLrl960qX
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]140Please respect copyright.ＰＥＮＡＮＡGwdI9nGFaq
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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