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:40Please respect copyright.ＰＥＮＡＮＡlToSQaItrI
----------http range request-------40Please respect copyright.ＰＥＮＡＮＡ4YA9yyr2Ix
GET /zen.iso HTTP/1.140Please respect copyright.ＰＥＮＡＮＡvkeSk0si3z
Range: bytes=100-102340Please respect copyright.ＰＥＮＡＮＡzicSHG4QSO
If-Match: "534231200_1732277116"40Please respect copyright.ＰＥＮＡＮＡAaAbahlTTK
......40Please respect copyright.ＰＥＮＡＮＡvgilrTB6R6
-----------------------------------40Please respect copyright.ＰＥＮＡＮＡpZGScYzIAp
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:40Please respect copyright.ＰＥＮＡＮＡqc1oa6R4Gf
----------------html-----------------40Please respect copyright.ＰＥＮＡＮＡMN6qV1KmRg
<a href="/zen.iso">zen.iso</a>40Please respect copyright.ＰＥＮＡＮＡ2l4zH7Hvtc
-------------------------------------40Please respect copyright.ＰＥＮＡＮＡiNnf4ednN2
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:40Please respect copyright.ＰＥＮＡＮＡYPT1DAqDAJ
----------get request------------40Please respect copyright.ＰＥＮＡＮＡ33YF4hbxqY
GET /zen.iso HTTP/1.140Please respect copyright.ＰＥＮＡＮＡQXQVnI0nBC
......40Please respect copyright.ＰＥＮＡＮＡ7iqGWOqKUq
---------------------------------
40Please respect copyright.ＰＥＮＡＮＡCsmteMMFga
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:40Please respect copyright.ＰＥＮＡＮＡqsh5mL634D
----------------server response--------------40Please respect copyright.ＰＥＮＡＮＡibAu87ZrcM
HTTP/1.1 200 OK40Please respect copyright.ＰＥＮＡＮＡpqBbOv05KT
Date: Sat, 23 Nov 2024 06:29:19 GMT40Please respect copyright.ＰＥＮＡＮＡtRxrA8QEmT
accept-ranges: bytes40Please respect copyright.ＰＥＮＡＮＡt9oypTJRok
content-range: bytes 0-534231199/53423120040Please respect copyright.ＰＥＮＡＮＡMjQV2ni69I
ETag: "534231200_1732277116"40Please respect copyright.ＰＥＮＡＮＡ5aCuZ7th8w
Content-Disposition: attachment; filename="zen.iso"40Please respect copyright.ＰＥＮＡＮＡwElNrJKUhF
Content-Length: 53423120040Please respect copyright.ＰＥＮＡＮＡVuTOX1RON9
Content-Type: application/octet-stream
(http response body is the whole file content)40Please respect copyright.ＰＥＮＡＮＡsTSvd1egHU
-------------------------------------------
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:40Please respect copyright.ＰＥＮＡＮＡ22F6pGJOao
---------------http server send big file content by multiple times--------------40Please respect copyright.ＰＥＮＡＮＡedmsFcgR6x
const int buf_size=20000000; // buf size is about 20M40Please respect copyright.ＰＥＮＡＮＡHafbwxvrED
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers40Please respect copyright.ＰＥＮＡＮＡPYSwPYnLQf
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 big40Please respect copyright.ＰＥＮＡＮＡTGRj5y8tDF
while(there_is_remain_file_content_to_send)40Please respect copyright.ＰＥＮＡＮＡCc7HoZAhmM
{ 40Please respect copyright.ＰＥＮＡＮＡPdSDOxTxGY
read_remain_file_content_to_send_buf(file, send_buf);40Please respect copyright.ＰＥＮＡＮＡbE0T6QgsMa
send_result = send(client_socket, send_buf);40Please respect copyright.ＰＥＮＡＮＡdoWLdSkJhq
if(send_result == fail) return; else continue;40Please respect copyright.ＰＥＮＡＮＡIWJk3IKBWl
}
// a way to make etag for a file using its file size and last modifed time40Please respect copyright.ＰＥＮＡＮＡnRjiC7ToVI
const char *file_etag(const char *path) 40Please respect copyright.ＰＥＮＡＮＡuuVkbcke37
{40Please respect copyright.ＰＥＮＡＮＡvFMUXznWiL
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;40Please respect copyright.ＰＥＮＡＮＡDZXDbDUiLQ
}40Please respect copyright.ＰＥＮＡＮＡ22u5NwHGKm
-------------------------------------------------40Please respect copyright.ＰＥＮＡＮＡmwIlTMPBok
"accept-ranges: bytes" tells client web browser that the http server support range.40Please respect copyright.ＰＥＮＡＮＡOUF36NDpyW
"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).40Please respect copyright.ＰＥＮＡＮＡAYY6HWllqj
'ETag: "534231200_1732277116"' tells client web browser the file version.40Please respect copyright.ＰＥＮＡＮＡsJxePMxdUS
