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:159Please respect copyright.ＰＥＮＡＮＡ7FhKHa8twz
----------http range request-------159Please respect copyright.ＰＥＮＡＮＡcsWHHi3w5i
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡzrFBfDzrg2
Range: bytes=100-1023159Please respect copyright.ＰＥＮＡＮＡEhSumVGvKi
If-Match: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡdCaBox4CsU
......159Please respect copyright.ＰＥＮＡＮＡ5JsTCFPoXs
-----------------------------------159Please respect copyright.ＰＥＮＡＮＡvmwiZ5Kwtc
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:159Please respect copyright.ＰＥＮＡＮＡTvXWGsMTOy
----------------html-----------------159Please respect copyright.ＰＥＮＡＮＡuiEhRbIO2m
<a href="/zen.iso">zen.iso</a>159Please respect copyright.ＰＥＮＡＮＡEOdqatPGEs
-------------------------------------159Please respect copyright.ＰＥＮＡＮＡiszxlPsTNO
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:159Please respect copyright.ＰＥＮＡＮＡKuteGabzxt
----------get request------------159Please respect copyright.ＰＥＮＡＮＡT9WR47ks0M
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡlgHUTjj3NK
......159Please respect copyright.ＰＥＮＡＮＡXeFRZFhTEf
---------------------------------
159Please respect copyright.ＰＥＮＡＮＡjxDPemHfbo
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:159Please respect copyright.ＰＥＮＡＮＡiJ7toPkPfm
----------------server response--------------159Please respect copyright.ＰＥＮＡＮＡg8XdLKlvSE
HTTP/1.1 200 OK159Please respect copyright.ＰＥＮＡＮＡbvvQ0TvP2u
Date: Sat, 23 Nov 2024 06:29:19 GMT159Please respect copyright.ＰＥＮＡＮＡZPq85wY1Dw
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡSLaeON7Hnr
content-range: bytes 0-534231199/534231200159Please respect copyright.ＰＥＮＡＮＡnGmK1CQfP8
ETag: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡj9JOoTT1Js
Content-Disposition: attachment; filename="zen.iso"159Please respect copyright.ＰＥＮＡＮＡhosAsPlpKG
Content-Length: 534231200159Please respect copyright.ＰＥＮＡＮＡHjrRp8vzLa
Content-Type: application/octet-stream
(http response body is the whole file content)159Please respect copyright.ＰＥＮＡＮＡyYOjGxk6w4
-------------------------------------------
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:159Please respect copyright.ＰＥＮＡＮＡEvurIYKA3D
---------------http server send big file content by multiple times--------------159Please respect copyright.ＰＥＮＡＮＡrylM4VfGYa
const int buf_size=20000000; // buf size is about 20M159Please respect copyright.ＰＥＮＡＮＡLFVlMLLYv6
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers159Please respect copyright.ＰＥＮＡＮＡ0bDZtR4L8o
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 big159Please respect copyright.ＰＥＮＡＮＡjRTlHu1cEx
while(there_is_remain_file_content_to_send)159Please respect copyright.ＰＥＮＡＮＡGv7FMSGhh3
{ 159Please respect copyright.ＰＥＮＡＮＡlS1ZUQrqQB
read_remain_file_content_to_send_buf(file, send_buf);159Please respect copyright.ＰＥＮＡＮＡbkJpAxiD1b
send_result = send(client_socket, send_buf);159Please respect copyright.ＰＥＮＡＮＡKZmtoQLE5B
if(send_result == fail) return; else continue;159Please respect copyright.ＰＥＮＡＮＡV0LhvSr2qZ
}
// a way to make etag for a file using its file size and last modifed time159Please respect copyright.ＰＥＮＡＮＡgupjrZyj1j
const char *file_etag(const char *path) 159Please respect copyright.ＰＥＮＡＮＡmjTch5Y94k
{159Please respect copyright.ＰＥＮＡＮＡxgs2ewcn4G
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;159Please respect copyright.ＰＥＮＡＮＡcISvC1Z5Ka
}159Please respect copyright.ＰＥＮＡＮＡz71epEc3fW
-------------------------------------------------159Please respect copyright.ＰＥＮＡＮＡ6DJxjeJQxn
"accept-ranges: bytes" tells client web browser that the http server support range.159Please respect copyright.ＰＥＮＡＮＡ2RXWjYnR6s
"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).159Please respect copyright.ＰＥＮＡＮＡ69IF4YL3SS
'ETag: "534231200_1732277116"' tells client web browser the file version.159Please respect copyright.ＰＥＮＡＮＡkIrTcwpFkF
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.159Please respect copyright.ＰＥＮＡＮＡYdRvtg47e6
