明慈淨行記 - 第68章《Coding Art》第20篇 The Implementation of Http Range for Supporting Big File Download Pause and Re

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:146Please respect copyright.ＰＥＮＡＮＡLmT89Twkkr
----------http range request-------146Please respect copyright.ＰＥＮＡＮＡYzEi7vf29o
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡzrGnUXH7gU
Range: bytes=100-1023146Please respect copyright.ＰＥＮＡＮＡ0pBa3SGltz
If-Match: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡEf978XvCq4
......146Please respect copyright.ＰＥＮＡＮＡgNcV2X6axz
-----------------------------------146Please respect copyright.ＰＥＮＡＮＡuzDYDD5JjC
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.

146Please respect copyright.ＰＥＮＡＮＡkcPvkZDpQJ
Step 1. make a link for file to download, for example:146Please respect copyright.ＰＥＮＡＮＡISkr2nzPHs
----------------html-----------------146Please respect copyright.ＰＥＮＡＮＡ93rdy82VQU
<a href="/zen.iso">zen.iso</a>146Please respect copyright.ＰＥＮＡＮＡ6mOk4yHLYF
-------------------------------------146Please respect copyright.ＰＥＮＡＮＡkB8kwv7gJG
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:146Please respect copyright.ＰＥＮＡＮＡtGtYgMWXXC
----------get request------------146Please respect copyright.ＰＥＮＡＮＡ6XcKdU6AV3
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡLvXELVhuL9
......146Please respect copyright.ＰＥＮＡＮＡ8dDhUOQ4zQ
---------------------------------

146Please respect copyright.ＰＥＮＡＮＡLKux4BUgTe
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:146Please respect copyright.ＰＥＮＡＮＡfh1sdAH8ez
----------------server response--------------146Please respect copyright.ＰＥＮＡＮＡ9v1LYKYiNl
HTTP/1.1 200 OK146Please respect copyright.ＰＥＮＡＮＡz7Ki3OLGiK
Date: Sat, 23 Nov 2024 06:29:19 GMT146Please respect copyright.ＰＥＮＡＮＡhMj1Hsm3Wy
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡ5xCwJUXV6C
content-range: bytes 0-534231199/534231200146Please respect copyright.ＰＥＮＡＮＡBypzu9PSpD
ETag: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡamR0X4M10P
Content-Disposition: attachment; filename="zen.iso"146Please respect copyright.ＰＥＮＡＮＡELPWT3LNKD
Content-Length: 534231200146Please respect copyright.ＰＥＮＡＮＡGrb8OZ8Iiy
Content-Type: application/octet-stream

(http response body is the whole file content)146Please respect copyright.ＰＥＮＡＮＡLQZQEuUEH5
-------------------------------------------

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:146Please respect copyright.ＰＥＮＡＮＡII3jJW4jPO
---------------http server send big file content by multiple times--------------146Please respect copyright.ＰＥＮＡＮＡg7982LsWAC
const int buf_size=20000000; // buf size is about 20M146Please respect copyright.ＰＥＮＡＮＡFVVFZ87mZu
int client_socket; char send_buf[buf_size]; FILE *file;

// first send http response headers146Please respect copyright.ＰＥＮＡＮＡKjnftXIbhS
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 big146Please respect copyright.ＰＥＮＡＮＡU3hPGmuFS8
while(there_is_remain_file_content_to_send)146Please respect copyright.ＰＥＮＡＮＡqTji4EOxPp
{ 146Please respect copyright.ＰＥＮＡＮＡ4hHU1fFpZH
read_remain_file_content_to_send_buf(file, send_buf);146Please respect copyright.ＰＥＮＡＮＡicjk1ALZlx
send_result = send(client_socket, send_buf);146Please respect copyright.ＰＥＮＡＮＡ9hlMMxMTTf
if(send_result == fail) return; else continue;146Please respect copyright.ＰＥＮＡＮＡasGDDk8SeZ
}

// a way to make etag for a file using its file size and last modifed time146Please respect copyright.ＰＥＮＡＮＡE8d3TDU7Xf
const char *file_etag(const char *path) 146Please respect copyright.ＰＥＮＡＮＡxht1v3la67
{146Please respect copyright.ＰＥＮＡＮＡvUnf9mm5UO
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;146Please respect copyright.ＰＥＮＡＮＡG8hEra0vab
}146Please respect copyright.ＰＥＮＡＮＡpV6MqdjOFR
-------------------------------------------------146Please respect copyright.ＰＥＮＡＮＡPpfcRgY5TC
"accept-ranges: bytes" tells client web browser that the http server support range.146Please respect copyright.ＰＥＮＡＮＡu3DagMeYJZ
"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).146Please respect copyright.ＰＥＮＡＮＡLxfR4NCdvJ
'ETag: "534231200_1732277116"' tells client web browser the file version.146Please respect copyright.ＰＥＮＡＮＡKAUBCzY1DH
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.146Please respect copyright.ＰＥＮＡＮＡK0LjDkTGHk
"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.146Please respect copyright.ＰＥＮＡＮＡ3feINYOXJb
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.

