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:108Please respect copyright.ＰＥＮＡＮＡKVz9DuJFSu
----------http range request-------108Please respect copyright.ＰＥＮＡＮＡmkcyhXRGQV
GET /zen.iso HTTP/1.1108Please respect copyright.ＰＥＮＡＮＡicgOMFmnhg
Range: bytes=100-1023108Please respect copyright.ＰＥＮＡＮＡljUnyw4gNf
If-Match: "534231200_1732277116"108Please respect copyright.ＰＥＮＡＮＡtkqwEOmVNF
......108Please respect copyright.ＰＥＮＡＮＡRfoYrTf91G
-----------------------------------108Please respect copyright.ＰＥＮＡＮＡM1zeImWNDT
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:108Please respect copyright.ＰＥＮＡＮＡjew93F6epE
----------------html-----------------108Please respect copyright.ＰＥＮＡＮＡdLwYGz8b2q
<a href="/zen.iso">zen.iso</a>108Please respect copyright.ＰＥＮＡＮＡ9HcL84iTdR
-------------------------------------108Please respect copyright.ＰＥＮＡＮＡcZETBv2Z4e
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:108Please respect copyright.ＰＥＮＡＮＡMbmZmgcNGy
----------get request------------108Please respect copyright.ＰＥＮＡＮＡY6wQYCMpED
GET /zen.iso HTTP/1.1108Please respect copyright.ＰＥＮＡＮＡiYZ7VgSfLd
......108Please respect copyright.ＰＥＮＡＮＡSiVQwx1DqH
---------------------------------
108Please respect copyright.ＰＥＮＡＮＡnbRMX6a9vx
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:108Please respect copyright.ＰＥＮＡＮＡiOCcam8YH0
----------------server response--------------108Please respect copyright.ＰＥＮＡＮＡelE2btPEpj
HTTP/1.1 200 OK108Please respect copyright.ＰＥＮＡＮＡvyXXMDAYDw
Date: Sat, 23 Nov 2024 06:29:19 GMT108Please respect copyright.ＰＥＮＡＮＡB1vq4H8TNJ
accept-ranges: bytes108Please respect copyright.ＰＥＮＡＮＡfAZInCfChF
content-range: bytes 0-534231199/534231200108Please respect copyright.ＰＥＮＡＮＡw0xjGz3cjF
ETag: "534231200_1732277116"108Please respect copyright.ＰＥＮＡＮＡLsIq8d0wvl
Content-Disposition: attachment; filename="zen.iso"108Please respect copyright.ＰＥＮＡＮＡ32VpeNncD0
Content-Length: 534231200108Please respect copyright.ＰＥＮＡＮＡsoidJJ5zL6
Content-Type: application/octet-stream
(http response body is the whole file content)108Please respect copyright.ＰＥＮＡＮＡlgtwWMH5JM
-------------------------------------------
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:108Please respect copyright.ＰＥＮＡＮＡR24W0MOzFQ
---------------http server send big file content by multiple times--------------108Please respect copyright.ＰＥＮＡＮＡWga9LyWXpC
const int buf_size=20000000; // buf size is about 20M108Please respect copyright.ＰＥＮＡＮＡKB1Mm3ILOy
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers108Please respect copyright.ＰＥＮＡＮＡN3Dcnd7pW4
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 big108Please respect copyright.ＰＥＮＡＮＡk5gbJlmtBp
while(there_is_remain_file_content_to_send)108Please respect copyright.ＰＥＮＡＮＡNik4wQt0N9
{ 108Please respect copyright.ＰＥＮＡＮＡRiMGtZyMmk
read_remain_file_content_to_send_buf(file, send_buf);108Please respect copyright.ＰＥＮＡＮＡMxfzi8hCbi
send_result = send(client_socket, send_buf);108Please respect copyright.ＰＥＮＡＮＡTOANaECiS5
if(send_result == fail) return; else continue;108Please respect copyright.ＰＥＮＡＮＡHgUPPKT66o
}
// a way to make etag for a file using its file size and last modifed time108Please respect copyright.ＰＥＮＡＮＡ15ae4JQ3O9
const char *file_etag(const char *path) 108Please respect copyright.ＰＥＮＡＮＡeyFvzyCYhd
{108Please respect copyright.ＰＥＮＡＮＡkDUAUUxHnb
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;108Please respect copyright.ＰＥＮＡＮＡCLpJurSbLG
}108Please respect copyright.ＰＥＮＡＮＡrMBCwgTzJz
-------------------------------------------------108Please respect copyright.ＰＥＮＡＮＡ30kQ0deUJX
"accept-ranges: bytes" tells client web browser that the http server support range.108Please respect copyright.ＰＥＮＡＮＡXRr7bdUOWY
"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).108Please respect copyright.ＰＥＮＡＮＡoakliyJV2y
'ETag: "534231200_1732277116"' tells client web browser the file version.108Please respect copyright.ＰＥＮＡＮＡU7hFrD9s5J
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.108Please respect copyright.ＰＥＮＡＮＡalLSEmTRup
