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:91Please respect copyright.ＰＥＮＡＮＡvGHXZDM0NZ
----------http range request-------91Please respect copyright.ＰＥＮＡＮＡyMGfuoN9BR
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡiNyDOoCup2
Range: bytes=100-102391Please respect copyright.ＰＥＮＡＮＡgeRnsEmF7L
If-Match: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡaeYBZvl6qy
......91Please respect copyright.ＰＥＮＡＮＡcN9ghZLIZB
-----------------------------------91Please respect copyright.ＰＥＮＡＮＡBAdXXsz8fO
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:91Please respect copyright.ＰＥＮＡＮＡ2Oxi1B42rD
----------------html-----------------91Please respect copyright.ＰＥＮＡＮＡan0TIbQCp7
<a href="/zen.iso">zen.iso</a>91Please respect copyright.ＰＥＮＡＮＡpGNiOeYe7I
-------------------------------------91Please respect copyright.ＰＥＮＡＮＡM9wGVsRsWQ
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:91Please respect copyright.ＰＥＮＡＮＡXPkX01SEMG
----------get request------------91Please respect copyright.ＰＥＮＡＮＡ6rtI35A1mF
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡzb92y0svOJ
......91Please respect copyright.ＰＥＮＡＮＡp9ZRplKzEW
---------------------------------
91Please respect copyright.ＰＥＮＡＮＡkn2XiLaedj
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:91Please respect copyright.ＰＥＮＡＮＡw6FrIp84Ff
----------------server response--------------91Please respect copyright.ＰＥＮＡＮＡ54i6nlQmXL
HTTP/1.1 200 OK91Please respect copyright.ＰＥＮＡＮＡMxoGDfq7La
Date: Sat, 23 Nov 2024 06:29:19 GMT91Please respect copyright.ＰＥＮＡＮＡLmcvwmaz0m
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡOuHcy76McJ
content-range: bytes 0-534231199/53423120091Please respect copyright.ＰＥＮＡＮＡWPze7W2BXB
ETag: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡCnfSvulu48
Content-Disposition: attachment; filename="zen.iso"91Please respect copyright.ＰＥＮＡＮＡ1kVVqIOxvp
Content-Length: 53423120091Please respect copyright.ＰＥＮＡＮＡ5Z1NgNS24w
Content-Type: application/octet-stream
(http response body is the whole file content)91Please respect copyright.ＰＥＮＡＮＡlLl9BT6bLm
-------------------------------------------
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:91Please respect copyright.ＰＥＮＡＮＡQ1bqV6eY4p
---------------http server send big file content by multiple times--------------91Please respect copyright.ＰＥＮＡＮＡyFZ2ttP41a
const int buf_size=20000000; // buf size is about 20M91Please respect copyright.ＰＥＮＡＮＡA21KCDQQow
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers91Please respect copyright.ＰＥＮＡＮＡuUwX9Nj80z
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 big91Please respect copyright.ＰＥＮＡＮＡ8bl7GiQXOL
while(there_is_remain_file_content_to_send)91Please respect copyright.ＰＥＮＡＮＡidlnQOq83y
{ 91Please respect copyright.ＰＥＮＡＮＡ25MOHANtN8
read_remain_file_content_to_send_buf(file, send_buf);91Please respect copyright.ＰＥＮＡＮＡVzmPD8l5R9
send_result = send(client_socket, send_buf);91Please respect copyright.ＰＥＮＡＮＡnolqBCZKQh
if(send_result == fail) return; else continue;91Please respect copyright.ＰＥＮＡＮＡEFhk2kf5Gb
}
// a way to make etag for a file using its file size and last modifed time91Please respect copyright.ＰＥＮＡＮＡBvRjT2N4o4
const char *file_etag(const char *path) 91Please respect copyright.ＰＥＮＡＮＡK5ZIp0sXTK
{91Please respect copyright.ＰＥＮＡＮＡFCZzHJ70QU
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;91Please respect copyright.ＰＥＮＡＮＡpehLI7nfgL
}91Please respect copyright.ＰＥＮＡＮＡnBTNeQ0UOw
-------------------------------------------------91Please respect copyright.ＰＥＮＡＮＡD2EanVceLG
"accept-ranges: bytes" tells client web browser that the http server support range.91Please respect copyright.ＰＥＮＡＮＡBrvanf0YD2
"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).91Please respect copyright.ＰＥＮＡＮＡuMukz1JbEd
'ETag: "534231200_1732277116"' tells client web browser the file version.91Please respect copyright.ＰＥＮＡＮＡVabpzxcIHC
