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:163Please respect copyright.ＰＥＮＡＮＡwPy893zgoH
----------http range request-------163Please respect copyright.ＰＥＮＡＮＡewLPkHtFCM
GET /zen.iso HTTP/1.1163Please respect copyright.ＰＥＮＡＮＡRITGPLiDoX
Range: bytes=100-1023163Please respect copyright.ＰＥＮＡＮＡISRsCiaQRQ
If-Match: "534231200_1732277116"163Please respect copyright.ＰＥＮＡＮＡPJJTsOmIWK
......163Please respect copyright.ＰＥＮＡＮＡrrpp6YYgmY
-----------------------------------163Please respect copyright.ＰＥＮＡＮＡGI2sDkxM9q
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:163Please respect copyright.ＰＥＮＡＮＡhxGHko80Tb
----------------html-----------------163Please respect copyright.ＰＥＮＡＮＡoPAFZPvFCI
<a href="/zen.iso">zen.iso</a>163Please respect copyright.ＰＥＮＡＮＡNb3Kbqfutr
-------------------------------------163Please respect copyright.ＰＥＮＡＮＡK2hex07C0F
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:163Please respect copyright.ＰＥＮＡＮＡEs1dJKR24j
----------get request------------163Please respect copyright.ＰＥＮＡＮＡHsKYQdsAAj
GET /zen.iso HTTP/1.1163Please respect copyright.ＰＥＮＡＮＡYXmKd9UuC6
......163Please respect copyright.ＰＥＮＡＮＡYyM0syirQI
---------------------------------
163Please respect copyright.ＰＥＮＡＮＡQgbgb6lKjs
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:163Please respect copyright.ＰＥＮＡＮＡlh0SaNrKGq
----------------server response--------------163Please respect copyright.ＰＥＮＡＮＡhqkfsjFv5r
HTTP/1.1 200 OK163Please respect copyright.ＰＥＮＡＮＡ63AvlcKrP8
Date: Sat, 23 Nov 2024 06:29:19 GMT163Please respect copyright.ＰＥＮＡＮＡL2ZyUHoPf6
accept-ranges: bytes163Please respect copyright.ＰＥＮＡＮＡV20ikJo1ZM
content-range: bytes 0-534231199/534231200163Please respect copyright.ＰＥＮＡＮＡhbIe4x5ASE
ETag: "534231200_1732277116"163Please respect copyright.ＰＥＮＡＮＡx78dgwZMWR
Content-Disposition: attachment; filename="zen.iso"163Please respect copyright.ＰＥＮＡＮＡ8b7c0oerYe
Content-Length: 534231200163Please respect copyright.ＰＥＮＡＮＡRUwvVYSe4R
Content-Type: application/octet-stream
(http response body is the whole file content)163Please respect copyright.ＰＥＮＡＮＡ19q9RMyilk
-------------------------------------------
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:163Please respect copyright.ＰＥＮＡＮＡHkOQFp77Ps
---------------http server send big file content by multiple times--------------163Please respect copyright.ＰＥＮＡＮＡW7yHNoPnOR
const int buf_size=20000000; // buf size is about 20M163Please respect copyright.ＰＥＮＡＮＡ5H7JomZRUj
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers163Please respect copyright.ＰＥＮＡＮＡ1oM7PmJe7D
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 big163Please respect copyright.ＰＥＮＡＮＡ5Ckr9VJCrr
while(there_is_remain_file_content_to_send)163Please respect copyright.ＰＥＮＡＮＡVyoEis8ULm
{ 163Please respect copyright.ＰＥＮＡＮＡTH6YoUsADS
read_remain_file_content_to_send_buf(file, send_buf);163Please respect copyright.ＰＥＮＡＮＡhVVVs8kq9r
send_result = send(client_socket, send_buf);163Please respect copyright.ＰＥＮＡＮＡtq0vgDH5NA
if(send_result == fail) return; else continue;163Please respect copyright.ＰＥＮＡＮＡCMObTmImOK
}
// a way to make etag for a file using its file size and last modifed time163Please respect copyright.ＰＥＮＡＮＡt9QWRcS7gI
const char *file_etag(const char *path) 163Please respect copyright.ＰＥＮＡＮＡ58fv5eY0GY
{163Please respect copyright.ＰＥＮＡＮＡdQ8yMk69JM
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;163Please respect copyright.ＰＥＮＡＮＡY5bQejB3u1
}163Please respect copyright.ＰＥＮＡＮＡyMt9wppw8O
-------------------------------------------------163Please respect copyright.ＰＥＮＡＮＡT4WogWUQXb
"accept-ranges: bytes" tells client web browser that the http server support range.163Please respect copyright.ＰＥＮＡＮＡeJutMnvn6E
"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).163Please respect copyright.ＰＥＮＡＮＡPEcdQmHt0R
'ETag: "534231200_1732277116"' tells client web browser the file version.163Please respect copyright.ＰＥＮＡＮＡrT9GOGFVpE
