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:75Please respect copyright.ＰＥＮＡＮＡmVH3iOdEk1
----------http range request-------75Please respect copyright.ＰＥＮＡＮＡl13bDXo4X8
GET /zen.iso HTTP/1.175Please respect copyright.ＰＥＮＡＮＡoBDbP0ODFX
Range: bytes=100-102375Please respect copyright.ＰＥＮＡＮＡDPEMIVEWCR
If-Match: "534231200_1732277116"75Please respect copyright.ＰＥＮＡＮＡxTcWCaTKTo
......75Please respect copyright.ＰＥＮＡＮＡktcJc3fpQf
-----------------------------------75Please respect copyright.ＰＥＮＡＮＡwnRGFT5RhL
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:75Please respect copyright.ＰＥＮＡＮＡd5OQbvkmXZ
----------------html-----------------75Please respect copyright.ＰＥＮＡＮＡKPIm3iVclV
<a href="/zen.iso">zen.iso</a>75Please respect copyright.ＰＥＮＡＮＡe2BznkdZUY
-------------------------------------75Please respect copyright.ＰＥＮＡＮＡrOoHGZnXS7
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:75Please respect copyright.ＰＥＮＡＮＡYsKA5ZbfLi
----------get request------------75Please respect copyright.ＰＥＮＡＮＡiNfxmDeaLf
GET /zen.iso HTTP/1.175Please respect copyright.ＰＥＮＡＮＡX01XqhfaIM
......75Please respect copyright.ＰＥＮＡＮＡC7GQuazoOk
---------------------------------
75Please respect copyright.ＰＥＮＡＮＡN3i3I6OhNg
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:75Please respect copyright.ＰＥＮＡＮＡvhrG8rTyRK
----------------server response--------------75Please respect copyright.ＰＥＮＡＮＡf0ifbaW0hd
HTTP/1.1 200 OK75Please respect copyright.ＰＥＮＡＮＡHs2Qf1lijK
Date: Sat, 23 Nov 2024 06:29:19 GMT75Please respect copyright.ＰＥＮＡＮＡcqbulT6kkR
accept-ranges: bytes75Please respect copyright.ＰＥＮＡＮＡgFQmYD0X8j
content-range: bytes 0-534231199/53423120075Please respect copyright.ＰＥＮＡＮＡsTJTHA051N
ETag: "534231200_1732277116"75Please respect copyright.ＰＥＮＡＮＡf0H2kSysLR
Content-Disposition: attachment; filename="zen.iso"75Please respect copyright.ＰＥＮＡＮＡSaXZFUtQLL
Content-Length: 53423120075Please respect copyright.ＰＥＮＡＮＡ4yt6hKOML7
Content-Type: application/octet-stream
(http response body is the whole file content)75Please respect copyright.ＰＥＮＡＮＡMm5rhLe14K
-------------------------------------------
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:75Please respect copyright.ＰＥＮＡＮＡMQTeG1pm19
---------------http server send big file content by multiple times--------------75Please respect copyright.ＰＥＮＡＮＡSgqDQbmgGU
const int buf_size=20000000; // buf size is about 20M75Please respect copyright.ＰＥＮＡＮＡxPoww81W93
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers75Please respect copyright.ＰＥＮＡＮＡd3RNkif48m
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 big75Please respect copyright.ＰＥＮＡＮＡ7XObEeKVPv
while(there_is_remain_file_content_to_send)75Please respect copyright.ＰＥＮＡＮＡMNQ3xV35Sh
{ 75Please respect copyright.ＰＥＮＡＮＡqcpYAaHBbN
read_remain_file_content_to_send_buf(file, send_buf);75Please respect copyright.ＰＥＮＡＮＡ45TGtfd07x
send_result = send(client_socket, send_buf);75Please respect copyright.ＰＥＮＡＮＡspZ3zfdpCm
if(send_result == fail) return; else continue;75Please respect copyright.ＰＥＮＡＮＡoEHXEryDeS
}
// a way to make etag for a file using its file size and last modifed time75Please respect copyright.ＰＥＮＡＮＡpNXYVEaAdo
const char *file_etag(const char *path) 75Please respect copyright.ＰＥＮＡＮＡPDb17fwUnn
{75Please respect copyright.ＰＥＮＡＮＡCoTr1gL79S
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;75Please respect copyright.ＰＥＮＡＮＡZPWuUhoc1F
}75Please respect copyright.ＰＥＮＡＮＡJBMdXFxxzD
-------------------------------------------------75Please respect copyright.ＰＥＮＡＮＡK2ht4u663j
"accept-ranges: bytes" tells client web browser that the http server support range.75Please respect copyright.ＰＥＮＡＮＡNO7C0JFKyu
"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).75Please respect copyright.ＰＥＮＡＮＡupzD2eTzWf
'ETag: "534231200_1732277116"' tells client web browser the file version.75Please respect copyright.ＰＥＮＡＮＡBnVu5Ibpou
