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:57Please respect copyright.ＰＥＮＡＮＡlioIZFfjNz
----------http range request-------57Please respect copyright.ＰＥＮＡＮＡfkvavuXitv
GET /zen.iso HTTP/1.157Please respect copyright.ＰＥＮＡＮＡdW95FMXa1f
Range: bytes=100-102357Please respect copyright.ＰＥＮＡＮＡOYIrFUdbnZ
If-Match: "534231200_1732277116"57Please respect copyright.ＰＥＮＡＮＡUOdxnM8lXr
......57Please respect copyright.ＰＥＮＡＮＡVUwmXZm0Fs
-----------------------------------57Please respect copyright.ＰＥＮＡＮＡKAkulekilZ
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:57Please respect copyright.ＰＥＮＡＮＡEN2TpcaX5o
----------------html-----------------57Please respect copyright.ＰＥＮＡＮＡ0wkvkfeCW7
<a href="/zen.iso">zen.iso</a>57Please respect copyright.ＰＥＮＡＮＡqk36QS8a1j
-------------------------------------57Please respect copyright.ＰＥＮＡＮＡX6HxvmnGBy
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:57Please respect copyright.ＰＥＮＡＮＡjAVEh8PeH7
----------get request------------57Please respect copyright.ＰＥＮＡＮＡHsRWH36JzM
GET /zen.iso HTTP/1.157Please respect copyright.ＰＥＮＡＮＡgiLifjmABF
......57Please respect copyright.ＰＥＮＡＮＡyOX4YMLFCc
---------------------------------
57Please respect copyright.ＰＥＮＡＮＡ8GdjheGrVi
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:57Please respect copyright.ＰＥＮＡＮＡ0TsnZun3U6
----------------server response--------------57Please respect copyright.ＰＥＮＡＮＡLNTiBYUNR5
HTTP/1.1 200 OK57Please respect copyright.ＰＥＮＡＮＡ3sCS5cQ6KH
Date: Sat, 23 Nov 2024 06:29:19 GMT57Please respect copyright.ＰＥＮＡＮＡERxmh2Ghz9
accept-ranges: bytes57Please respect copyright.ＰＥＮＡＮＡBEb6aMhoXI
content-range: bytes 0-534231199/53423120057Please respect copyright.ＰＥＮＡＮＡfxZqsjLeiH
ETag: "534231200_1732277116"57Please respect copyright.ＰＥＮＡＮＡ1YXvwfdFGe
Content-Disposition: attachment; filename="zen.iso"57Please respect copyright.ＰＥＮＡＮＡ03Mx5IufDY
Content-Length: 53423120057Please respect copyright.ＰＥＮＡＮＡGk7SktTyzy
Content-Type: application/octet-stream
(http response body is the whole file content)57Please respect copyright.ＰＥＮＡＮＡsgkj4zV6zT
-------------------------------------------
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:57Please respect copyright.ＰＥＮＡＮＡeQFiGTkQd6
---------------http server send big file content by multiple times--------------57Please respect copyright.ＰＥＮＡＮＡ04v4JHtyAM
const int buf_size=20000000; // buf size is about 20M57Please respect copyright.ＰＥＮＡＮＡl01h1pcPm2
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers57Please respect copyright.ＰＥＮＡＮＡjpS7xKj7Mc
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 big57Please respect copyright.ＰＥＮＡＮＡ8Zu970Zfi8
while(there_is_remain_file_content_to_send)57Please respect copyright.ＰＥＮＡＮＡxI7fxvOgqV
{ 57Please respect copyright.ＰＥＮＡＮＡHXYsmLwipd
read_remain_file_content_to_send_buf(file, send_buf);57Please respect copyright.ＰＥＮＡＮＡ1XZR4eNbbZ
send_result = send(client_socket, send_buf);57Please respect copyright.ＰＥＮＡＮＡHC0DwHs8hM
if(send_result == fail) return; else continue;57Please respect copyright.ＰＥＮＡＮＡzIPUaxX7tl
}
// a way to make etag for a file using its file size and last modifed time57Please respect copyright.ＰＥＮＡＮＡ6xA9FesWEM
const char *file_etag(const char *path) 57Please respect copyright.ＰＥＮＡＮＡ1nKMyIzApL
{57Please respect copyright.ＰＥＮＡＮＡlUk0yJJ0w3
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;57Please respect copyright.ＰＥＮＡＮＡ1tOLw1ctfL
}57Please respect copyright.ＰＥＮＡＮＡ459N6SK6Eg
-------------------------------------------------57Please respect copyright.ＰＥＮＡＮＡ3KLBoEG7RO
"accept-ranges: bytes" tells client web browser that the http server support range.57Please respect copyright.ＰＥＮＡＮＡYLnhtwydez
"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).57Please respect copyright.ＰＥＮＡＮＡ7FtipOj6l5
'ETag: "534231200_1732277116"' tells client web browser the file version.57Please respect copyright.ＰＥＮＡＮＡJHIprzpvhj