146Please respect copyright.ＰＥＮＡＮＡ2KTRcJI6a1
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.146Please respect copyright.ＰＥＮＡＮＡnvMSJAoaIR
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.)146Please respect copyright.ＰＥＮＡＮＡqcMPh62KHQ
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.

146Please respect copyright.ＰＥＮＡＮＡn2qkq3dqPA
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:146Please respect copyright.ＰＥＮＡＮＡEbj1fnf4oZ
---------resume range request---------146Please respect copyright.ＰＥＮＡＮＡ9Io7QGfPk4
GET /zen.iso HTTP/1.1146Please respect copyright.ＰＥＮＡＮＡQ30VuxrwKe
Range: bytes=243220480-146Please respect copyright.ＰＥＮＡＮＡQV8ftgRLzA
If-Match: "534231200_1732277116"146Please respect copyright.ＰＥＮＡＮＡLGGvfkyjgC
......146Please respect copyright.ＰＥＮＡＮＡjipqGXKacz
-------------------------------------146Please respect copyright.ＰＥＮＡＮＡjxFue5cpzH
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.146Please respect copyright.ＰＥＮＡＮＡMxe35ILzUH
'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:146Please respect copyright.ＰＥＮＡＮＡNuP63ihs1e
-------response content from resume point-------146Please respect copyright.ＰＥＮＡＮＡYSKXbWcArB
HTTP/1.1 206 Partial Content146Please respect copyright.ＰＥＮＡＮＡ5HtVFUJgfj
Date: Sat, 23 Nov 2024 06:56:17 GMT146Please respect copyright.ＰＥＮＡＮＡgHkX51rynd
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡFpMnx5xhJU
content-range: bytes 243220480-534231199/534231200146Please respect copyright.ＰＥＮＡＮＡA21uWCaoq4
Content-Length: 291010720146Please respect copyright.ＰＥＮＡＮＡJUsb8WQzoB
Content-Type: application/octet-stream146Please respect copyright.ＰＥＮＡＮＡaMJmYI9GyT
Content-Disposition: attachment; filename="zen.iso"146Please respect copyright.ＰＥＮＡＮＡw7tNf8HtWV
ETag: "534231200_1732277116"

(http response body is the file content from resume point to end)146Please respect copyright.ＰＥＮＡＮＡxkD1D3e7mH
------------------------------------------------146Please respect copyright.ＰＥＮＡＮＡfslurp1dRD
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:146Please respect copyright.ＰＥＮＡＮＡ26eKTKoQdn
-------response content from resume point-------146Please respect copyright.ＰＥＮＡＮＡehGOd5fRYZ
HTTP/1.1 200 OK146Please respect copyright.ＰＥＮＡＮＡUBzLqapMBd
Date: Sat, 23 Nov 2024 06:29:19 GMT146Please respect copyright.ＰＥＮＡＮＡcDisKmD7V7
accept-ranges: bytes146Please respect copyright.ＰＥＮＡＮＡAnUfDrKCN1
content-range: bytes 0-634231201/634231202146Please respect copyright.ＰＥＮＡＮＡE3Kq5n4SD6
ETag: "634231202_2732277118"146Please respect copyright.ＰＥＮＡＮＡWOl4ZwMuAL
Content-Disposition: attachment; filename="zen.iso"146Please respect copyright.ＰＥＮＡＮＡS7y3vssrcB
Content-Length: 634231202146Please respect copyright.ＰＥＮＡＮＡluV23elcFU
Content-Type: application/octet-stream

(http response body is the whole file content)146Please respect copyright.ＰＥＮＡＮＡkOA3s8F51L
------------------------------------------------146Please respect copyright.ＰＥＮＡＮＡ6IDGHRoMnj
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.

146Please respect copyright.ＰＥＮＡＮＡi170B8GxKe
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:146Please respect copyright.ＰＥＮＡＮＡlsL3p7f1cq
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?

146Please respect copyright.ＰＥＮＡＮＡ0J54azpByl
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146Please respect copyright.ＰＥＮＡＮＡDVDz6XxD05
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146Please respect copyright.ＰＥＮＡＮＡFIUWf1Ih2p
https://gemini.google.com/

感谢互联网上无私奉献的参考资146Please respect copyright.ＰＥＮＡＮＡia2PtS6lce
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests146Please respect copyright.ＰＥＮＡＮＡXuX20Hhqhu
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range146Please respect copyright.ＰＥＮＡＮＡ6qbLy5Aoj1
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag146Please respect copyright.ＰＥＮＡＮＡazcVuVeBGo
https://www.rfc-editor.org/rfc/rfc7233146Please respect copyright.ＰＥＮＡＮＡt1163SDRuV
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/

146Please respect copyright.ＰＥＮＡＮＡl5C9BHfJo8
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]146Please respect copyright.ＰＥＮＡＮＡl8OwNSI7Tt
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]

如果您對C/C++ programming, Linux, website development, Vue, Git, vscode感興趣，邀請您加入「Linux/C/C++ Website Development」微信群，請加我的微信（si_jinmin）以便拉您进群。

ns216.73.216.224da2

介面主題:

字體大小:

行距:

段距:

自動加載下一章