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:155Please respect copyright.ＰＥＮＡＮＡivXR6UUvzE
----------http range request-------155Please respect copyright.ＰＥＮＡＮＡ1OHs1FmiHR
GET /zen.iso HTTP/1.1155Please respect copyright.ＰＥＮＡＮＡx9b3LeT6gq
Range: bytes=100-1023155Please respect copyright.ＰＥＮＡＮＡk1SIkoML78
If-Match: "534231200_1732277116"155Please respect copyright.ＰＥＮＡＮＡTtHLKRrDek
......155Please respect copyright.ＰＥＮＡＮＡ0WgmSYUx4r
-----------------------------------155Please respect copyright.ＰＥＮＡＮＡzNauxIsU8V
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:155Please respect copyright.ＰＥＮＡＮＡJ28rNJJe1B
----------------html-----------------155Please respect copyright.ＰＥＮＡＮＡi7D0OMcCpe
<a href="/zen.iso">zen.iso</a>155Please respect copyright.ＰＥＮＡＮＡqnOUJcVQoZ
-------------------------------------155Please respect copyright.ＰＥＮＡＮＡoN9RFGP2HQ
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:155Please respect copyright.ＰＥＮＡＮＡWhOzBwNmPT
----------get request------------155Please respect copyright.ＰＥＮＡＮＡMRGQD2uteA
GET /zen.iso HTTP/1.1155Please respect copyright.ＰＥＮＡＮＡokUYtzYfd8
......155Please respect copyright.ＰＥＮＡＮＡq92eBM2DXA
---------------------------------
155Please respect copyright.ＰＥＮＡＮＡPb4ieSukbp
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:155Please respect copyright.ＰＥＮＡＮＡQ5PVIcXM34
----------------server response--------------155Please respect copyright.ＰＥＮＡＮＡ3kGcZJLAeE
HTTP/1.1 200 OK155Please respect copyright.ＰＥＮＡＮＡv87XtND4Ni
Date: Sat, 23 Nov 2024 06:29:19 GMT155Please respect copyright.ＰＥＮＡＮＡSAGs0J4jOG
accept-ranges: bytes155Please respect copyright.ＰＥＮＡＮＡnkmb6l2UJw
content-range: bytes 0-534231199/534231200155Please respect copyright.ＰＥＮＡＮＡl7EVIdGoMH
ETag: "534231200_1732277116"155Please respect copyright.ＰＥＮＡＮＡ12eRmFqxQ8
Content-Disposition: attachment; filename="zen.iso"155Please respect copyright.ＰＥＮＡＮＡJYOhcPQvCF
Content-Length: 534231200155Please respect copyright.ＰＥＮＡＮＡt4uQox5aPf
Content-Type: application/octet-stream
(http response body is the whole file content)155Please respect copyright.ＰＥＮＡＮＡk6mBXjvpFD
-------------------------------------------
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:155Please respect copyright.ＰＥＮＡＮＡ8pL4QomsZi
---------------http server send big file content by multiple times--------------155Please respect copyright.ＰＥＮＡＮＡHt7kKq0qMz
const int buf_size=20000000; // buf size is about 20M155Please respect copyright.ＰＥＮＡＮＡtISHiwmwqa
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers155Please respect copyright.ＰＥＮＡＮＡUXgJcWFoHp
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 big155Please respect copyright.ＰＥＮＡＮＡCY3G92IQJD
while(there_is_remain_file_content_to_send)155Please respect copyright.ＰＥＮＡＮＡSnAO2n6udJ
{ 155Please respect copyright.ＰＥＮＡＮＡiYYauwBHeY
read_remain_file_content_to_send_buf(file, send_buf);155Please respect copyright.ＰＥＮＡＮＡYUXgty7WIc
send_result = send(client_socket, send_buf);155Please respect copyright.ＰＥＮＡＮＡ12TFfB7VsX
if(send_result == fail) return; else continue;155Please respect copyright.ＰＥＮＡＮＡzfO6cLjtlI
}
// a way to make etag for a file using its file size and last modifed time155Please respect copyright.ＰＥＮＡＮＡ0hK8qBDgZJ
const char *file_etag(const char *path) 155Please respect copyright.ＰＥＮＡＮＡjd5wBcMWdc
{155Please respect copyright.ＰＥＮＡＮＡ04JGPWQErB
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;155Please respect copyright.ＰＥＮＡＮＡszyj1NpXrO
}155Please respect copyright.ＰＥＮＡＮＡzRJn8UcTzg
-------------------------------------------------155Please respect copyright.ＰＥＮＡＮＡcTjMw5Zm3M
"accept-ranges: bytes" tells client web browser that the http server support range.155Please respect copyright.ＰＥＮＡＮＡAjrzfocDHi
"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).155Please respect copyright.ＰＥＮＡＮＡVGrVXO5nlO
'ETag: "534231200_1732277116"' tells client web browser the file version.155Please respect copyright.ＰＥＮＡＮＡLoplOXVX0R