"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.108Please respect copyright.ＰＥＮＡＮＡbANMxodHJr
"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.108Please respect copyright.ＰＥＮＡＮＡffzCkv1QqU
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.)108Please respect copyright.ＰＥＮＡＮＡfhtnIUqunx
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:108Please respect copyright.ＰＥＮＡＮＡ9KEeO4lWpP
---------resume range request---------108Please respect copyright.ＰＥＮＡＮＡ4WpS687jPC
GET /zen.iso HTTP/1.1108Please respect copyright.ＰＥＮＡＮＡWmZfg9bkxX
Range: bytes=243220480-108Please respect copyright.ＰＥＮＡＮＡ9Lp8CupR0c
If-Match: "534231200_1732277116"108Please respect copyright.ＰＥＮＡＮＡgPGyGndzyU
......108Please respect copyright.ＰＥＮＡＮＡXakJ3I2fVS
-------------------------------------108Please respect copyright.ＰＥＮＡＮＡHQU2WzFK57
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.108Please respect copyright.ＰＥＮＡＮＡpzdK3tqGKn
'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:108Please respect copyright.ＰＥＮＡＮＡRoQ7fB11sA
-------response content from resume point-------108Please respect copyright.ＰＥＮＡＮＡORhxfqiqqD
HTTP/1.1 206 Partial Content108Please respect copyright.ＰＥＮＡＮＡOV48TYGUpQ
Date: Sat, 23 Nov 2024 06:56:17 GMT108Please respect copyright.ＰＥＮＡＮＡQESXJBVco5
accept-ranges: bytes108Please respect copyright.ＰＥＮＡＮＡxCCwKLSDud
content-range: bytes 243220480-534231199/534231200108Please respect copyright.ＰＥＮＡＮＡxKUhZcITMV
Content-Length: 291010720108Please respect copyright.ＰＥＮＡＮＡFs8BjVkGIn
Content-Type: application/octet-stream108Please respect copyright.ＰＥＮＡＮＡOyC4NNzJdm
Content-Disposition: attachment; filename="zen.iso"108Please respect copyright.ＰＥＮＡＮＡxFoogLVucm
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)108Please respect copyright.ＰＥＮＡＮＡeLREWL1zCW
------------------------------------------------108Please respect copyright.ＰＥＮＡＮＡlTf8PorZIq
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:108Please respect copyright.ＰＥＮＡＮＡA4yLNeNeBu
-------response content from resume point-------108Please respect copyright.ＰＥＮＡＮＡG2n2Qp4ijn
HTTP/1.1 200 OK108Please respect copyright.ＰＥＮＡＮＡtGRrAqi5V0
Date: Sat, 23 Nov 2024 06:29:19 GMT108Please respect copyright.ＰＥＮＡＮＡzWnxb9GJ6L
accept-ranges: bytes108Please respect copyright.ＰＥＮＡＮＡDrdp9lo15P
content-range: bytes 0-634231201/634231202108Please respect copyright.ＰＥＮＡＮＡGkRVagJBVz
ETag: "634231202_2732277118"108Please respect copyright.ＰＥＮＡＮＡJuJ6ntfOON
Content-Disposition: attachment; filename="zen.iso"108Please respect copyright.ＰＥＮＡＮＡ4oEVIiDmEL
Content-Length: 634231202108Please respect copyright.ＰＥＮＡＮＡ48Vct9Prfb
Content-Type: application/octet-stream
(http response body is the whole file content)108Please respect copyright.ＰＥＮＡＮＡEY7oTVfyRP
------------------------------------------------108Please respect copyright.ＰＥＮＡＮＡmS1gYMvAxB
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
108Please respect copyright.ＰＥＮＡＮＡjvKr2tiNtI
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:108Please respect copyright.ＰＥＮＡＮＡqLcSr1Kus8
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
108Please respect copyright.ＰＥＮＡＮＡWqfbPF08Qj
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108Please respect copyright.ＰＥＮＡＮＡ5oaLKSvCcP
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108Please respect copyright.ＰＥＮＡＮＡNSeuxJTzb4
https://gemini.google.com/
感谢互联网上无私奉献的参考资108Please respect copyright.ＰＥＮＡＮＡhqD1NzZ3ud
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests108Please respect copyright.ＰＥＮＡＮＡc5yXGupkft
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range108Please respect copyright.ＰＥＮＡＮＡWUbOsDZvxo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag108Please respect copyright.ＰＥＮＡＮＡc7ocCDImQR
https://www.rfc-editor.org/rfc/rfc7233108Please respect copyright.ＰＥＮＡＮＡ93eFMWsIa7
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]108Please respect copyright.ＰＥＮＡＮＡL62Ey4ufA4
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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