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.91Please respect copyright.ＰＥＮＡＮＡJWpdtZJSZl
"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.91Please respect copyright.ＰＥＮＡＮＡWV2iNMsdCW
"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.91Please respect copyright.ＰＥＮＡＮＡ2bIVVe3p2X
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.)91Please respect copyright.ＰＥＮＡＮＡ3wUkygnzFL
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:91Please respect copyright.ＰＥＮＡＮＡYNa8lSks7T
---------resume range request---------91Please respect copyright.ＰＥＮＡＮＡzoIy1oyeKf
GET /zen.iso HTTP/1.191Please respect copyright.ＰＥＮＡＮＡnrKKpncQTP
Range: bytes=243220480-91Please respect copyright.ＰＥＮＡＮＡW9zRdmxnGs
If-Match: "534231200_1732277116"91Please respect copyright.ＰＥＮＡＮＡ9FcFUC69FU
......91Please respect copyright.ＰＥＮＡＮＡMGWbyLD26N
-------------------------------------91Please respect copyright.ＰＥＮＡＮＡqzIxrYk71g
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.91Please respect copyright.ＰＥＮＡＮＡJCqoIvATgU
'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:91Please respect copyright.ＰＥＮＡＮＡC3Pds8lXFB
-------response content from resume point-------91Please respect copyright.ＰＥＮＡＮＡZFHkaqrzCg
HTTP/1.1 206 Partial Content91Please respect copyright.ＰＥＮＡＮＡQTVceWo8XJ
Date: Sat, 23 Nov 2024 06:56:17 GMT91Please respect copyright.ＰＥＮＡＮＡFIsna95siV
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡrjAm47rcRt
content-range: bytes 243220480-534231199/53423120091Please respect copyright.ＰＥＮＡＮＡZNSf5Fy0VL
Content-Length: 29101072091Please respect copyright.ＰＥＮＡＮＡ8XP5kvdMOF
Content-Type: application/octet-stream91Please respect copyright.ＰＥＮＡＮＡp0yP5Qd2dk
Content-Disposition: attachment; filename="zen.iso"91Please respect copyright.ＰＥＮＡＮＡ7Mk19ovR4W
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)91Please respect copyright.ＰＥＮＡＮＡklTnymMlLP
------------------------------------------------91Please respect copyright.ＰＥＮＡＮＡ6WeDlsREhC
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:91Please respect copyright.ＰＥＮＡＮＡ7U1XWbGUP4
-------response content from resume point-------91Please respect copyright.ＰＥＮＡＮＡKtpSvqPGT2
HTTP/1.1 200 OK91Please respect copyright.ＰＥＮＡＮＡBo7PJXSsCj
Date: Sat, 23 Nov 2024 06:29:19 GMT91Please respect copyright.ＰＥＮＡＮＡJI7y8YEJhg
accept-ranges: bytes91Please respect copyright.ＰＥＮＡＮＡAS4pNpslDZ
content-range: bytes 0-634231201/63423120291Please respect copyright.ＰＥＮＡＮＡr8yMNDqElt
ETag: "634231202_2732277118"91Please respect copyright.ＰＥＮＡＮＡazDRK0AjBy
Content-Disposition: attachment; filename="zen.iso"91Please respect copyright.ＰＥＮＡＮＡWiDeqlLd5j
Content-Length: 63423120291Please respect copyright.ＰＥＮＡＮＡALoNMydyds
Content-Type: application/octet-stream
(http response body is the whole file content)91Please respect copyright.ＰＥＮＡＮＡDpQ9gRRqaK
------------------------------------------------91Please respect copyright.ＰＥＮＡＮＡryabNMd6kE
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:91Please respect copyright.ＰＥＮＡＮＡxn9nR3bh0J
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
91Please respect copyright.ＰＥＮＡＮＡ6gHLLNDjxt
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91Please respect copyright.ＰＥＮＡＮＡkQe03vc87x
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91Please respect copyright.ＰＥＮＡＮＡ1NsjQztSgC
https://gemini.google.com/
感谢互联网上无私奉献的参考资91Please respect copyright.ＰＥＮＡＮＡ3M1TqgqAXh
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests91Please respect copyright.ＰＥＮＡＮＡm76yqU0Hsd
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range91Please respect copyright.ＰＥＮＡＮＡqtHjwmYiWK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag91Please respect copyright.ＰＥＮＡＮＡ7oziGUIFVX
https://www.rfc-editor.org/rfc/rfc723391Please respect copyright.ＰＥＮＡＮＡbu184rVeYg
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]91Please respect copyright.ＰＥＮＡＮＡnzSUY4YRZs
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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