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:39Please respect copyright.ＰＥＮＡＮＡ5FuhZ1I7Mf
----------http range request-------39Please respect copyright.ＰＥＮＡＮＡ2gZm17pKii
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡvVI8iqBqQp
Range: bytes=100-102339Please respect copyright.ＰＥＮＡＮＡpb6YK5L0nq
If-Match: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡlOmcwLjAEx
......39Please respect copyright.ＰＥＮＡＮＡbb4iVjTMVS
-----------------------------------39Please respect copyright.ＰＥＮＡＮＡvvAzXtaNgZ
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:39Please respect copyright.ＰＥＮＡＮＡ2GVxAgMsj8
----------------html-----------------39Please respect copyright.ＰＥＮＡＮＡWgmYfmBGQx
<a href="/zen.iso">zen.iso</a>39Please respect copyright.ＰＥＮＡＮＡpFuP6CCxFw
-------------------------------------39Please respect copyright.ＰＥＮＡＮＡm7GFerNXfx
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:39Please respect copyright.ＰＥＮＡＮＡYkC504IVGw
----------get request------------39Please respect copyright.ＰＥＮＡＮＡkbZQ4gVGld
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡ6kfhpr12nJ
......39Please respect copyright.ＰＥＮＡＮＡHLDq1tesEx
---------------------------------
39Please respect copyright.ＰＥＮＡＮＡi4Wkd88j3D
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:39Please respect copyright.ＰＥＮＡＮＡzenz3fAcmX
----------------server response--------------39Please respect copyright.ＰＥＮＡＮＡAs1fImwnJx
HTTP/1.1 200 OK39Please respect copyright.ＰＥＮＡＮＡ2zCac26Mlo
Date: Sat, 23 Nov 2024 06:29:19 GMT39Please respect copyright.ＰＥＮＡＮＡ3QhQzn7fDT
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡOI3Nd8yhht
content-range: bytes 0-534231199/53423120039Please respect copyright.ＰＥＮＡＮＡbCGhO9Hn34
ETag: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡNL57zhbALe
Content-Disposition: attachment; filename="zen.iso"39Please respect copyright.ＰＥＮＡＮＡOa7DSIIe2L
Content-Length: 53423120039Please respect copyright.ＰＥＮＡＮＡ2dcRBvp4ro
Content-Type: application/octet-stream
(http response body is the whole file content)39Please respect copyright.ＰＥＮＡＮＡo38EkHwGuN
-------------------------------------------
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:39Please respect copyright.ＰＥＮＡＮＡLyroy3EbwY
---------------http server send big file content by multiple times--------------39Please respect copyright.ＰＥＮＡＮＡgYFX5iJuqc
const int buf_size=20000000; // buf size is about 20M39Please respect copyright.ＰＥＮＡＮＡ0U2ulSCkmb
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers39Please respect copyright.ＰＥＮＡＮＡqgTeKByOSa
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 big39Please respect copyright.ＰＥＮＡＮＡhLNapgQSs4
while(there_is_remain_file_content_to_send)39Please respect copyright.ＰＥＮＡＮＡkRpjLenWo5
{ 39Please respect copyright.ＰＥＮＡＮＡNWCoCa7nBJ
read_remain_file_content_to_send_buf(file, send_buf);39Please respect copyright.ＰＥＮＡＮＡpCi9kBWtPe
send_result = send(client_socket, send_buf);39Please respect copyright.ＰＥＮＡＮＡxK7Nj2dhsz
if(send_result == fail) return; else continue;39Please respect copyright.ＰＥＮＡＮＡBlOOq04Znm
}
// a way to make etag for a file using its file size and last modifed time39Please respect copyright.ＰＥＮＡＮＡ0hZpC7b1Mu
const char *file_etag(const char *path) 39Please respect copyright.ＰＥＮＡＮＡMzqM6avqJL
{39Please respect copyright.ＰＥＮＡＮＡqRBuluoXGw
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;39Please respect copyright.ＰＥＮＡＮＡ85lbF1B4CQ
}39Please respect copyright.ＰＥＮＡＮＡgJl3hLGjVX
-------------------------------------------------39Please respect copyright.ＰＥＮＡＮＡamvETq5HY1
"accept-ranges: bytes" tells client web browser that the http server support range.39Please respect copyright.ＰＥＮＡＮＡqwb8fymGbC
"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).39Please respect copyright.ＰＥＮＡＮＡ3GQmUupHdy
'ETag: "534231200_1732277116"' tells client web browser the file version.39Please respect copyright.ＰＥＮＡＮＡCLQYKf6Rpk
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.39Please respect copyright.ＰＥＮＡＮＡEMfklfvE1V
"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.39Please respect copyright.ＰＥＮＡＮＡnQNGCn7x8P
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
39Please respect copyright.ＰＥＮＡＮＡM6WOt3TmcE
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.39Please respect copyright.ＰＥＮＡＮＡGs8sO9Yb0z
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.)39Please respect copyright.ＰＥＮＡＮＡZIfWkR0DNY
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.
