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:73Please respect copyright.ＰＥＮＡＮＡ6y0FhVEBSv
----------http range request-------73Please respect copyright.ＰＥＮＡＮＡbJ95KeqI1k
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡaV5Rnimxmm
Range: bytes=100-102373Please respect copyright.ＰＥＮＡＮＡ9FwKZvsxiW
If-Match: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡHpBoVvE37G
......73Please respect copyright.ＰＥＮＡＮＡRhIj4dRxgi
-----------------------------------73Please respect copyright.ＰＥＮＡＮＡtpggskubJi
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:73Please respect copyright.ＰＥＮＡＮＡnsN55PMU8U
----------------html-----------------73Please respect copyright.ＰＥＮＡＮＡIt1Bt6tgqE
<a href="/zen.iso">zen.iso</a>73Please respect copyright.ＰＥＮＡＮＡr1CEOVIfZq
-------------------------------------73Please respect copyright.ＰＥＮＡＮＡt628Pe9pUD
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:73Please respect copyright.ＰＥＮＡＮＡBPcMVFnfdq
----------get request------------73Please respect copyright.ＰＥＮＡＮＡZkITAkfUwh
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡWSEkHM0CbS
......73Please respect copyright.ＰＥＮＡＮＡvUUTHQLvUJ
---------------------------------
73Please respect copyright.ＰＥＮＡＮＡj45tiv0YVB
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:73Please respect copyright.ＰＥＮＡＮＡyoOmez3m9t
----------------server response--------------73Please respect copyright.ＰＥＮＡＮＡOgVlrpyTJE
HTTP/1.1 200 OK73Please respect copyright.ＰＥＮＡＮＡiB2iDOyAnO
Date: Sat, 23 Nov 2024 06:29:19 GMT73Please respect copyright.ＰＥＮＡＮＡFqd5shFqU5
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡjM19UOBRtc
content-range: bytes 0-534231199/53423120073Please respect copyright.ＰＥＮＡＮＡT27b5NNgqI
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡ6I9Up6FQMt
Content-Disposition: attachment; filename="zen.iso"73Please respect copyright.ＰＥＮＡＮＡlow8mUuBYu
Content-Length: 53423120073Please respect copyright.ＰＥＮＡＮＡRmKFSl8Gzg
Content-Type: application/octet-stream
(http response body is the whole file content)73Please respect copyright.ＰＥＮＡＮＡSsyljJI4Xt
-------------------------------------------
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:73Please respect copyright.ＰＥＮＡＮＡPGdh2R9jma
---------------http server send big file content by multiple times--------------73Please respect copyright.ＰＥＮＡＮＡDwXzXwDeLK
const int buf_size=20000000; // buf size is about 20M73Please respect copyright.ＰＥＮＡＮＡ3K0YzNn2cw
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers73Please respect copyright.ＰＥＮＡＮＡjwE9kUfeOy
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 big73Please respect copyright.ＰＥＮＡＮＡUjaNF0Ral3
while(there_is_remain_file_content_to_send)73Please respect copyright.ＰＥＮＡＮＡM2L6tEGX67
{ 73Please respect copyright.ＰＥＮＡＮＡbJlrC3Ye9d
read_remain_file_content_to_send_buf(file, send_buf);73Please respect copyright.ＰＥＮＡＮＡCjZKG6ZGDK
send_result = send(client_socket, send_buf);73Please respect copyright.ＰＥＮＡＮＡiEKIi0O0Tq
if(send_result == fail) return; else continue;73Please respect copyright.ＰＥＮＡＮＡzArdznl5MQ
}
// a way to make etag for a file using its file size and last modifed time73Please respect copyright.ＰＥＮＡＮＡ8HA3nECsk5
const char *file_etag(const char *path) 73Please respect copyright.ＰＥＮＡＮＡCxgTYCP5TA
{73Please respect copyright.ＰＥＮＡＮＡB0WzlnIlpQ
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;73Please respect copyright.ＰＥＮＡＮＡKITonaaxOM
}73Please respect copyright.ＰＥＮＡＮＡtdEte7dxrF
-------------------------------------------------73Please respect copyright.ＰＥＮＡＮＡdbKljZvPtH
"accept-ranges: bytes" tells client web browser that the http server support range.73Please respect copyright.ＰＥＮＡＮＡPNdk9xRwfE
"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).73Please respect copyright.ＰＥＮＡＮＡejhCbYilut
'ETag: "534231200_1732277116"' tells client web browser the file version.73Please respect copyright.ＰＥＮＡＮＡrVZlC23HOr