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.75Please respect copyright.ＰＥＮＡＮＡCRtlrbADJ3
"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.75Please respect copyright.ＰＥＮＡＮＡO5UwwygeEO
"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.75Please respect copyright.ＰＥＮＡＮＡWYiRKjLSpq
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.)75Please respect copyright.ＰＥＮＡＮＡtZumYMkai3
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:75Please respect copyright.ＰＥＮＡＮＡ8XyNPkQeTl
---------resume range request---------75Please respect copyright.ＰＥＮＡＮＡb3Bu4tRc6w
GET /zen.iso HTTP/1.175Please respect copyright.ＰＥＮＡＮＡrLnPPkU5h4
Range: bytes=243220480-75Please respect copyright.ＰＥＮＡＮＡiMqTSPHxP5
If-Match: "534231200_1732277116"75Please respect copyright.ＰＥＮＡＮＡIoyb8NCI89
......75Please respect copyright.ＰＥＮＡＮＡWqAjt6RxRB
-------------------------------------75Please respect copyright.ＰＥＮＡＮＡfhjFMnm8Ja
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.75Please respect copyright.ＰＥＮＡＮＡJVw9g31ALe
'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:75Please respect copyright.ＰＥＮＡＮＡoboBXyWgPn
-------response content from resume point-------75Please respect copyright.ＰＥＮＡＮＡHias6VoEMV
HTTP/1.1 206 Partial Content75Please respect copyright.ＰＥＮＡＮＡOi6HeayyxJ
Date: Sat, 23 Nov 2024 06:56:17 GMT75Please respect copyright.ＰＥＮＡＮＡ57JDf2vAm9
accept-ranges: bytes75Please respect copyright.ＰＥＮＡＮＡ5Ay72ofCK4
content-range: bytes 243220480-534231199/53423120075Please respect copyright.ＰＥＮＡＮＡdEhkBtngul
Content-Length: 29101072075Please respect copyright.ＰＥＮＡＮＡNt8PsZ4FF3
Content-Type: application/octet-stream75Please respect copyright.ＰＥＮＡＮＡlWPlUtf7gk
Content-Disposition: attachment; filename="zen.iso"75Please respect copyright.ＰＥＮＡＮＡkMqRoQNY6a
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)75Please respect copyright.ＰＥＮＡＮＡI8ScnhyPAK
------------------------------------------------75Please respect copyright.ＰＥＮＡＮＡFKHVTkddjF
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:75Please respect copyright.ＰＥＮＡＮＡkTHb35EgPc
-------response content from resume point-------75Please respect copyright.ＰＥＮＡＮＡYaBJZspToe
HTTP/1.1 200 OK75Please respect copyright.ＰＥＮＡＮＡ5LbivUxDOb
Date: Sat, 23 Nov 2024 06:29:19 GMT75Please respect copyright.ＰＥＮＡＮＡBYKpbz8kOQ
accept-ranges: bytes75Please respect copyright.ＰＥＮＡＮＡtTdCTpKUXp
content-range: bytes 0-634231201/63423120275Please respect copyright.ＰＥＮＡＮＡUkNo7hJSRy
ETag: "634231202_2732277118"75Please respect copyright.ＰＥＮＡＮＡYnZEOBbnFl
Content-Disposition: attachment; filename="zen.iso"75Please respect copyright.ＰＥＮＡＮＡaoBJOHXk1O
Content-Length: 63423120275Please respect copyright.ＰＥＮＡＮＡk8D0gudeOQ
Content-Type: application/octet-stream
(http response body is the whole file content)75Please respect copyright.ＰＥＮＡＮＡy3GJLlVkCs
------------------------------------------------75Please respect copyright.ＰＥＮＡＮＡX9PJsF84fD
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:75Please respect copyright.ＰＥＮＡＮＡi0WtYmo7Jf
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
75Please respect copyright.ＰＥＮＡＮＡgOjBmIdmAS
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75Please respect copyright.ＰＥＮＡＮＡVZYYPf1wGD
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75Please respect copyright.ＰＥＮＡＮＡyf8r45VcHG
https://gemini.google.com/
感谢互联网上无私奉献的参考资75Please respect copyright.ＰＥＮＡＮＡxCxYX5F5fp
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests75Please respect copyright.ＰＥＮＡＮＡ5VYHzLjaXC
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range75Please respect copyright.ＰＥＮＡＮＡN5OyapTV2i
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag75Please respect copyright.ＰＥＮＡＮＡTI4SNR3uGl
https://www.rfc-editor.org/rfc/rfc723375Please respect copyright.ＰＥＮＡＮＡxFftIkYd1y
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.com75Please respect copyright.ＰＥＮＡＮＡl7zzBi1VWz
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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