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:80Please respect copyright.ＰＥＮＡＮＡIa126goHnL
----------http range request-------80Please respect copyright.ＰＥＮＡＮＡNkUZAtrd3u
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡYI14wl1GEl
Range: bytes=100-102380Please respect copyright.ＰＥＮＡＮＡPbBKCAGcHR
If-Match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡQmUeSfHMbj
......80Please respect copyright.ＰＥＮＡＮＡEPch3gHAXJ
-----------------------------------80Please respect copyright.ＰＥＮＡＮＡXpBxRmARhZ
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:80Please respect copyright.ＰＥＮＡＮＡH3m321B56s
----------------html-----------------80Please respect copyright.ＰＥＮＡＮＡm9Ntckx8Xp
<a href="/zen.iso">zen.iso</a>80Please respect copyright.ＰＥＮＡＮＡf6GnPxLzmg
-------------------------------------80Please respect copyright.ＰＥＮＡＮＡuXdy7Ms0uJ
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:80Please respect copyright.ＰＥＮＡＮＡmG28l2fuUt
----------get request------------80Please respect copyright.ＰＥＮＡＮＡzow9yhIX0o
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡEosmVd9CnT
......80Please respect copyright.ＰＥＮＡＮＡBTyLO1FJvf
---------------------------------
80Please respect copyright.ＰＥＮＡＮＡsPMusRyuB5
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:80Please respect copyright.ＰＥＮＡＮＡYb5qzUVglC
----------------server response--------------80Please respect copyright.ＰＥＮＡＮＡa5RqeKE4WA
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡNSbUa33Gic
Date: Sat, 23 Nov 2024 06:29:19 GMT80Please respect copyright.ＰＥＮＡＮＡVGAcBCjmh9
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡyrA4yNTDhg
content-range: bytes 0-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡLoj7ElUaqx
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡqIShdJKsTq
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡAz5tpXJJ0e
Content-Length: 53423120080Please respect copyright.ＰＥＮＡＮＡ583RyeUn57
Content-Type: application/octet-stream
(http response body is the whole file content)80Please respect copyright.ＰＥＮＡＮＡnYNYd0tFrQ
-------------------------------------------
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:80Please respect copyright.ＰＥＮＡＮＡJakO8Z8d8l
---------------http server send big file content by multiple times--------------80Please respect copyright.ＰＥＮＡＮＡTsaGK4EB2n
const int buf_size=20000000; // buf size is about 20M80Please respect copyright.ＰＥＮＡＮＡcLA41bJD1l
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers80Please respect copyright.ＰＥＮＡＮＡMmtvv1bvzV
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 big80Please respect copyright.ＰＥＮＡＮＡU7m0m55aXW
while(there_is_remain_file_content_to_send)80Please respect copyright.ＰＥＮＡＮＡwpFqKC7peZ
{ 80Please respect copyright.ＰＥＮＡＮＡPBfIcSZzDm
read_remain_file_content_to_send_buf(file, send_buf);80Please respect copyright.ＰＥＮＡＮＡOracZ2JaDP
send_result = send(client_socket, send_buf);80Please respect copyright.ＰＥＮＡＮＡzWDC1K6XxQ
if(send_result == fail) return; else continue;80Please respect copyright.ＰＥＮＡＮＡosv9XeK2iP
}
// a way to make etag for a file using its file size and last modifed time80Please respect copyright.ＰＥＮＡＮＡWTElTHXaBA
const char *file_etag(const char *path) 80Please respect copyright.ＰＥＮＡＮＡIYy1afh2kR
{80Please respect copyright.ＰＥＮＡＮＡMLjeUS1aMJ
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;80Please respect copyright.ＰＥＮＡＮＡqs3JEAS7mH
}80Please respect copyright.ＰＥＮＡＮＡEJh1ZMdwdp
-------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡzlvig1YIi9
"accept-ranges: bytes" tells client web browser that the http server support range.80Please respect copyright.ＰＥＮＡＮＡ7wBGpc2v8B
"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).80Please respect copyright.ＰＥＮＡＮＡw8JLrQXv80
'ETag: "534231200_1732277116"' tells client web browser the file version.80Please respect copyright.ＰＥＮＡＮＡ3DUALMZYVN
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.80Please respect copyright.ＰＥＮＡＮＡtivYkIf94F