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.155Please respect copyright.ＰＥＮＡＮＡNCU12ysRKS
"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.155Please respect copyright.ＰＥＮＡＮＡcUTFNSO1Pr
"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.155Please respect copyright.ＰＥＮＡＮＡYVxQNzfCuV
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.)155Please respect copyright.ＰＥＮＡＮＡdPr0fcj0hl
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:155Please respect copyright.ＰＥＮＡＮＡHy60Vly3Xn
---------resume range request---------155Please respect copyright.ＰＥＮＡＮＡFYDj2ZSvg0
GET /zen.iso HTTP/1.1155Please respect copyright.ＰＥＮＡＮＡlgDuPJec51
Range: bytes=243220480-155Please respect copyright.ＰＥＮＡＮＡX5ZS1dtuCT
If-Match: "534231200_1732277116"155Please respect copyright.ＰＥＮＡＮＡPzpxqJNcCR
......155Please respect copyright.ＰＥＮＡＮＡL7kdH4BbeE
-------------------------------------155Please respect copyright.ＰＥＮＡＮＡAtYk1xMZsZ
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.155Please respect copyright.ＰＥＮＡＮＡpO7aj1soYG
'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:155Please respect copyright.ＰＥＮＡＮＡ2aqudXnMVd
-------response content from resume point-------155Please respect copyright.ＰＥＮＡＮＡy0mJqQ69Fn
HTTP/1.1 206 Partial Content155Please respect copyright.ＰＥＮＡＮＡRgM07OuBb6
Date: Sat, 23 Nov 2024 06:56:17 GMT155Please respect copyright.ＰＥＮＡＮＡfDI2EeeKnw
accept-ranges: bytes155Please respect copyright.ＰＥＮＡＮＡ2wPhQtnz5T
content-range: bytes 243220480-534231199/534231200155Please respect copyright.ＰＥＮＡＮＡbe2uCkygFR
Content-Length: 291010720155Please respect copyright.ＰＥＮＡＮＡdCKDGU3nqJ
Content-Type: application/octet-stream155Please respect copyright.ＰＥＮＡＮＡRw7mADwGL0
Content-Disposition: attachment; filename="zen.iso"155Please respect copyright.ＰＥＮＡＮＡJtGLDL83Yj
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)155Please respect copyright.ＰＥＮＡＮＡDcaikPINMv
------------------------------------------------155Please respect copyright.ＰＥＮＡＮＡjphiz4Sjxl
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:155Please respect copyright.ＰＥＮＡＮＡpS6yMbcv5d
-------response content from resume point-------155Please respect copyright.ＰＥＮＡＮＡ8ZEqOx3TE1
HTTP/1.1 200 OK155Please respect copyright.ＰＥＮＡＮＡJFTEaROMk4
Date: Sat, 23 Nov 2024 06:29:19 GMT155Please respect copyright.ＰＥＮＡＮＡ04xJdSnNI6
accept-ranges: bytes155Please respect copyright.ＰＥＮＡＮＡVj9YxxZlr0
content-range: bytes 0-634231201/634231202155Please respect copyright.ＰＥＮＡＮＡA12p7F85vr
ETag: "634231202_2732277118"155Please respect copyright.ＰＥＮＡＮＡQxt7SmquWq
Content-Disposition: attachment; filename="zen.iso"155Please respect copyright.ＰＥＮＡＮＡIbFs0tpi5N
Content-Length: 634231202155Please respect copyright.ＰＥＮＡＮＡvu1Fd3Cxo1
Content-Type: application/octet-stream
(http response body is the whole file content)155Please respect copyright.ＰＥＮＡＮＡvUC9VGGVk0
------------------------------------------------155Please respect copyright.ＰＥＮＡＮＡO7YC9k6A1M
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:155Please respect copyright.ＰＥＮＡＮＡn65KfaJTNg
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
155Please respect copyright.ＰＥＮＡＮＡNITFcp6VkC
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155Please respect copyright.ＰＥＮＡＮＡXmpl65cKar
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155Please respect copyright.ＰＥＮＡＮＡsMvx7ksuYt
https://gemini.google.com/
感谢互联网上无私奉献的参考资155Please respect copyright.ＰＥＮＡＮＡEJsdxEUKU6
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests155Please respect copyright.ＰＥＮＡＮＡFOp4In9Jen
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range155Please respect copyright.ＰＥＮＡＮＡWmQlLR8JIG
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag155Please respect copyright.ＰＥＮＡＮＡj6i2qmz8A2
https://www.rfc-editor.org/rfc/rfc7233155Please respect copyright.ＰＥＮＡＮＡN8I13l675b
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]155Please respect copyright.ＰＥＮＡＮＡJ9ZENsSHRX
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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