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.163Please respect copyright.ＰＥＮＡＮＡCdSURHEvsH
"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.163Please respect copyright.ＰＥＮＡＮＡwUWy48vMsH
"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.163Please respect copyright.ＰＥＮＡＮＡchN240b4yX
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.)163Please respect copyright.ＰＥＮＡＮＡ6caCxdqoik
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:163Please respect copyright.ＰＥＮＡＮＡQOPgaBlASh
---------resume range request---------163Please respect copyright.ＰＥＮＡＮＡ6YoGWD4RT4
GET /zen.iso HTTP/1.1163Please respect copyright.ＰＥＮＡＮＡBIryblG8Ah
Range: bytes=243220480-163Please respect copyright.ＰＥＮＡＮＡBDFkjJSD5j
If-Match: "534231200_1732277116"163Please respect copyright.ＰＥＮＡＮＡy0tf5KF8Hc
......163Please respect copyright.ＰＥＮＡＮＡS3QVN5Zsk5
-------------------------------------163Please respect copyright.ＰＥＮＡＮＡQIqBqVHi4Z
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.163Please respect copyright.ＰＥＮＡＮＡ0FWV4GltnW
'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:163Please respect copyright.ＰＥＮＡＮＡTPzLwkGYNk
-------response content from resume point-------163Please respect copyright.ＰＥＮＡＮＡAUbQji7Uzj
HTTP/1.1 206 Partial Content163Please respect copyright.ＰＥＮＡＮＡ2PYCq2D5GR
Date: Sat, 23 Nov 2024 06:56:17 GMT163Please respect copyright.ＰＥＮＡＮＡLCTYmWNf9H
accept-ranges: bytes163Please respect copyright.ＰＥＮＡＮＡXlO3TbpDXf
content-range: bytes 243220480-534231199/534231200163Please respect copyright.ＰＥＮＡＮＡNbnJqWAqpz
Content-Length: 291010720163Please respect copyright.ＰＥＮＡＮＡmlFJhrUiLz
Content-Type: application/octet-stream163Please respect copyright.ＰＥＮＡＮＡFC6kthHzxk
Content-Disposition: attachment; filename="zen.iso"163Please respect copyright.ＰＥＮＡＮＡweZ5xrr59j
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)163Please respect copyright.ＰＥＮＡＮＡlRggNi2n69
------------------------------------------------163Please respect copyright.ＰＥＮＡＮＡJ9p1IgYwUt
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:163Please respect copyright.ＰＥＮＡＮＡFeSBP9sUw6
-------response content from resume point-------163Please respect copyright.ＰＥＮＡＮＡhladqqcP0F
HTTP/1.1 200 OK163Please respect copyright.ＰＥＮＡＮＡFzNSIjWnds
Date: Sat, 23 Nov 2024 06:29:19 GMT163Please respect copyright.ＰＥＮＡＮＡml5O7wnli4
accept-ranges: bytes163Please respect copyright.ＰＥＮＡＮＡ0LuzG9RdEd
content-range: bytes 0-634231201/634231202163Please respect copyright.ＰＥＮＡＮＡxw01dtjGP1
ETag: "634231202_2732277118"163Please respect copyright.ＰＥＮＡＮＡ6o4tRsb3r4
Content-Disposition: attachment; filename="zen.iso"163Please respect copyright.ＰＥＮＡＮＡy2i1NX1k4K
Content-Length: 634231202163Please respect copyright.ＰＥＮＡＮＡQ8m0YsxvNL
Content-Type: application/octet-stream
(http response body is the whole file content)163Please respect copyright.ＰＥＮＡＮＡf2ygm4x4Dn
------------------------------------------------163Please respect copyright.ＰＥＮＡＮＡ6n1jpwispf
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:163Please respect copyright.ＰＥＮＡＮＡwNRsWJwuZE
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
163Please respect copyright.ＰＥＮＡＮＡ203Nzwos32
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163Please respect copyright.ＰＥＮＡＮＡwKWYd0i4Mr
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163Please respect copyright.ＰＥＮＡＮＡe72AngTV1j
https://gemini.google.com/
感谢互联网上无私奉献的参考资163Please respect copyright.ＰＥＮＡＮＡ1uwnXMcxNx
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests163Please respect copyright.ＰＥＮＡＮＡvfW1tqvgJA
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range163Please respect copyright.ＰＥＮＡＮＡfI3HE8Espi
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag163Please respect copyright.ＰＥＮＡＮＡ1DLN3L28Ix
https://www.rfc-editor.org/rfc/rfc7233163Please respect copyright.ＰＥＮＡＮＡNnONhHRMjl
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]163Please respect copyright.ＰＥＮＡＮＡpVnl3QvPG4
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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