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:128Please respect copyright.ＰＥＮＡＮＡf1EERcZL7N
----------http range request-------128Please respect copyright.ＰＥＮＡＮＡV5N0cPlj36
GET /zen.iso HTTP/1.1128Please respect copyright.ＰＥＮＡＮＡSYdGsaDaUM
Range: bytes=100-1023128Please respect copyright.ＰＥＮＡＮＡ552gBZYAZ5
If-Match: "534231200_1732277116"128Please respect copyright.ＰＥＮＡＮＡ3ef9DZxHpH
......128Please respect copyright.ＰＥＮＡＮＡ66dGUpXBrI
-----------------------------------128Please respect copyright.ＰＥＮＡＮＡnnqliR002W
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:128Please respect copyright.ＰＥＮＡＮＡyaajgYhl1Z
----------------html-----------------128Please respect copyright.ＰＥＮＡＮＡ47LZkV4mbJ
<a href="/zen.iso">zen.iso</a>128Please respect copyright.ＰＥＮＡＮＡWff5USBoD9
-------------------------------------128Please respect copyright.ＰＥＮＡＮＡD6z5cumKzX
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:128Please respect copyright.ＰＥＮＡＮＡKUFzOVS3ax
----------get request------------128Please respect copyright.ＰＥＮＡＮＡULeDKNajqm
GET /zen.iso HTTP/1.1128Please respect copyright.ＰＥＮＡＮＡrNLoekzAlw
......128Please respect copyright.ＰＥＮＡＮＡYjxLYLfHXo
---------------------------------
128Please respect copyright.ＰＥＮＡＮＡx6oszmW2vc
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:128Please respect copyright.ＰＥＮＡＮＡYbWLiLSmST
----------------server response--------------128Please respect copyright.ＰＥＮＡＮＡNeHYeN35CI
HTTP/1.1 200 OK128Please respect copyright.ＰＥＮＡＮＡu9eZQtrToQ
Date: Sat, 23 Nov 2024 06:29:19 GMT128Please respect copyright.ＰＥＮＡＮＡwo09ee3aDv
accept-ranges: bytes128Please respect copyright.ＰＥＮＡＮＡIWB501WZNU
content-range: bytes 0-534231199/534231200128Please respect copyright.ＰＥＮＡＮＡ7tSbr0z4PU
ETag: "534231200_1732277116"128Please respect copyright.ＰＥＮＡＮＡPWijfF7Uxx
Content-Disposition: attachment; filename="zen.iso"128Please respect copyright.ＰＥＮＡＮＡvQ1pz0RE76
Content-Length: 534231200128Please respect copyright.ＰＥＮＡＮＡozqB2H9ixu
Content-Type: application/octet-stream
(http response body is the whole file content)128Please respect copyright.ＰＥＮＡＮＡZbkdkyf9WK
-------------------------------------------
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:128Please respect copyright.ＰＥＮＡＮＡH8nXay1rva
---------------http server send big file content by multiple times--------------128Please respect copyright.ＰＥＮＡＮＡCyN7okFP5d
const int buf_size=20000000; // buf size is about 20M128Please respect copyright.ＰＥＮＡＮＡjRw4enBhcv
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers128Please respect copyright.ＰＥＮＡＮＡn1AY95W2zx
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 big128Please respect copyright.ＰＥＮＡＮＡQhrSDJ84E9
while(there_is_remain_file_content_to_send)128Please respect copyright.ＰＥＮＡＮＡsD5vAtG0Hn
{ 128Please respect copyright.ＰＥＮＡＮＡu9pICUt0fi
read_remain_file_content_to_send_buf(file, send_buf);128Please respect copyright.ＰＥＮＡＮＡlPITpdqBgp
send_result = send(client_socket, send_buf);128Please respect copyright.ＰＥＮＡＮＡfqEeXybikv
if(send_result == fail) return; else continue;128Please respect copyright.ＰＥＮＡＮＡ8nHwy8eNWA
}
// a way to make etag for a file using its file size and last modifed time128Please respect copyright.ＰＥＮＡＮＡQriKl6zUCd
const char *file_etag(const char *path) 128Please respect copyright.ＰＥＮＡＮＡ5c2qkFDL3B
{128Please respect copyright.ＰＥＮＡＮＡJeic4awQTJ
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;128Please respect copyright.ＰＥＮＡＮＡ0DDiWteZlW
}128Please respect copyright.ＰＥＮＡＮＡqUvWsjSqFw
-------------------------------------------------128Please respect copyright.ＰＥＮＡＮＡX151dSvMGi
"accept-ranges: bytes" tells client web browser that the http server support range.128Please respect copyright.ＰＥＮＡＮＡ20rUzB1Bpe
"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).128Please respect copyright.ＰＥＮＡＮＡkHs5z9PSh2
'ETag: "534231200_1732277116"' tells client web browser the file version.128Please respect copyright.ＰＥＮＡＮＡ7TMIHA7mif