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.40Please respect copyright.ＰＥＮＡＮＡNLapH03UrJ
"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.40Please respect copyright.ＰＥＮＡＮＡNTXXowcrZ0
"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.40Please respect copyright.ＰＥＮＡＮＡhD2CCVeip8
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.)40Please respect copyright.ＰＥＮＡＮＡxXDD2oJIzt
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:40Please respect copyright.ＰＥＮＡＮＡVUavQTb86K
---------resume range request---------40Please respect copyright.ＰＥＮＡＮＡEQm2jqQxaV
GET /zen.iso HTTP/1.140Please respect copyright.ＰＥＮＡＮＡbdllH1qXhw
Range: bytes=243220480-40Please respect copyright.ＰＥＮＡＮＡN5fMrPoMCT
If-Match: "534231200_1732277116"40Please respect copyright.ＰＥＮＡＮＡ749GDSV4iE
......40Please respect copyright.ＰＥＮＡＮＡM1ZHWpeYXh
-------------------------------------40Please respect copyright.ＰＥＮＡＮＡrPZ0yHYwNG
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.40Please respect copyright.ＰＥＮＡＮＡLkLPjqmruG
'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:40Please respect copyright.ＰＥＮＡＮＡP29Su5Z109
-------response content from resume point-------40Please respect copyright.ＰＥＮＡＮＡhOJhSyDZSu
HTTP/1.1 206 Partial Content40Please respect copyright.ＰＥＮＡＮＡQS7BwIyGW7
Date: Sat, 23 Nov 2024 06:56:17 GMT40Please respect copyright.ＰＥＮＡＮＡ6kzqpfmjLN
accept-ranges: bytes40Please respect copyright.ＰＥＮＡＮＡXWQMBEoyQo
content-range: bytes 243220480-534231199/53423120040Please respect copyright.ＰＥＮＡＮＡwaosFonXM6
Content-Length: 29101072040Please respect copyright.ＰＥＮＡＮＡvcWCc6x6Fb
Content-Type: application/octet-stream40Please respect copyright.ＰＥＮＡＮＡ8BRG9GLPCJ
Content-Disposition: attachment; filename="zen.iso"40Please respect copyright.ＰＥＮＡＮＡMuKoAvzh6r
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)40Please respect copyright.ＰＥＮＡＮＡXzFjaqJAUh
------------------------------------------------40Please respect copyright.ＰＥＮＡＮＡIrhGDclFmb
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:40Please respect copyright.ＰＥＮＡＮＡw3vSIyhJJh
-------response content from resume point-------40Please respect copyright.ＰＥＮＡＮＡErls0PHt2E
HTTP/1.1 200 OK40Please respect copyright.ＰＥＮＡＮＡhkv6KFH0Kq
Date: Sat, 23 Nov 2024 06:29:19 GMT40Please respect copyright.ＰＥＮＡＮＡipOIrEyAbU
accept-ranges: bytes40Please respect copyright.ＰＥＮＡＮＡ9v1zDOCX1L
content-range: bytes 0-634231201/63423120240Please respect copyright.ＰＥＮＡＮＡGFLIQ09wzw
ETag: "634231202_2732277118"40Please respect copyright.ＰＥＮＡＮＡxOSq3DQpdf
Content-Disposition: attachment; filename="zen.iso"40Please respect copyright.ＰＥＮＡＮＡNciVQPbsuJ
Content-Length: 63423120240Please respect copyright.ＰＥＮＡＮＡUQC7UywPpK
Content-Type: application/octet-stream
(http response body is the whole file content)40Please respect copyright.ＰＥＮＡＮＡEDIowlMy6f
------------------------------------------------40Please respect copyright.ＰＥＮＡＮＡMUiA5BRbJ5
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:40Please respect copyright.ＰＥＮＡＮＡPT3PNrnaEu
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40Please respect copyright.ＰＥＮＡＮＡzWR1YMx5IL
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40Please respect copyright.ＰＥＮＡＮＡprjUCyQykH
https://gemini.google.com/
感谢互联网上无私奉献的参考资40Please respect copyright.ＰＥＮＡＮＡI9oJQRMRxO
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests40Please respect copyright.ＰＥＮＡＮＡIkg2idrlNl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range40Please respect copyright.ＰＥＮＡＮＡBtUn0LJnI8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag40Please respect copyright.ＰＥＮＡＮＡCHiVn3YAHn
https://www.rfc-editor.org/rfc/rfc723340Please respect copyright.ＰＥＮＡＮＡChZHFkWB5t
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: si.jinmin@gmail.com40Please respect copyright.ＰＥＮＡＮＡlo9nGDlWw1
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: si.jinmin@gmail.com
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