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:69Please respect copyright.ＰＥＮＡＮＡQAgfNgdhyb
----------http range request-------69Please respect copyright.ＰＥＮＡＮＡrqSlkeOTLC
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡ9wWtB1cpoJ
Range: bytes=100-102369Please respect copyright.ＰＥＮＡＮＡA9PSJ5DYLZ
If-Match: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡl2WKyywznH
......69Please respect copyright.ＰＥＮＡＮＡmKbpYMCYQF
-----------------------------------69Please respect copyright.ＰＥＮＡＮＡsA6KNmfX4d
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:69Please respect copyright.ＰＥＮＡＮＡsVuhrnKrGZ
----------------html-----------------69Please respect copyright.ＰＥＮＡＮＡHFpZO5p9Fq
<a href="/zen.iso">zen.iso</a>69Please respect copyright.ＰＥＮＡＮＡ2Md65rvQF5
-------------------------------------69Please respect copyright.ＰＥＮＡＮＡwXGXSo2vbv
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:69Please respect copyright.ＰＥＮＡＮＡgXMp57DYgD
----------get request------------69Please respect copyright.ＰＥＮＡＮＡELBKFPO7Dn
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡrzMYAF4pDU
......69Please respect copyright.ＰＥＮＡＮＡvG02F51FLN
---------------------------------
69Please respect copyright.ＰＥＮＡＮＡWXMF6rP47E
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:69Please respect copyright.ＰＥＮＡＮＡkWPKYK0lDA
----------------server response--------------69Please respect copyright.ＰＥＮＡＮＡaYsvSiS3vV
HTTP/1.1 200 OK69Please respect copyright.ＰＥＮＡＮＡFzppLNvVkf
Date: Sat, 23 Nov 2024 06:29:19 GMT69Please respect copyright.ＰＥＮＡＮＡeq9nSfDZD8
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡ7zIVfKPtop
content-range: bytes 0-534231199/53423120069Please respect copyright.ＰＥＮＡＮＡiO2Wrfbbbz
ETag: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡ5REVjgflGk
Content-Disposition: attachment; filename="zen.iso"69Please respect copyright.ＰＥＮＡＮＡn6hQNKT4GU
Content-Length: 53423120069Please respect copyright.ＰＥＮＡＮＡhBzAqTHwWS
Content-Type: application/octet-stream
(http response body is the whole file content)69Please respect copyright.ＰＥＮＡＮＡnKSN078aO5
-------------------------------------------
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:69Please respect copyright.ＰＥＮＡＮＡYtv2m56HYS
---------------http server send big file content by multiple times--------------69Please respect copyright.ＰＥＮＡＮＡsjMBAEomDa
const int buf_size=20000000; // buf size is about 20M69Please respect copyright.ＰＥＮＡＮＡWRGkHf0fjW
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers69Please respect copyright.ＰＥＮＡＮＡ2oDogKpzCk
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 big69Please respect copyright.ＰＥＮＡＮＡrWutIfPy8y
while(there_is_remain_file_content_to_send)69Please respect copyright.ＰＥＮＡＮＡOUBTsJeizh
{ 69Please respect copyright.ＰＥＮＡＮＡWdlpY78ZZE
read_remain_file_content_to_send_buf(file, send_buf);69Please respect copyright.ＰＥＮＡＮＡEjJU7Bt9bJ
send_result = send(client_socket, send_buf);69Please respect copyright.ＰＥＮＡＮＡnDtLF3JP7h
if(send_result == fail) return; else continue;69Please respect copyright.ＰＥＮＡＮＡgcGGIEnE1l
}
// a way to make etag for a file using its file size and last modifed time69Please respect copyright.ＰＥＮＡＮＡZ0INbYkkeB
const char *file_etag(const char *path) 69Please respect copyright.ＰＥＮＡＮＡeBBsLvcbj6
{69Please respect copyright.ＰＥＮＡＮＡHxYmf51QaD
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;69Please respect copyright.ＰＥＮＡＮＡFXpMN2eeU2
}69Please respect copyright.ＰＥＮＡＮＡBDmnuMCv2H
-------------------------------------------------69Please respect copyright.ＰＥＮＡＮＡeC8F9dnkMF
"accept-ranges: bytes" tells client web browser that the http server support range.69Please respect copyright.ＰＥＮＡＮＡCGqvI0d9oU
"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).69Please respect copyright.ＰＥＮＡＮＡqh2WDQQxk1
'ETag: "534231200_1732277116"' tells client web browser the file version.69Please respect copyright.ＰＥＮＡＮＡDb0AxpMNtH
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.69Please respect copyright.ＰＥＮＡＮＡ3C81zz8fs0