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.128Please respect copyright.ＰＥＮＡＮＡ71lTIQcLL8
"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.128Please respect copyright.ＰＥＮＡＮＡLTLvffH4bZ
"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.128Please respect copyright.ＰＥＮＡＮＡKajrLRK8Ih
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.)128Please respect copyright.ＰＥＮＡＮＡHtIVg3wYAV
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:128Please respect copyright.ＰＥＮＡＮＡwpjzjRgBro
---------resume range request---------128Please respect copyright.ＰＥＮＡＮＡKPkFx6zabv
GET /zen.iso HTTP/1.1128Please respect copyright.ＰＥＮＡＮＡq96VUkNa83
Range: bytes=243220480-128Please respect copyright.ＰＥＮＡＮＡSD5G0JeC0r
If-Match: "534231200_1732277116"128Please respect copyright.ＰＥＮＡＮＡAoSpcypOhm
......128Please respect copyright.ＰＥＮＡＮＡTSPIBnKZhl
-------------------------------------128Please respect copyright.ＰＥＮＡＮＡQcdMtUz1R5
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.128Please respect copyright.ＰＥＮＡＮＡnmlHbflhGf
'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:128Please respect copyright.ＰＥＮＡＮＡIT89GhXq9O
-------response content from resume point-------128Please respect copyright.ＰＥＮＡＮＡueYZZwpr9q
HTTP/1.1 206 Partial Content128Please respect copyright.ＰＥＮＡＮＡE3xb08oce0
Date: Sat, 23 Nov 2024 06:56:17 GMT128Please respect copyright.ＰＥＮＡＮＡ4n0pSn3Gvv
accept-ranges: bytes128Please respect copyright.ＰＥＮＡＮＡO04pZjZZYf
content-range: bytes 243220480-534231199/534231200128Please respect copyright.ＰＥＮＡＮＡnys5768JNL
Content-Length: 291010720128Please respect copyright.ＰＥＮＡＮＡiBbA6Otm9N
Content-Type: application/octet-stream128Please respect copyright.ＰＥＮＡＮＡz258O8fij8
Content-Disposition: attachment; filename="zen.iso"128Please respect copyright.ＰＥＮＡＮＡ1f2ImMwyg0
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)128Please respect copyright.ＰＥＮＡＮＡSy5zS9iul5
------------------------------------------------128Please respect copyright.ＰＥＮＡＮＡgsHYSV46V9
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:128Please respect copyright.ＰＥＮＡＮＡnKP1Ve6cOY
-------response content from resume point-------128Please respect copyright.ＰＥＮＡＮＡsivOFW3J0X
HTTP/1.1 200 OK128Please respect copyright.ＰＥＮＡＮＡ3XCpQmAXJ7
Date: Sat, 23 Nov 2024 06:29:19 GMT128Please respect copyright.ＰＥＮＡＮＡoEagFQMvrd
accept-ranges: bytes128Please respect copyright.ＰＥＮＡＮＡ5ykIijSDel
content-range: bytes 0-634231201/634231202128Please respect copyright.ＰＥＮＡＮＡtR88PhsNOR
ETag: "634231202_2732277118"128Please respect copyright.ＰＥＮＡＮＡ4uaBEanTfl
Content-Disposition: attachment; filename="zen.iso"128Please respect copyright.ＰＥＮＡＮＡTz9W7xahIk
Content-Length: 634231202128Please respect copyright.ＰＥＮＡＮＡkJ2uGU9B6H
Content-Type: application/octet-stream
(http response body is the whole file content)128Please respect copyright.ＰＥＮＡＮＡNf4tNylL43
------------------------------------------------128Please respect copyright.ＰＥＮＡＮＡsXu2GpCQBv
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:128Please respect copyright.ＰＥＮＡＮＡEzc4l5LHSB
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
128Please respect copyright.ＰＥＮＡＮＡH3ed7OZkuc
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128Please respect copyright.ＰＥＮＡＮＡkUUwwGTQCk
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128Please respect copyright.ＰＥＮＡＮＡOOSvw38HcU
https://gemini.google.com/
感谢互联网上无私奉献的参考资128Please respect copyright.ＰＥＮＡＮＡnaQwS9MmAQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests128Please respect copyright.ＰＥＮＡＮＡWaaiXDHK9l
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range128Please respect copyright.ＰＥＮＡＮＡwUT3mMjjbP
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag128Please respect copyright.ＰＥＮＡＮＡu3uOyWuUNL
https://www.rfc-editor.org/rfc/rfc7233128Please respect copyright.ＰＥＮＡＮＡO8sI5SJ828
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]128Please respect copyright.ＰＥＮＡＮＡqQ7VVzqtia
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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