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:133Please respect copyright.ＰＥＮＡＮＡUvxxOWuWmU
----------http range request-------133Please respect copyright.ＰＥＮＡＮＡigViHN2ua5
GET /zen.iso HTTP/1.1133Please respect copyright.ＰＥＮＡＮＡkADMBsnDCM
Range: bytes=100-1023133Please respect copyright.ＰＥＮＡＮＡVn2cwUWh1w
If-Match: "534231200_1732277116"133Please respect copyright.ＰＥＮＡＮＡY0Ow3k5FZJ
......133Please respect copyright.ＰＥＮＡＮＡBsIyEcZWAf
-----------------------------------133Please respect copyright.ＰＥＮＡＮＡjK2yLTRXDG
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:133Please respect copyright.ＰＥＮＡＮＡdufBSIIYAA
----------------html-----------------133Please respect copyright.ＰＥＮＡＮＡ0OCrig0479
<a href="/zen.iso">zen.iso</a>133Please respect copyright.ＰＥＮＡＮＡsc13yoPI9K
-------------------------------------133Please respect copyright.ＰＥＮＡＮＡM2gfu0rYv2
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:133Please respect copyright.ＰＥＮＡＮＡ7lEgGrZ3lQ
----------get request------------133Please respect copyright.ＰＥＮＡＮＡBmyfPvIvTk
GET /zen.iso HTTP/1.1133Please respect copyright.ＰＥＮＡＮＡVayFjMpMLl
......133Please respect copyright.ＰＥＮＡＮＡV712A7wR4Q
---------------------------------
133Please respect copyright.ＰＥＮＡＮＡ3rVzL7tDw0
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:133Please respect copyright.ＰＥＮＡＮＡPGxwKF4FJY
----------------server response--------------133Please respect copyright.ＰＥＮＡＮＡ5Hjfut7RSp
HTTP/1.1 200 OK133Please respect copyright.ＰＥＮＡＮＡAptD7UPRZe
Date: Sat, 23 Nov 2024 06:29:19 GMT133Please respect copyright.ＰＥＮＡＮＡRlIEe3PoKH
accept-ranges: bytes133Please respect copyright.ＰＥＮＡＮＡzidUcznh7f
content-range: bytes 0-534231199/534231200133Please respect copyright.ＰＥＮＡＮＡpkhayRiqyy
ETag: "534231200_1732277116"133Please respect copyright.ＰＥＮＡＮＡAJRIoYwyIn
Content-Disposition: attachment; filename="zen.iso"133Please respect copyright.ＰＥＮＡＮＡs2zqmlUp2e
Content-Length: 534231200133Please respect copyright.ＰＥＮＡＮＡH6azZdnSkG
Content-Type: application/octet-stream
(http response body is the whole file content)133Please respect copyright.ＰＥＮＡＮＡR6ojlgNGhd
-------------------------------------------
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:133Please respect copyright.ＰＥＮＡＮＡvhLESFlW82
---------------http server send big file content by multiple times--------------133Please respect copyright.ＰＥＮＡＮＡaN91Apvjqr
const int buf_size=20000000; // buf size is about 20M133Please respect copyright.ＰＥＮＡＮＡlWsg8msUdG
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers133Please respect copyright.ＰＥＮＡＮＡJrC8I6Oiks
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 big133Please respect copyright.ＰＥＮＡＮＡN4tAY7i6qN
while(there_is_remain_file_content_to_send)133Please respect copyright.ＰＥＮＡＮＡHX7gHAmniH
{ 133Please respect copyright.ＰＥＮＡＮＡrKYAZnCWLn
read_remain_file_content_to_send_buf(file, send_buf);133Please respect copyright.ＰＥＮＡＮＡvpSzDrNurn
send_result = send(client_socket, send_buf);133Please respect copyright.ＰＥＮＡＮＡQw6kX8ZJpx
if(send_result == fail) return; else continue;133Please respect copyright.ＰＥＮＡＮＡLCuf6cWm0w
}
// a way to make etag for a file using its file size and last modifed time133Please respect copyright.ＰＥＮＡＮＡRlDtN58GSC
const char *file_etag(const char *path) 133Please respect copyright.ＰＥＮＡＮＡ8kGA5qkX86
{133Please respect copyright.ＰＥＮＡＮＡQYIkEbn6lf
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;133Please respect copyright.ＰＥＮＡＮＡdr9CmOGcHZ
}133Please respect copyright.ＰＥＮＡＮＡAHIK9BwDXo
-------------------------------------------------133Please respect copyright.ＰＥＮＡＮＡtXPl7LLo6D
"accept-ranges: bytes" tells client web browser that the http server support range.133Please respect copyright.ＰＥＮＡＮＡLfhw4j6jXE
"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).133Please respect copyright.ＰＥＮＡＮＡb6UVHHnyRD
'ETag: "534231200_1732277116"' tells client web browser the file version.133Please respect copyright.ＰＥＮＡＮＡsyXD66U80s
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.133Please respect copyright.ＰＥＮＡＮＡBDD77xvsBk