"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.159Please respect copyright.ＰＥＮＡＮＡov4EAUin4i
"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.159Please respect copyright.ＰＥＮＡＮＡ65mCJkO2iQ
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.)159Please respect copyright.ＰＥＮＡＮＡDifCsPHRTz
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:159Please respect copyright.ＰＥＮＡＮＡaGQ8A7R8iT
---------resume range request---------159Please respect copyright.ＰＥＮＡＮＡCn3eTYDhzU
GET /zen.iso HTTP/1.1159Please respect copyright.ＰＥＮＡＮＡJSFmu2Lxth
Range: bytes=243220480-159Please respect copyright.ＰＥＮＡＮＡkY2EcA3dfU
If-Match: "534231200_1732277116"159Please respect copyright.ＰＥＮＡＮＡJl5cPRF9I6
......159Please respect copyright.ＰＥＮＡＮＡoaDoxzLod4
-------------------------------------159Please respect copyright.ＰＥＮＡＮＡWJ3aQsFYRi
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.159Please respect copyright.ＰＥＮＡＮＡOqRLHwgIBH
'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:159Please respect copyright.ＰＥＮＡＮＡVCrYR7lJpy
-------response content from resume point-------159Please respect copyright.ＰＥＮＡＮＡMBWJhkoRQG
HTTP/1.1 206 Partial Content159Please respect copyright.ＰＥＮＡＮＡYb3Vku3PeJ
Date: Sat, 23 Nov 2024 06:56:17 GMT159Please respect copyright.ＰＥＮＡＮＡGBFqX9OkKD
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡbwK4AEASMi
content-range: bytes 243220480-534231199/534231200159Please respect copyright.ＰＥＮＡＮＡE2sOojYI4g
Content-Length: 291010720159Please respect copyright.ＰＥＮＡＮＡSz7EPk3Fja
Content-Type: application/octet-stream159Please respect copyright.ＰＥＮＡＮＡ9UVQGUNzHh
Content-Disposition: attachment; filename="zen.iso"159Please respect copyright.ＰＥＮＡＮＡ1VvswiL1hL
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)159Please respect copyright.ＰＥＮＡＮＡhMcLsFTOww
------------------------------------------------159Please respect copyright.ＰＥＮＡＮＡSPfXjNGY9p
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:159Please respect copyright.ＰＥＮＡＮＡB8dl12v42U
-------response content from resume point-------159Please respect copyright.ＰＥＮＡＮＡeAQF3zhN5d
HTTP/1.1 200 OK159Please respect copyright.ＰＥＮＡＮＡKfEO3T3grt
Date: Sat, 23 Nov 2024 06:29:19 GMT159Please respect copyright.ＰＥＮＡＮＡxyoGk9nbvT
accept-ranges: bytes159Please respect copyright.ＰＥＮＡＮＡEDGGyRFlyU
content-range: bytes 0-634231201/634231202159Please respect copyright.ＰＥＮＡＮＡi2akfX9PPZ
ETag: "634231202_2732277118"159Please respect copyright.ＰＥＮＡＮＡCIek0Mz6Cj
Content-Disposition: attachment; filename="zen.iso"159Please respect copyright.ＰＥＮＡＮＡmjwaf3vKv6
Content-Length: 634231202159Please respect copyright.ＰＥＮＡＮＡ290PLoat0V
Content-Type: application/octet-stream
(http response body is the whole file content)159Please respect copyright.ＰＥＮＡＮＡkrsb2c5pWN
------------------------------------------------159Please respect copyright.ＰＥＮＡＮＡCjhbDz1z58
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:159Please respect copyright.ＰＥＮＡＮＡrgwcbMX5Fl
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
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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159Please respect copyright.ＰＥＮＡＮＡ0d3Skb5x4Z
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159Please respect copyright.ＰＥＮＡＮＡvS5vBQZH8k
https://gemini.google.com/
感谢互联网上无私奉献的参考资159Please respect copyright.ＰＥＮＡＮＡQ2L40sO1Dl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests159Please respect copyright.ＰＥＮＡＮＡEzBaMecOlv
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range159Please respect copyright.ＰＥＮＡＮＡm9AV6LJifl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag159Please respect copyright.ＰＥＮＡＮＡlyfPvFHAM1
https://www.rfc-editor.org/rfc/rfc7233159Please respect copyright.ＰＥＮＡＮＡrPh0bVaQH4
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]159Please respect copyright.ＰＥＮＡＮＡq7Q0heOnZY
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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