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.57Please respect copyright.ＰＥＮＡＮＡArE0QXtDP4
"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.57Please respect copyright.ＰＥＮＡＮＡRmTkmHNYHS
"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.57Please respect copyright.ＰＥＮＡＮＡmyZNE7SMNj
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.)57Please respect copyright.ＰＥＮＡＮＡRwfIg0zjIV
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:57Please respect copyright.ＰＥＮＡＮＡwZs8Okkpqk
---------resume range request---------57Please respect copyright.ＰＥＮＡＮＡIuMXewm94b
GET /zen.iso HTTP/1.157Please respect copyright.ＰＥＮＡＮＡoOzhS89ue4
Range: bytes=243220480-57Please respect copyright.ＰＥＮＡＮＡkF9lE6pG7U
If-Match: "534231200_1732277116"57Please respect copyright.ＰＥＮＡＮＡ9lK0obIefM
......57Please respect copyright.ＰＥＮＡＮＡv6BRj46Gx1
-------------------------------------57Please respect copyright.ＰＥＮＡＮＡUhHFIH9iF2
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.57Please respect copyright.ＰＥＮＡＮＡF2eSXZu0Ev
'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:57Please respect copyright.ＰＥＮＡＮＡhHmrJDWMVl
-------response content from resume point-------57Please respect copyright.ＰＥＮＡＮＡNvLdZXJvNV
HTTP/1.1 206 Partial Content57Please respect copyright.ＰＥＮＡＮＡqIBdgA4HuQ
Date: Sat, 23 Nov 2024 06:56:17 GMT57Please respect copyright.ＰＥＮＡＮＡiIJeHgqZFM
accept-ranges: bytes57Please respect copyright.ＰＥＮＡＮＡuI2MlnPT7g
content-range: bytes 243220480-534231199/53423120057Please respect copyright.ＰＥＮＡＮＡMt3Y5XKAdT
Content-Length: 29101072057Please respect copyright.ＰＥＮＡＮＡ5t7dSzRrK0
Content-Type: application/octet-stream57Please respect copyright.ＰＥＮＡＮＡiCZ53GA6UY
Content-Disposition: attachment; filename="zen.iso"57Please respect copyright.ＰＥＮＡＮＡ4xQc7DFE5o
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)57Please respect copyright.ＰＥＮＡＮＡDiKvbKgZzC
------------------------------------------------57Please respect copyright.ＰＥＮＡＮＡKsUheRApSP
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:57Please respect copyright.ＰＥＮＡＮＡnNqnXngyPH
-------response content from resume point-------57Please respect copyright.ＰＥＮＡＮＡkMR03euIBW
HTTP/1.1 200 OK57Please respect copyright.ＰＥＮＡＮＡXguyty7sqp
Date: Sat, 23 Nov 2024 06:29:19 GMT57Please respect copyright.ＰＥＮＡＮＡtTFAzUn200
accept-ranges: bytes57Please respect copyright.ＰＥＮＡＮＡcu2mFOQANa
content-range: bytes 0-634231201/63423120257Please respect copyright.ＰＥＮＡＮＡurgaFlkyeE
ETag: "634231202_2732277118"57Please respect copyright.ＰＥＮＡＮＡF0HfxnRNgM
Content-Disposition: attachment; filename="zen.iso"57Please respect copyright.ＰＥＮＡＮＡo4IJNVg3aj
Content-Length: 63423120257Please respect copyright.ＰＥＮＡＮＡDNuDMvXSON
Content-Type: application/octet-stream
(http response body is the whole file content)57Please respect copyright.ＰＥＮＡＮＡOTclWQGbKY
------------------------------------------------57Please respect copyright.ＰＥＮＡＮＡTbehbHbUBo
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:57Please respect copyright.ＰＥＮＡＮＡwKfKLIeUDN
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
57Please respect copyright.ＰＥＮＡＮＡHUmmtPoKSN
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57Please respect copyright.ＰＥＮＡＮＡFD5KmVYma6
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57Please respect copyright.ＰＥＮＡＮＡEV235kn72k
https://gemini.google.com/
感谢互联网上无私奉献的参考资57Please respect copyright.ＰＥＮＡＮＡ9lp5dE89xJ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests57Please respect copyright.ＰＥＮＡＮＡqftiMofesK
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range57Please respect copyright.ＰＥＮＡＮＡIungluQNYy
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag57Please respect copyright.ＰＥＮＡＮＡWwJTmQ9xTU
https://www.rfc-editor.org/rfc/rfc723357Please respect copyright.ＰＥＮＡＮＡJYNPJBnQQL
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.com57Please respect copyright.ＰＥＮＡＮＡiWSnpEMo8c
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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