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:198Please respect copyright.ＰＥＮＡＮＡite4QAadRz
----------http range request-------198Please respect copyright.ＰＥＮＡＮＡMs5CH9bp5P
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡmTrln2lfQk
Range: bytes=100-1023198Please respect copyright.ＰＥＮＡＮＡr7sqkHhrne
If-Match: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡFN0E5bli07
......198Please respect copyright.ＰＥＮＡＮＡwDUD17zO5R
-----------------------------------198Please respect copyright.ＰＥＮＡＮＡ3iyidF0Pa4
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:198Please respect copyright.ＰＥＮＡＮＡFBs1e31oq4
----------------html-----------------198Please respect copyright.ＰＥＮＡＮＡfFEDE8YLLW
<a href="/zen.iso">zen.iso</a>198Please respect copyright.ＰＥＮＡＮＡQlgKBm7tGe
-------------------------------------198Please respect copyright.ＰＥＮＡＮＡJS5yVBApxi
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:198Please respect copyright.ＰＥＮＡＮＡuYKt15NkA0
----------get request------------198Please respect copyright.ＰＥＮＡＮＡGkaZ1AC5rp
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡgmQFwwi1cc
......198Please respect copyright.ＰＥＮＡＮＡdd42aarcCF
---------------------------------
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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:198Please respect copyright.ＰＥＮＡＮＡpnRdIRen71
----------------server response--------------198Please respect copyright.ＰＥＮＡＮＡMYf6aiiBtt
HTTP/1.1 200 OK198Please respect copyright.ＰＥＮＡＮＡ1beXTnenzU
Date: Sat, 23 Nov 2024 06:29:19 GMT198Please respect copyright.ＰＥＮＡＮＡzc9K8pLZ1H
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡViSOWTkZki
content-range: bytes 0-534231199/534231200198Please respect copyright.ＰＥＮＡＮＡPYW0CB067X
ETag: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡzL0rBNzuhs
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡyt9QphtA3J
Content-Length: 534231200198Please respect copyright.ＰＥＮＡＮＡc1g4i8Hagk
Content-Type: application/octet-stream
(http response body is the whole file content)198Please respect copyright.ＰＥＮＡＮＡ9XVa4Bvf9z
-------------------------------------------
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:198Please respect copyright.ＰＥＮＡＮＡX9q4qFiCcn
---------------http server send big file content by multiple times--------------198Please respect copyright.ＰＥＮＡＮＡnSCP1pCNCg
const int buf_size=20000000; // buf size is about 20M198Please respect copyright.ＰＥＮＡＮＡlTkkRbYM7o
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers198Please respect copyright.ＰＥＮＡＮＡohOIrddT2c
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 big198Please respect copyright.ＰＥＮＡＮＡakjTZpim2e
while(there_is_remain_file_content_to_send)198Please respect copyright.ＰＥＮＡＮＡmb457Aq3ed
{ 198Please respect copyright.ＰＥＮＡＮＡOjDOiwCWWB
read_remain_file_content_to_send_buf(file, send_buf);198Please respect copyright.ＰＥＮＡＮＡAWHfpLQ9y3
send_result = send(client_socket, send_buf);198Please respect copyright.ＰＥＮＡＮＡGHdLServ36
if(send_result == fail) return; else continue;198Please respect copyright.ＰＥＮＡＮＡMxSElirZvm
}
// a way to make etag for a file using its file size and last modifed time198Please respect copyright.ＰＥＮＡＮＡoTFLuzRAE9
const char *file_etag(const char *path) 198Please respect copyright.ＰＥＮＡＮＡGEH6WnCtbP
{198Please respect copyright.ＰＥＮＡＮＡk6RRynH0ZQ
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;198Please respect copyright.ＰＥＮＡＮＡFJQ4RjbUKh
}198Please respect copyright.ＰＥＮＡＮＡ9z1ttP9AJ2
-------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡmrVmsTvZy9
"accept-ranges: bytes" tells client web browser that the http server support range.198Please respect copyright.ＰＥＮＡＮＡPQTliJuN1I
"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).198Please respect copyright.ＰＥＮＡＮＡAFBIahl2DW
'ETag: "534231200_1732277116"' tells client web browser the file version.198Please respect copyright.ＰＥＮＡＮＡTlwGFiVw5T
