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:160Please respect copyright.ＰＥＮＡＮＡIK3WXfoaPR
----------http range request-------160Please respect copyright.ＰＥＮＡＮＡ0IjBhUALd1
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡW62NqIOg5k
Range: bytes=100-1023160Please respect copyright.ＰＥＮＡＮＡh1bndekb05
If-Match: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡNCNuIDNBDb
......160Please respect copyright.ＰＥＮＡＮＡYebySKeWIV
-----------------------------------160Please respect copyright.ＰＥＮＡＮＡ6YY6mq1fZm
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:160Please respect copyright.ＰＥＮＡＮＡKSB0Fahaow
----------------html-----------------160Please respect copyright.ＰＥＮＡＮＡSITNZfbUHH
<a href="/zen.iso">zen.iso</a>160Please respect copyright.ＰＥＮＡＮＡmWuhW99b5R
-------------------------------------160Please respect copyright.ＰＥＮＡＮＡrNSalzaX43
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:160Please respect copyright.ＰＥＮＡＮＡbqIycVNZbb
----------get request------------160Please respect copyright.ＰＥＮＡＮＡMsq0zo4tGq
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡweXsJ03zkg
......160Please respect copyright.ＰＥＮＡＮＡELHqFN2kx1
---------------------------------
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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:160Please respect copyright.ＰＥＮＡＮＡDUO6jaDNuJ
----------------server response--------------160Please respect copyright.ＰＥＮＡＮＡRslav7QlfG
HTTP/1.1 200 OK160Please respect copyright.ＰＥＮＡＮＡqa3QENACs7
Date: Sat, 23 Nov 2024 06:29:19 GMT160Please respect copyright.ＰＥＮＡＮＡgdoh2Mwzsw
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡtN3nOuvdzV
content-range: bytes 0-534231199/534231200160Please respect copyright.ＰＥＮＡＮＡAcgbDgGVdL
ETag: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡLC3u0wofdp
Content-Disposition: attachment; filename="zen.iso"160Please respect copyright.ＰＥＮＡＮＡ60CW8j0YUQ
Content-Length: 534231200160Please respect copyright.ＰＥＮＡＮＡuTwRezj1MZ
Content-Type: application/octet-stream
(http response body is the whole file content)160Please respect copyright.ＰＥＮＡＮＡvbDJxV0dQH
-------------------------------------------
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:160Please respect copyright.ＰＥＮＡＮＡUK90mJSDL2
---------------http server send big file content by multiple times--------------160Please respect copyright.ＰＥＮＡＮＡSj7FQptcpr
const int buf_size=20000000; // buf size is about 20M160Please respect copyright.ＰＥＮＡＮＡHVFFLUvBJr
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers160Please respect copyright.ＰＥＮＡＮＡ5RkGAhLZXr
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 big160Please respect copyright.ＰＥＮＡＮＡIKYBGpYu2R
while(there_is_remain_file_content_to_send)160Please respect copyright.ＰＥＮＡＮＡzCH58qXerq
{ 160Please respect copyright.ＰＥＮＡＮＡubg4s3taxn
read_remain_file_content_to_send_buf(file, send_buf);160Please respect copyright.ＰＥＮＡＮＡpGrremmw7U
send_result = send(client_socket, send_buf);160Please respect copyright.ＰＥＮＡＮＡaEiw7ZtJY1
if(send_result == fail) return; else continue;160Please respect copyright.ＰＥＮＡＮＡ4sJ0SCMCDL
}
// a way to make etag for a file using its file size and last modifed time160Please respect copyright.ＰＥＮＡＮＡ9X2oRKD6xY
const char *file_etag(const char *path) 160Please respect copyright.ＰＥＮＡＮＡ3vROiVCnT3
{160Please respect copyright.ＰＥＮＡＮＡdxmeBInXDM
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;160Please respect copyright.ＰＥＮＡＮＡYiSyrO7Tyl
}160Please respect copyright.ＰＥＮＡＮＡ9eCWAg38tP
-------------------------------------------------160Please respect copyright.ＰＥＮＡＮＡglTHRY8t56
"accept-ranges: bytes" tells client web browser that the http server support range.160Please respect copyright.ＰＥＮＡＮＡpRI7Tx5cZV
"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).160Please respect copyright.ＰＥＮＡＮＡmndZMnd5c0
'ETag: "534231200_1732277116"' tells client web browser the file version.160Please respect copyright.ＰＥＮＡＮＡBcQqgHbY95
