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:114Please respect copyright.ＰＥＮＡＮＡTx0RKBGfTx
----------http range request-------114Please respect copyright.ＰＥＮＡＮＡE0eIAHg8w9
GET /zen.iso HTTP/1.1114Please respect copyright.ＰＥＮＡＮＡ05quwg42Pu
Range: bytes=100-1023114Please respect copyright.ＰＥＮＡＮＡz2QN8j7HWP
If-Match: "534231200_1732277116"114Please respect copyright.ＰＥＮＡＮＡ1c4DxZs6pd
......114Please respect copyright.ＰＥＮＡＮＡ0AsGCBgGrm
-----------------------------------114Please respect copyright.ＰＥＮＡＮＡD0s46n3ZCr
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:114Please respect copyright.ＰＥＮＡＮＡgdC2CmB42E
----------------html-----------------114Please respect copyright.ＰＥＮＡＮＡz9cWa4Ezle
<a href="/zen.iso">zen.iso</a>114Please respect copyright.ＰＥＮＡＮＡTcueiy1TJw
-------------------------------------114Please respect copyright.ＰＥＮＡＮＡ0bWSVUImq8
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:114Please respect copyright.ＰＥＮＡＮＡM1q8sIvqM9
----------get request------------114Please respect copyright.ＰＥＮＡＮＡCqf1h9uJvx
GET /zen.iso HTTP/1.1114Please respect copyright.ＰＥＮＡＮＡMsKeuSltXp
......114Please respect copyright.ＰＥＮＡＮＡojBlqOXzi0
---------------------------------
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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:114Please respect copyright.ＰＥＮＡＮＡrnb7Dj9tRC
----------------server response--------------114Please respect copyright.ＰＥＮＡＮＡ48LvgqZSXE
HTTP/1.1 200 OK114Please respect copyright.ＰＥＮＡＮＡBZ3wjZp9j2
Date: Sat, 23 Nov 2024 06:29:19 GMT114Please respect copyright.ＰＥＮＡＮＡZCyFCuec7s
accept-ranges: bytes114Please respect copyright.ＰＥＮＡＮＡnSvGqSAKGA
content-range: bytes 0-534231199/534231200114Please respect copyright.ＰＥＮＡＮＡ338A36FiiB
ETag: "534231200_1732277116"114Please respect copyright.ＰＥＮＡＮＡQ7NG7R2USM
Content-Disposition: attachment; filename="zen.iso"114Please respect copyright.ＰＥＮＡＮＡRACPhY2n1V
Content-Length: 534231200114Please respect copyright.ＰＥＮＡＮＡ7UTk8y3zds
Content-Type: application/octet-stream
(http response body is the whole file content)114Please respect copyright.ＰＥＮＡＮＡBfQM5123RF
-------------------------------------------
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:114Please respect copyright.ＰＥＮＡＮＡTlWdji4mcB
---------------http server send big file content by multiple times--------------114Please respect copyright.ＰＥＮＡＮＡFHg8U4jUIy
const int buf_size=20000000; // buf size is about 20M114Please respect copyright.ＰＥＮＡＮＡAtY0dBJdF4
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers114Please respect copyright.ＰＥＮＡＮＡj0hwLDTx7x
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 big114Please respect copyright.ＰＥＮＡＮＡRdamLjdAxF
while(there_is_remain_file_content_to_send)114Please respect copyright.ＰＥＮＡＮＡmyRw5uxq3F
{ 114Please respect copyright.ＰＥＮＡＮＡSTC8JuBAtZ
read_remain_file_content_to_send_buf(file, send_buf);114Please respect copyright.ＰＥＮＡＮＡlrKd9rO8gZ
send_result = send(client_socket, send_buf);114Please respect copyright.ＰＥＮＡＮＡusqOQgOFJt
if(send_result == fail) return; else continue;114Please respect copyright.ＰＥＮＡＮＡxWeJx5An62
}
// a way to make etag for a file using its file size and last modifed time114Please respect copyright.ＰＥＮＡＮＡQwFX7ux3FP
const char *file_etag(const char *path) 114Please respect copyright.ＰＥＮＡＮＡAVDmj6tlDT
{114Please respect copyright.ＰＥＮＡＮＡSk2UxTyqvT
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;114Please respect copyright.ＰＥＮＡＮＡepG0ORJEO1
}114Please respect copyright.ＰＥＮＡＮＡ9KOwdLAsIT
-------------------------------------------------114Please respect copyright.ＰＥＮＡＮＡ7N4KwDtZvy
"accept-ranges: bytes" tells client web browser that the http server support range.114Please respect copyright.ＰＥＮＡＮＡHvraIkbcx4
"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).114Please respect copyright.ＰＥＮＡＮＡM4UMRnwryv
'ETag: "534231200_1732277116"' tells client web browser the file version.114Please respect copyright.ＰＥＮＡＮＡxc8lCYbg1H