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.73Please respect copyright.ＰＥＮＡＮＡnO4YKjK0gp
"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.73Please respect copyright.ＰＥＮＡＮＡ6KEzT7mbDj
"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.73Please respect copyright.ＰＥＮＡＮＡykvZfAPZxY
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.)73Please respect copyright.ＰＥＮＡＮＡDZqhoBXhFi
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:73Please respect copyright.ＰＥＮＡＮＡHBmx4GGK28
---------resume range request---------73Please respect copyright.ＰＥＮＡＮＡia6v6zSJJK
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡJen1iVedMP
Range: bytes=243220480-73Please respect copyright.ＰＥＮＡＮＡUUCOr4FQaT
If-Match: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡbwMOrfsNME
......73Please respect copyright.ＰＥＮＡＮＡRXlhsRvhYw
-------------------------------------73Please respect copyright.ＰＥＮＡＮＡuIUu7iI2U6
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.73Please respect copyright.ＰＥＮＡＮＡmDKksN65vT
'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:73Please respect copyright.ＰＥＮＡＮＡEOFZ253wAB
-------response content from resume point-------73Please respect copyright.ＰＥＮＡＮＡr9A4cZP6MC
HTTP/1.1 206 Partial Content73Please respect copyright.ＰＥＮＡＮＡuua9qBDxL0
Date: Sat, 23 Nov 2024 06:56:17 GMT73Please respect copyright.ＰＥＮＡＮＡKCii8SXC94
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡ7mxLSDyRoY
content-range: bytes 243220480-534231199/53423120073Please respect copyright.ＰＥＮＡＮＡM80EEScByY
Content-Length: 29101072073Please respect copyright.ＰＥＮＡＮＡeh3yaas60M
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡWcQgBgTSXR
Content-Disposition: attachment; filename="zen.iso"73Please respect copyright.ＰＥＮＡＮＡhQkw8WTaQo
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)73Please respect copyright.ＰＥＮＡＮＡDkp08jgL6u
------------------------------------------------73Please respect copyright.ＰＥＮＡＮＡStsGUpFouG
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:73Please respect copyright.ＰＥＮＡＮＡWmhdBCpszr
-------response content from resume point-------73Please respect copyright.ＰＥＮＡＮＡsubPsW1MU6
HTTP/1.1 200 OK73Please respect copyright.ＰＥＮＡＮＡeE8SjDb5CA
Date: Sat, 23 Nov 2024 06:29:19 GMT73Please respect copyright.ＰＥＮＡＮＡ95nozToft7
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡ7sFnzJCfEl
content-range: bytes 0-634231201/63423120273Please respect copyright.ＰＥＮＡＮＡIR5wGcIq4e
ETag: "634231202_2732277118"73Please respect copyright.ＰＥＮＡＮＡCifNo29n0E
Content-Disposition: attachment; filename="zen.iso"73Please respect copyright.ＰＥＮＡＮＡoMen5veggg
Content-Length: 63423120273Please respect copyright.ＰＥＮＡＮＡVG7DUPFcOn
Content-Type: application/octet-stream
(http response body is the whole file content)73Please respect copyright.ＰＥＮＡＮＡn3Z447ZfS3
------------------------------------------------73Please respect copyright.ＰＥＮＡＮＡrgmsrZQf4C
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:73Please respect copyright.ＰＥＮＡＮＡzgXzheKL9Y
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
73Please respect copyright.ＰＥＮＡＮＡzcDs8pTGot
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73Please respect copyright.ＰＥＮＡＮＡ1TcohMpBfc
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73Please respect copyright.ＰＥＮＡＮＡFSbHCnLnLQ
https://gemini.google.com/
感谢互联网上无私奉献的参考资73Please respect copyright.ＰＥＮＡＮＡwPTXgngjKY
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests73Please respect copyright.ＰＥＮＡＮＡ5vX1DwknKv
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range73Please respect copyright.ＰＥＮＡＮＡINc6zgtSjP
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag73Please respect copyright.ＰＥＮＡＮＡASgpeyYqIJ
https://www.rfc-editor.org/rfc/rfc723373Please respect copyright.ＰＥＮＡＮＡTRBCFogh0P
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]73Please respect copyright.ＰＥＮＡＮＡ88xgfN28Jg
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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