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.160Please respect copyright.ＰＥＮＡＮＡDmYZbtK1uk
"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.160Please respect copyright.ＰＥＮＡＮＡD5gIIn4vMp
"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.160Please respect copyright.ＰＥＮＡＮＡL30dByaI2r
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.)160Please respect copyright.ＰＥＮＡＮＡ5Gdf4z8rgB
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:160Please respect copyright.ＰＥＮＡＮＡeJH3vWh9bP
---------resume range request---------160Please respect copyright.ＰＥＮＡＮＡrWUaNCVPDx
GET /zen.iso HTTP/1.1160Please respect copyright.ＰＥＮＡＮＡeaiL0H1O7c
Range: bytes=243220480-160Please respect copyright.ＰＥＮＡＮＡ03YEW9f59w
If-Match: "534231200_1732277116"160Please respect copyright.ＰＥＮＡＮＡcXpzyyMXWj
......160Please respect copyright.ＰＥＮＡＮＡWmMxwNdnMe
-------------------------------------160Please respect copyright.ＰＥＮＡＮＡuMIFyjVLMI
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.160Please respect copyright.ＰＥＮＡＮＡichqJ7ctvE
'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:160Please respect copyright.ＰＥＮＡＮＡaOScaASz8Y
-------response content from resume point-------160Please respect copyright.ＰＥＮＡＮＡJGKIF8Sb1R
HTTP/1.1 206 Partial Content160Please respect copyright.ＰＥＮＡＮＡdnQAqeN4WQ
Date: Sat, 23 Nov 2024 06:56:17 GMT160Please respect copyright.ＰＥＮＡＮＡy0GhqpqmFh
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡ7d4BjuSldQ
content-range: bytes 243220480-534231199/534231200160Please respect copyright.ＰＥＮＡＮＡqo2UKZRNUd
Content-Length: 291010720160Please respect copyright.ＰＥＮＡＮＡB5TZEPL5Kx
Content-Type: application/octet-stream160Please respect copyright.ＰＥＮＡＮＡOwOnJlE6NB
Content-Disposition: attachment; filename="zen.iso"160Please respect copyright.ＰＥＮＡＮＡo5MrSUXdkq
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)160Please respect copyright.ＰＥＮＡＮＡsqebDGl8Ka
------------------------------------------------160Please respect copyright.ＰＥＮＡＮＡbLeYgLIXtf
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:160Please respect copyright.ＰＥＮＡＮＡCFx8YB3FnR
-------response content from resume point-------160Please respect copyright.ＰＥＮＡＮＡaHxNb5JfAk
HTTP/1.1 200 OK160Please respect copyright.ＰＥＮＡＮＡCAB5gYit26
Date: Sat, 23 Nov 2024 06:29:19 GMT160Please respect copyright.ＰＥＮＡＮＡRS1rXVEN9K
accept-ranges: bytes160Please respect copyright.ＰＥＮＡＮＡAKrvmKGZsl
content-range: bytes 0-634231201/634231202160Please respect copyright.ＰＥＮＡＮＡVp6O5AV5tE
ETag: "634231202_2732277118"160Please respect copyright.ＰＥＮＡＮＡVgw1HX0GWY
Content-Disposition: attachment; filename="zen.iso"160Please respect copyright.ＰＥＮＡＮＡlC6I1xB5TM
Content-Length: 634231202160Please respect copyright.ＰＥＮＡＮＡ33ToCDWkXp
Content-Type: application/octet-stream
(http response body is the whole file content)160Please respect copyright.ＰＥＮＡＮＡDCxkeCRyL5
------------------------------------------------160Please respect copyright.ＰＥＮＡＮＡOZZqd8TvGc
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:160Please respect copyright.ＰＥＮＡＮＡ5NBCGJuDLx
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160Please respect copyright.ＰＥＮＡＮＡtWiPypCuGI
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160Please respect copyright.ＰＥＮＡＮＡGfprmZyjLC
https://gemini.google.com/
感谢互联网上无私奉献的参考资160Please respect copyright.ＰＥＮＡＮＡ6oTDDAavbo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests160Please respect copyright.ＰＥＮＡＮＡbAlhKbbSnx
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range160Please respect copyright.ＰＥＮＡＮＡwVsbJVSaVu
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag160Please respect copyright.ＰＥＮＡＮＡifRVj7Sk06
https://www.rfc-editor.org/rfc/rfc7233160Please respect copyright.ＰＥＮＡＮＡfr4LajAQ7w
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]160Please respect copyright.ＰＥＮＡＮＡUGR1Jl1A9D
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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