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.114Please respect copyright.ＰＥＮＡＮＡadnFwu2zqM
"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.114Please respect copyright.ＰＥＮＡＮＡoayxzqiLHF
"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.114Please respect copyright.ＰＥＮＡＮＡ50hSwTmqJy
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.)114Please respect copyright.ＰＥＮＡＮＡuCRilxJAPs
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:114Please respect copyright.ＰＥＮＡＮＡ1Nlo8RBPsd
---------resume range request---------114Please respect copyright.ＰＥＮＡＮＡy0qt3NxlLf
GET /zen.iso HTTP/1.1114Please respect copyright.ＰＥＮＡＮＡd5MAkNWJMO
Range: bytes=243220480-114Please respect copyright.ＰＥＮＡＮＡsohAMyrjgV
If-Match: "534231200_1732277116"114Please respect copyright.ＰＥＮＡＮＡ4UGBWsAkMx
......114Please respect copyright.ＰＥＮＡＮＡ7Lrjs9blXP
-------------------------------------114Please respect copyright.ＰＥＮＡＮＡmaw44DJkMp
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.114Please respect copyright.ＰＥＮＡＮＡ2CTVJIFOSo
'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:114Please respect copyright.ＰＥＮＡＮＡ8f0bvVb0hb
-------response content from resume point-------114Please respect copyright.ＰＥＮＡＮＡok18h2z7I0
HTTP/1.1 206 Partial Content114Please respect copyright.ＰＥＮＡＮＡesPL3Qaqku
Date: Sat, 23 Nov 2024 06:56:17 GMT114Please respect copyright.ＰＥＮＡＮＡMSQE3lZbWJ
accept-ranges: bytes114Please respect copyright.ＰＥＮＡＮＡrJkfFKqIU1
content-range: bytes 243220480-534231199/534231200114Please respect copyright.ＰＥＮＡＮＡo0Md8HO7kf
Content-Length: 291010720114Please respect copyright.ＰＥＮＡＮＡslxKNdIkQg
Content-Type: application/octet-stream114Please respect copyright.ＰＥＮＡＮＡmTTahdseTS
Content-Disposition: attachment; filename="zen.iso"114Please respect copyright.ＰＥＮＡＮＡ6lGoo2j1Iu
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)114Please respect copyright.ＰＥＮＡＮＡQj1KmCvqXV
------------------------------------------------114Please respect copyright.ＰＥＮＡＮＡKpmag6kxVi
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:114Please respect copyright.ＰＥＮＡＮＡ2fR0VagCVs
-------response content from resume point-------114Please respect copyright.ＰＥＮＡＮＡi9eMYOlT0x
HTTP/1.1 200 OK114Please respect copyright.ＰＥＮＡＮＡADvIxSYzT1
Date: Sat, 23 Nov 2024 06:29:19 GMT114Please respect copyright.ＰＥＮＡＮＡoB1yqhNFAF
accept-ranges: bytes114Please respect copyright.ＰＥＮＡＮＡHoGfz6pvGL
content-range: bytes 0-634231201/634231202114Please respect copyright.ＰＥＮＡＮＡUwuGQrz26t
ETag: "634231202_2732277118"114Please respect copyright.ＰＥＮＡＮＡaB48BrUhbU
Content-Disposition: attachment; filename="zen.iso"114Please respect copyright.ＰＥＮＡＮＡkOzvBn1kot
Content-Length: 634231202114Please respect copyright.ＰＥＮＡＮＡXCUakgGwn0
Content-Type: application/octet-stream
(http response body is the whole file content)114Please respect copyright.ＰＥＮＡＮＡGhs3lWRVCX
------------------------------------------------114Please respect copyright.ＰＥＮＡＮＡuTIdCdv2ms
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:114Please respect copyright.ＰＥＮＡＮＡw8YIt2nCh7
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114Please respect copyright.ＰＥＮＡＮＡvTG9l5bUrr
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114Please respect copyright.ＰＥＮＡＮＡ30sPUHfwQa
https://gemini.google.com/
感谢互联网上无私奉献的参考资114Please respect copyright.ＰＥＮＡＮＡbJ3txoGKzt
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests114Please respect copyright.ＰＥＮＡＮＡEJzaE0ifFm
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range114Please respect copyright.ＰＥＮＡＮＡTkDWFaukC7
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag114Please respect copyright.ＰＥＮＡＮＡ4gWgYqCkXF
https://www.rfc-editor.org/rfc/rfc7233114Please respect copyright.ＰＥＮＡＮＡ2zKhqJt99r
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]114Please respect copyright.ＰＥＮＡＮＡrKHLxu1NuZ
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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