39Please respect copyright.ＰＥＮＡＮＡMgwd5kUbws
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:39Please respect copyright.ＰＥＮＡＮＡiINgbG9AVV
---------resume range request---------39Please respect copyright.ＰＥＮＡＮＡM1Uy7nTVrU
GET /zen.iso HTTP/1.139Please respect copyright.ＰＥＮＡＮＡrQLMRVVGr2
Range: bytes=243220480-39Please respect copyright.ＰＥＮＡＮＡvsol5YyVFE
If-Match: "534231200_1732277116"39Please respect copyright.ＰＥＮＡＮＡHUHIAsurgr
......39Please respect copyright.ＰＥＮＡＮＡAUNoyfOs0Z
-------------------------------------39Please respect copyright.ＰＥＮＡＮＡ7AhLMnUbYF
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.39Please respect copyright.ＰＥＮＡＮＡuYNMnXcbA8
'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:39Please respect copyright.ＰＥＮＡＮＡbB2jW2jVUO
-------response content from resume point-------39Please respect copyright.ＰＥＮＡＮＡRsohhjE2Rc
HTTP/1.1 206 Partial Content39Please respect copyright.ＰＥＮＡＮＡCHGPVmcYkq
Date: Sat, 23 Nov 2024 06:56:17 GMT39Please respect copyright.ＰＥＮＡＮＡ2c7ycwllEn
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡdwBuk8P3s5
content-range: bytes 243220480-534231199/53423120039Please respect copyright.ＰＥＮＡＮＡGiWEZho65g
Content-Length: 29101072039Please respect copyright.ＰＥＮＡＮＡ5rVNmMFKl3
Content-Type: application/octet-stream39Please respect copyright.ＰＥＮＡＮＡXueY3tu1fU
Content-Disposition: attachment; filename="zen.iso"39Please respect copyright.ＰＥＮＡＮＡtdNpE2xP22
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)39Please respect copyright.ＰＥＮＡＮＡnI9FjRd2L1
------------------------------------------------39Please respect copyright.ＰＥＮＡＮＡOWub5UgZnO
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:39Please respect copyright.ＰＥＮＡＮＡ7ziyJvZvHx
-------response content from resume point-------39Please respect copyright.ＰＥＮＡＮＡd84cXKvWt8
HTTP/1.1 200 OK39Please respect copyright.ＰＥＮＡＮＡEyRBPjFROf
Date: Sat, 23 Nov 2024 06:29:19 GMT39Please respect copyright.ＰＥＮＡＮＡzqU6b1uNR7
accept-ranges: bytes39Please respect copyright.ＰＥＮＡＮＡVWhosB2irJ
content-range: bytes 0-634231201/63423120239Please respect copyright.ＰＥＮＡＮＡp0WNkdEoEz
ETag: "634231202_2732277118"39Please respect copyright.ＰＥＮＡＮＡiZXLE6KbqU
Content-Disposition: attachment; filename="zen.iso"39Please respect copyright.ＰＥＮＡＮＡUnIQMD29ia
Content-Length: 63423120239Please respect copyright.ＰＥＮＡＮＡX9VfzoYpLf
Content-Type: application/octet-stream
(http response body is the whole file content)39Please respect copyright.ＰＥＮＡＮＡm6LBkG9ROM
------------------------------------------------39Please respect copyright.ＰＥＮＡＮＡPhVtoVkxqd
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
39Please respect copyright.ＰＥＮＡＮＡbS4Lzw6Mdr
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:39Please respect copyright.ＰＥＮＡＮＡXZTAgIxiyr
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
39Please respect copyright.ＰＥＮＡＮＡ27ElGVVWGD
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39Please respect copyright.ＰＥＮＡＮＡjUG0NKcpSS
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39Please respect copyright.ＰＥＮＡＮＡ3iWht7JvH4
https://gemini.google.com/
感谢互联网上无私奉献的参考资39Please respect copyright.ＰＥＮＡＮＡiODZF2lDd3
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests39Please respect copyright.ＰＥＮＡＮＡcHSncasH8j
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range39Please respect copyright.ＰＥＮＡＮＡeSUAptdcEj
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag39Please respect copyright.ＰＥＮＡＮＡ2quZNC9QF4
https://www.rfc-editor.org/rfc/rfc723339Please respect copyright.ＰＥＮＡＮＡ3gNrqcFwdG
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/
39Please respect copyright.ＰＥＮＡＮＡQAtkh6i8cb
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.com39Please respect copyright.ＰＥＮＡＮＡeRKhhfFFpQ
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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