"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.133Please respect copyright.ＰＥＮＡＮＡDH3ealmtDj
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
133Please respect copyright.ＰＥＮＡＮＡIIs30JLmtD
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.133Please respect copyright.ＰＥＮＡＮＡBVlQuUSjcq
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.)133Please respect copyright.ＰＥＮＡＮＡIrhKBc911L
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:133Please respect copyright.ＰＥＮＡＮＡnzGt8KbRTd
---------resume range request---------133Please respect copyright.ＰＥＮＡＮＡl6YAT5K7o6
GET /zen.iso HTTP/1.1133Please respect copyright.ＰＥＮＡＮＡPXzxAKTIPI
Range: bytes=243220480-133Please respect copyright.ＰＥＮＡＮＡns4u7GxbCS
If-Match: "534231200_1732277116"133Please respect copyright.ＰＥＮＡＮＡXAe7BpgjiV
......133Please respect copyright.ＰＥＮＡＮＡxj3F1gB4MQ
-------------------------------------133Please respect copyright.ＰＥＮＡＮＡk0imcBic1O
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.133Please respect copyright.ＰＥＮＡＮＡBrPTQkvt5L
'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:133Please respect copyright.ＰＥＮＡＮＡphegqVcuR2
-------response content from resume point-------133Please respect copyright.ＰＥＮＡＮＡo4JLZLTWi3
HTTP/1.1 206 Partial Content133Please respect copyright.ＰＥＮＡＮＡogfSvrlQuA
Date: Sat, 23 Nov 2024 06:56:17 GMT133Please respect copyright.ＰＥＮＡＮＡzapuLYjlHA
accept-ranges: bytes133Please respect copyright.ＰＥＮＡＮＡHv7VQ0BUYM
content-range: bytes 243220480-534231199/534231200133Please respect copyright.ＰＥＮＡＮＡLGYeeYo31w
Content-Length: 291010720133Please respect copyright.ＰＥＮＡＮＡVNabyvi76K
Content-Type: application/octet-stream133Please respect copyright.ＰＥＮＡＮＡ3VUb1RFjyR
Content-Disposition: attachment; filename="zen.iso"133Please respect copyright.ＰＥＮＡＮＡV5qaGmE1sG
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)133Please respect copyright.ＰＥＮＡＮＡudRXRvm6T2
------------------------------------------------133Please respect copyright.ＰＥＮＡＮＡRL3lMi0JzZ
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:133Please respect copyright.ＰＥＮＡＮＡprLiipr0mg
-------response content from resume point-------133Please respect copyright.ＰＥＮＡＮＡd5s9kRan3d
HTTP/1.1 200 OK133Please respect copyright.ＰＥＮＡＮＡ7gVzyrkPDh
Date: Sat, 23 Nov 2024 06:29:19 GMT133Please respect copyright.ＰＥＮＡＮＡoqN0ZShqo9
accept-ranges: bytes133Please respect copyright.ＰＥＮＡＮＡQrYSvFDaOR
content-range: bytes 0-634231201/634231202133Please respect copyright.ＰＥＮＡＮＡyVrc5t8Nch
ETag: "634231202_2732277118"133Please respect copyright.ＰＥＮＡＮＡrPQIA5sZNy
Content-Disposition: attachment; filename="zen.iso"133Please respect copyright.ＰＥＮＡＮＡpNN1u4dvOx
Content-Length: 634231202133Please respect copyright.ＰＥＮＡＮＡaOYWFLouNG
Content-Type: application/octet-stream
(http response body is the whole file content)133Please respect copyright.ＰＥＮＡＮＡcyhVChwEIH
------------------------------------------------133Please respect copyright.ＰＥＮＡＮＡxzDRFfRIEN
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:133Please respect copyright.ＰＥＮＡＮＡ29PWnVK3YM
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
133Please respect copyright.ＰＥＮＡＮＡI4UKTXFEeY
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133Please respect copyright.ＰＥＮＡＮＡDD549h9TLK
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133Please respect copyright.ＰＥＮＡＮＡonZtonjdW4
https://gemini.google.com/
感谢互联网上无私奉献的参考资133Please respect copyright.ＰＥＮＡＮＡRZ4luHmOWU
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests133Please respect copyright.ＰＥＮＡＮＡknnR4AefBJ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range133Please respect copyright.ＰＥＮＡＮＡ4XmgB4caF5
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag133Please respect copyright.ＰＥＮＡＮＡ9gcQTo8ZZy
https://www.rfc-editor.org/rfc/rfc7233133Please respect copyright.ＰＥＮＡＮＡHO4vsTkZDX
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]133Please respect copyright.ＰＥＮＡＮＡQbBZcg16Oa
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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