"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.80Please respect copyright.ＰＥＮＡＮＡVPtHXzxk1g
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
80Please respect copyright.ＰＥＮＡＮＡEN1lcnvicu
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.80Please respect copyright.ＰＥＮＡＮＡyJH2Cvv7SU
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.)80Please respect copyright.ＰＥＮＡＮＡQVQcQJE7jn
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:80Please respect copyright.ＰＥＮＡＮＡtIopdFU6oa
---------resume range request---------80Please respect copyright.ＰＥＮＡＮＡAJoICsD2o5
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡiGqCrvOdaZ
Range: bytes=243220480-80Please respect copyright.ＰＥＮＡＮＡNjc0nLJtJ8
If-Match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡ0YgI0Zdmki
......80Please respect copyright.ＰＥＮＡＮＡLaSEcBzfSb
-------------------------------------80Please respect copyright.ＰＥＮＡＮＡPzCjMIGaUr
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.80Please respect copyright.ＰＥＮＡＮＡv91QLwYJz0
'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:80Please respect copyright.ＰＥＮＡＮＡzqci62CKty
-------response content from resume point-------80Please respect copyright.ＰＥＮＡＮＡwH5TFYBumY
HTTP/1.1 206 Partial Content80Please respect copyright.ＰＥＮＡＮＡm51rcGPsEs
Date: Sat, 23 Nov 2024 06:56:17 GMT80Please respect copyright.ＰＥＮＡＮＡ3nS1LnQP4Z
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡxQRnETLdrp
content-range: bytes 243220480-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡy3cp3vGVQw
Content-Length: 29101072080Please respect copyright.ＰＥＮＡＮＡvK5zfNtxFI
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡTB3LR99Sm2
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡLUzhlvlk8j
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)80Please respect copyright.ＰＥＮＡＮＡIJ2lFIAMyi
------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡj7NQxqNrHC
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:80Please respect copyright.ＰＥＮＡＮＡi1BQfHiYvT
-------response content from resume point-------80Please respect copyright.ＰＥＮＡＮＡbJnjdWAcLc
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡDvPQ5rwDsa
Date: Sat, 23 Nov 2024 06:29:19 GMT80Please respect copyright.ＰＥＮＡＮＡtqqNwbNlgi
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡoMxseXg0xf
content-range: bytes 0-634231201/63423120280Please respect copyright.ＰＥＮＡＮＡV0dZXpc0D2
ETag: "634231202_2732277118"80Please respect copyright.ＰＥＮＡＮＡCmqBobm3xR
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡkRivvsWrYL
Content-Length: 63423120280Please respect copyright.ＰＥＮＡＮＡDdXASphJhI
Content-Type: application/octet-stream
(http response body is the whole file content)80Please respect copyright.ＰＥＮＡＮＡO8AbFZCPfr
------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡUsjSaaN85g
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
80Please respect copyright.ＰＥＮＡＮＡXHVuxGpUaM
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:80Please respect copyright.ＰＥＮＡＮＡL5oPLDdou0
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
80Please respect copyright.ＰＥＮＡＮＡUJa1r861S7
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80Please respect copyright.ＰＥＮＡＮＡJ0jxnDWnue
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80Please respect copyright.ＰＥＮＡＮＡP7hWJ9Q2Cr
https://gemini.google.com/
感谢互联网上无私奉献的参考资80Please respect copyright.ＰＥＮＡＮＡ00PY12F5ho
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests80Please respect copyright.ＰＥＮＡＮＡY8iLKrlITe
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range80Please respect copyright.ＰＥＮＡＮＡmAfjJ3aZ7K
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag80Please respect copyright.ＰＥＮＡＮＡDG8Zp7425n
https://www.rfc-editor.org/rfc/rfc723380Please respect copyright.ＰＥＮＡＮＡCSfY6cAVyY
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]80Please respect copyright.ＰＥＮＡＮＡCkqvjDNCG9
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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