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:95Please respect copyright.ＰＥＮＡＮＡy17KDKOmrM
----------http range request-------95Please respect copyright.ＰＥＮＡＮＡPJjX5XkUhq
GET /zen.iso HTTP/1.195Please respect copyright.ＰＥＮＡＮＡ0eZpDHA9cZ
Range: bytes=100-102395Please respect copyright.ＰＥＮＡＮＡsdAwBybihf
If-Match: "534231200_1732277116"95Please respect copyright.ＰＥＮＡＮＡFk6GFs5dc8
......95Please respect copyright.ＰＥＮＡＮＡPeiM65qkJ0
-----------------------------------95Please respect copyright.ＰＥＮＡＮＡhmKzCuR699
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:95Please respect copyright.ＰＥＮＡＮＡN4vYCbgFgB
----------------html-----------------95Please respect copyright.ＰＥＮＡＮＡTImiHLDhjP
<a href="/zen.iso">zen.iso</a>95Please respect copyright.ＰＥＮＡＮＡ53LA6IHeoJ
-------------------------------------95Please respect copyright.ＰＥＮＡＮＡztaEleSlOU
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:95Please respect copyright.ＰＥＮＡＮＡWljc9esYul
----------get request------------95Please respect copyright.ＰＥＮＡＮＡGSCWH28v7U
GET /zen.iso HTTP/1.195Please respect copyright.ＰＥＮＡＮＡuBYTjegV7z
......95Please respect copyright.ＰＥＮＡＮＡrZ2SRxKR5X
---------------------------------
95Please respect copyright.ＰＥＮＡＮＡyXOAr4V1N4
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:95Please respect copyright.ＰＥＮＡＮＡDhHzyuaCTp
----------------server response--------------95Please respect copyright.ＰＥＮＡＮＡKDm7CXlsSq
HTTP/1.1 200 OK95Please respect copyright.ＰＥＮＡＮＡ0TVDPmDYaY
Date: Sat, 23 Nov 2024 06:29:19 GMT95Please respect copyright.ＰＥＮＡＮＡoxzjf1NroV
accept-ranges: bytes95Please respect copyright.ＰＥＮＡＮＡ86LkxUFZwE
content-range: bytes 0-534231199/53423120095Please respect copyright.ＰＥＮＡＮＡWDFjdPIJgK
ETag: "534231200_1732277116"95Please respect copyright.ＰＥＮＡＮＡMBAxdhNwZ9
Content-Disposition: attachment; filename="zen.iso"95Please respect copyright.ＰＥＮＡＮＡoOiub1jCSb
Content-Length: 53423120095Please respect copyright.ＰＥＮＡＮＡIVduqh0Elw
Content-Type: application/octet-stream
(http response body is the whole file content)95Please respect copyright.ＰＥＮＡＮＡtkVtmjJ1fe
-------------------------------------------
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:95Please respect copyright.ＰＥＮＡＮＡ8qJWXXa0M0
---------------http server send big file content by multiple times--------------95Please respect copyright.ＰＥＮＡＮＡU4Rl73aDlt
const int buf_size=20000000; // buf size is about 20M95Please respect copyright.ＰＥＮＡＮＡ098v2V1yeT
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers95Please respect copyright.ＰＥＮＡＮＡXwejvM2XDP
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 big95Please respect copyright.ＰＥＮＡＮＡDUZwOKOMMc
while(there_is_remain_file_content_to_send)95Please respect copyright.ＰＥＮＡＮＡ0x2rqOPlsB
{ 95Please respect copyright.ＰＥＮＡＮＡtronTvaJi2
read_remain_file_content_to_send_buf(file, send_buf);95Please respect copyright.ＰＥＮＡＮＡuJVhsLD2Zw
send_result = send(client_socket, send_buf);95Please respect copyright.ＰＥＮＡＮＡ5hMJlv8KFY
if(send_result == fail) return; else continue;95Please respect copyright.ＰＥＮＡＮＡYp9tFeyqu3
}
// a way to make etag for a file using its file size and last modifed time95Please respect copyright.ＰＥＮＡＮＡ3V8E37CB6k
const char *file_etag(const char *path) 95Please respect copyright.ＰＥＮＡＮＡqtNV01Y38y
{95Please respect copyright.ＰＥＮＡＮＡ2w1GbEORPD
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;95Please respect copyright.ＰＥＮＡＮＡFZ7mc55xWZ
}95Please respect copyright.ＰＥＮＡＮＡ8sLGKA8l9E
-------------------------------------------------95Please respect copyright.ＰＥＮＡＮＡ8Q5uMjJ2f1
"accept-ranges: bytes" tells client web browser that the http server support range.95Please respect copyright.ＰＥＮＡＮＡe6Ha6sQosm
"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).95Please respect copyright.ＰＥＮＡＮＡ5hN3MNQT7G
'ETag: "534231200_1732277116"' tells client web browser the file version.95Please respect copyright.ＰＥＮＡＮＡqNZKkj91Im
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.95Please respect copyright.ＰＥＮＡＮＡbhRX745dGX