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.198Please respect copyright.ＰＥＮＡＮＡgUpEDzlKxN
"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.198Please respect copyright.ＰＥＮＡＮＡJsuZ6gxA9w
"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.198Please respect copyright.ＰＥＮＡＮＡHFoup0U6SZ
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.)198Please respect copyright.ＰＥＮＡＮＡcUvJTKBhfA
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:198Please respect copyright.ＰＥＮＡＮＡ3T3NBjXUTk
---------resume range request---------198Please respect copyright.ＰＥＮＡＮＡkwrYkLsADe
GET /zen.iso HTTP/1.1198Please respect copyright.ＰＥＮＡＮＡ2DtIXM5cKc
Range: bytes=243220480-198Please respect copyright.ＰＥＮＡＮＡtVk2qEDegV
If-Match: "534231200_1732277116"198Please respect copyright.ＰＥＮＡＮＡxnoDn2ZB5y
......198Please respect copyright.ＰＥＮＡＮＡ5UVHXPk5c0
-------------------------------------198Please respect copyright.ＰＥＮＡＮＡ5pgH0VYrFZ
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.198Please respect copyright.ＰＥＮＡＮＡO2KsFE4fI5
'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:198Please respect copyright.ＰＥＮＡＮＡiiQ7yebj95
-------response content from resume point-------198Please respect copyright.ＰＥＮＡＮＡrIBbwqyEFB
HTTP/1.1 206 Partial Content198Please respect copyright.ＰＥＮＡＮＡmKR9mDdZtA
Date: Sat, 23 Nov 2024 06:56:17 GMT198Please respect copyright.ＰＥＮＡＮＡQ0KejRT77d
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡe4ktvWjK72
content-range: bytes 243220480-534231199/534231200198Please respect copyright.ＰＥＮＡＮＡyMtLYZgOpo
Content-Length: 291010720198Please respect copyright.ＰＥＮＡＮＡUXf8ptR0zq
Content-Type: application/octet-stream198Please respect copyright.ＰＥＮＡＮＡuR1nqOTySh
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡekaLgGXsu6
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)198Please respect copyright.ＰＥＮＡＮＡZ1KNKymvU9
------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡpqjah0MrD9
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:198Please respect copyright.ＰＥＮＡＮＡ4fxrIBjEFv
-------response content from resume point-------198Please respect copyright.ＰＥＮＡＮＡPjGxl6v2VO
HTTP/1.1 200 OK198Please respect copyright.ＰＥＮＡＮＡnLybEg0t3i
Date: Sat, 23 Nov 2024 06:29:19 GMT198Please respect copyright.ＰＥＮＡＮＡAm0OimnEf3
accept-ranges: bytes198Please respect copyright.ＰＥＮＡＮＡydjf94ynEe
content-range: bytes 0-634231201/634231202198Please respect copyright.ＰＥＮＡＮＡTA31mr0xRW
ETag: "634231202_2732277118"198Please respect copyright.ＰＥＮＡＮＡVf9fzfo8pJ
Content-Disposition: attachment; filename="zen.iso"198Please respect copyright.ＰＥＮＡＮＡMtCDWuM2FE
Content-Length: 634231202198Please respect copyright.ＰＥＮＡＮＡBmQXtUr89t
Content-Type: application/octet-stream
(http response body is the whole file content)198Please respect copyright.ＰＥＮＡＮＡCgl4EYLX5v
------------------------------------------------198Please respect copyright.ＰＥＮＡＮＡfQAkvZ4tc2
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:198Please respect copyright.ＰＥＮＡＮＡ7DniIsKG6t
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
198Please respect copyright.ＰＥＮＡＮＡFL2lyUSefQ
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198Please respect copyright.ＰＥＮＡＮＡ5eYDo3skSo
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198Please respect copyright.ＰＥＮＡＮＡ9R0w8DPXjC
https://gemini.google.com/
感谢互联网上无私奉献的参考资198Please respect copyright.ＰＥＮＡＮＡRUt2d6zlcx
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests198Please respect copyright.ＰＥＮＡＮＡFHsAkiekY5
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range198Please respect copyright.ＰＥＮＡＮＡE5DA6uBeIC
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag198Please respect copyright.ＰＥＮＡＮＡvUCn3wcNJI
https://www.rfc-editor.org/rfc/rfc7233198Please respect copyright.ＰＥＮＡＮＡg8lgvJVXVK
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]198Please respect copyright.ＰＥＮＡＮＡBXRGcvXNcq
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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