"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.69Please respect copyright.ＰＥＮＡＮＡkG7uwsPJ7U
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
69Please respect copyright.ＰＥＮＡＮＡXMpDX0hP3L
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.69Please respect copyright.ＰＥＮＡＮＡEaNt7q9qNq
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.)69Please respect copyright.ＰＥＮＡＮＡJZfdvrJXWK
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:69Please respect copyright.ＰＥＮＡＮＡHfy8yL6H7p
---------resume range request---------69Please respect copyright.ＰＥＮＡＮＡDW8IRQNExC
GET /zen.iso HTTP/1.169Please respect copyright.ＰＥＮＡＮＡ7Z1qdZ40Qn
Range: bytes=243220480-69Please respect copyright.ＰＥＮＡＮＡ6sIC7QbCvB
If-Match: "534231200_1732277116"69Please respect copyright.ＰＥＮＡＮＡdORYq6MI7y
......69Please respect copyright.ＰＥＮＡＮＡRO40IlIM2X
-------------------------------------69Please respect copyright.ＰＥＮＡＮＡhG9joajISl
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.69Please respect copyright.ＰＥＮＡＮＡaCaRSzxfRd
'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:69Please respect copyright.ＰＥＮＡＮＡJphGbNuh6p
-------response content from resume point-------69Please respect copyright.ＰＥＮＡＮＡ98gm18oiB8
HTTP/1.1 206 Partial Content69Please respect copyright.ＰＥＮＡＮＡFvtsFxJqTy
Date: Sat, 23 Nov 2024 06:56:17 GMT69Please respect copyright.ＰＥＮＡＮＡ9gJoEaSmTi
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡBQXPwTsWn8
content-range: bytes 243220480-534231199/53423120069Please respect copyright.ＰＥＮＡＮＡeJ1e8AoEGy
Content-Length: 29101072069Please respect copyright.ＰＥＮＡＮＡBfiQr66mn9
Content-Type: application/octet-stream69Please respect copyright.ＰＥＮＡＮＡjLwwBfYfrh
Content-Disposition: attachment; filename="zen.iso"69Please respect copyright.ＰＥＮＡＮＡkBIkdaK5Fk
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)69Please respect copyright.ＰＥＮＡＮＡHUz6dW4sig
------------------------------------------------69Please respect copyright.ＰＥＮＡＮＡZoZUja5NXR
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:69Please respect copyright.ＰＥＮＡＮＡIvm1AtZNrx
-------response content from resume point-------69Please respect copyright.ＰＥＮＡＮＡfwqXlvqe0F
HTTP/1.1 200 OK69Please respect copyright.ＰＥＮＡＮＡr8wcdsRcr6
Date: Sat, 23 Nov 2024 06:29:19 GMT69Please respect copyright.ＰＥＮＡＮＡbmADlbO9AG
accept-ranges: bytes69Please respect copyright.ＰＥＮＡＮＡzbffUYadQz
content-range: bytes 0-634231201/63423120269Please respect copyright.ＰＥＮＡＮＡvcVue0fuXK
ETag: "634231202_2732277118"69Please respect copyright.ＰＥＮＡＮＡsvaOXJ8lTA
Content-Disposition: attachment; filename="zen.iso"69Please respect copyright.ＰＥＮＡＮＡChK88cu5wZ
Content-Length: 63423120269Please respect copyright.ＰＥＮＡＮＡQoKV0p8s3m
Content-Type: application/octet-stream
(http response body is the whole file content)69Please respect copyright.ＰＥＮＡＮＡZAMC2sSayr
------------------------------------------------69Please respect copyright.ＰＥＮＡＮＡfJ77ktduzc
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
69Please respect copyright.ＰＥＮＡＮＡN7pWceBNRA
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:69Please respect copyright.ＰＥＮＡＮＡYEMeB76Nvv
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
69Please respect copyright.ＰＥＮＡＮＡrLJYEPZcLc
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69Please respect copyright.ＰＥＮＡＮＡzkTqVgwKl9
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69Please respect copyright.ＰＥＮＡＮＡ0rEFh1aJwH
https://gemini.google.com/
感谢互联网上无私奉献的参考资69Please respect copyright.ＰＥＮＡＮＡeQwI33MEs8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests69Please respect copyright.ＰＥＮＡＮＡpsRRw2zePT
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range69Please respect copyright.ＰＥＮＡＮＡE2yCItDYQX
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag69Please respect copyright.ＰＥＮＡＮＡZLtNt4W9OF
https://www.rfc-editor.org/rfc/rfc723369Please respect copyright.ＰＥＮＡＮＡNmRGg1V4yu
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]69Please respect copyright.ＰＥＮＡＮＡuF0p5yKbS3
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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