"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.95Please respect copyright.ＰＥＮＡＮＡNKG7BOSe2F
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
95Please respect copyright.ＰＥＮＡＮＡReBiavA1bU
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.95Please respect copyright.ＰＥＮＡＮＡRfE1sD258U
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.)95Please respect copyright.ＰＥＮＡＮＡl1KID5UZTP
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:95Please respect copyright.ＰＥＮＡＮＡ34CgDjfe1L
---------resume range request---------95Please respect copyright.ＰＥＮＡＮＡW95OBotIZ5
GET /zen.iso HTTP/1.195Please respect copyright.ＰＥＮＡＮＡDwqa7cCmow
Range: bytes=243220480-95Please respect copyright.ＰＥＮＡＮＡiwQx4we83o
If-Match: "534231200_1732277116"95Please respect copyright.ＰＥＮＡＮＡb30rvavJAs
......95Please respect copyright.ＰＥＮＡＮＡk5FQx2idkY
-------------------------------------95Please respect copyright.ＰＥＮＡＮＡsocAuq3Bou
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.95Please respect copyright.ＰＥＮＡＮＡIGoO1NCSca
'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:95Please respect copyright.ＰＥＮＡＮＡBFKIsNuF1P
-------response content from resume point-------95Please respect copyright.ＰＥＮＡＮＡKhW76G8IOp
HTTP/1.1 206 Partial Content95Please respect copyright.ＰＥＮＡＮＡP3Yc7IlYde
Date: Sat, 23 Nov 2024 06:56:17 GMT95Please respect copyright.ＰＥＮＡＮＡs7YpLHFi9S
accept-ranges: bytes95Please respect copyright.ＰＥＮＡＮＡBcY2BPlb8M
content-range: bytes 243220480-534231199/53423120095Please respect copyright.ＰＥＮＡＮＡ7tfGkaavy5
Content-Length: 29101072095Please respect copyright.ＰＥＮＡＮＡUcz6nP3qH4
Content-Type: application/octet-stream95Please respect copyright.ＰＥＮＡＮＡw8VQAodRGF
Content-Disposition: attachment; filename="zen.iso"95Please respect copyright.ＰＥＮＡＮＡBQWSiirZHE
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)95Please respect copyright.ＰＥＮＡＮＡjwd4S8Rilm
------------------------------------------------95Please respect copyright.ＰＥＮＡＮＡQoa9PYVWFZ
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:95Please respect copyright.ＰＥＮＡＮＡOF6Og5Cikc
-------response content from resume point-------95Please respect copyright.ＰＥＮＡＮＡILBj9NO2TR
HTTP/1.1 200 OK95Please respect copyright.ＰＥＮＡＮＡzHwTcJ3RVU
Date: Sat, 23 Nov 2024 06:29:19 GMT95Please respect copyright.ＰＥＮＡＮＡD0KUHQOhKI
accept-ranges: bytes95Please respect copyright.ＰＥＮＡＮＡ6BnGkpO29d
content-range: bytes 0-634231201/63423120295Please respect copyright.ＰＥＮＡＮＡ9mT4yRYqi1
ETag: "634231202_2732277118"95Please respect copyright.ＰＥＮＡＮＡjMRyEqhP9c
Content-Disposition: attachment; filename="zen.iso"95Please respect copyright.ＰＥＮＡＮＡSOLekFk6A7
Content-Length: 63423120295Please respect copyright.ＰＥＮＡＮＡy9ChOK6Wjo
Content-Type: application/octet-stream
(http response body is the whole file content)95Please respect copyright.ＰＥＮＡＮＡMPV4BmzMgR
------------------------------------------------95Please respect copyright.ＰＥＮＡＮＡhaRqnrUP2x
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
95Please respect copyright.ＰＥＮＡＮＡfgMABan6Yg
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:95Please respect copyright.ＰＥＮＡＮＡZlyHEfbypH
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
95Please respect copyright.ＰＥＮＡＮＡeP2umUYaBM
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95Please respect copyright.ＰＥＮＡＮＡRXYTkGtr8h
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95Please respect copyright.ＰＥＮＡＮＡi6O7ENSpcb
https://gemini.google.com/
感谢互联网上无私奉献的参考资95Please respect copyright.ＰＥＮＡＮＡUxfGLP2Iyp
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests95Please respect copyright.ＰＥＮＡＮＡQUxPyJugwq
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range95Please respect copyright.ＰＥＮＡＮＡkR2vekq9He
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag95Please respect copyright.ＰＥＮＡＮＡJpyzOa7IJl
https://www.rfc-editor.org/rfc/rfc723395Please respect copyright.ＰＥＮＡＮＡSbV9T9sFoF
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]95Please respect copyright.ＰＥＮＡＮＡ5oJ8h90YfO
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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