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:132Please respect copyright.ＰＥＮＡＮＡ2tz4ddFKgD
----------http range request-------132Please respect copyright.ＰＥＮＡＮＡpWO6gENYkZ
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡ5pANPekR4T
Range: bytes=100-1023132Please respect copyright.ＰＥＮＡＮＡ0eK7HxaTov
If-Match: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡiefHiPx14M
......132Please respect copyright.ＰＥＮＡＮＡHeTVaxisNr
-----------------------------------132Please respect copyright.ＰＥＮＡＮＡAMsfY1EtTy
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:132Please respect copyright.ＰＥＮＡＮＡtxtbPjdvRD
----------------html-----------------132Please respect copyright.ＰＥＮＡＮＡQbh6aqvksP
<a href="/zen.iso">zen.iso</a>132Please respect copyright.ＰＥＮＡＮＡhsKrVBW9dY
-------------------------------------132Please respect copyright.ＰＥＮＡＮＡf690N7EKxo
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:132Please respect copyright.ＰＥＮＡＮＡV40cYHtVxG
----------get request------------132Please respect copyright.ＰＥＮＡＮＡ9qvUSlx3PE
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡcxogWhm2c3
......132Please respect copyright.ＰＥＮＡＮＡJQU84O9A7m
---------------------------------
132Please respect copyright.ＰＥＮＡＮＡT4kOozacR6
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:132Please respect copyright.ＰＥＮＡＮＡ8CDJlI5SrE
----------------server response--------------132Please respect copyright.ＰＥＮＡＮＡHFw7wsKu5l
HTTP/1.1 200 OK132Please respect copyright.ＰＥＮＡＮＡy2owZlqDnV
Date: Sat, 23 Nov 2024 06:29:19 GMT132Please respect copyright.ＰＥＮＡＮＡa20PztYkHe
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡp3fJwngnoG
content-range: bytes 0-534231199/534231200132Please respect copyright.ＰＥＮＡＮＡkD299PpIZn
ETag: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡcoCqQkRPin
Content-Disposition: attachment; filename="zen.iso"132Please respect copyright.ＰＥＮＡＮＡAg3jOFwxQC
Content-Length: 534231200132Please respect copyright.ＰＥＮＡＮＡzJxSZBx4aR
Content-Type: application/octet-stream
(http response body is the whole file content)132Please respect copyright.ＰＥＮＡＮＡAddGDmcnEo
-------------------------------------------
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:132Please respect copyright.ＰＥＮＡＮＡaPER7x5ifX
---------------http server send big file content by multiple times--------------132Please respect copyright.ＰＥＮＡＮＡgaP8e9RVI0
const int buf_size=20000000; // buf size is about 20M132Please respect copyright.ＰＥＮＡＮＡuDtN3VCHs7
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers132Please respect copyright.ＰＥＮＡＮＡTQRgsh8Ak2
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 big132Please respect copyright.ＰＥＮＡＮＡj1pvwPDyO9
while(there_is_remain_file_content_to_send)132Please respect copyright.ＰＥＮＡＮＡ4XSilPO1GI
{ 132Please respect copyright.ＰＥＮＡＮＡTRXkZJcTLe
read_remain_file_content_to_send_buf(file, send_buf);132Please respect copyright.ＰＥＮＡＮＡv5FNnTMeI8
send_result = send(client_socket, send_buf);132Please respect copyright.ＰＥＮＡＮＡueeQrVGRFb
if(send_result == fail) return; else continue;132Please respect copyright.ＰＥＮＡＮＡMqrYvGTYNA
}
// a way to make etag for a file using its file size and last modifed time132Please respect copyright.ＰＥＮＡＮＡWp0XL76Sta
const char *file_etag(const char *path) 132Please respect copyright.ＰＥＮＡＮＡXvzh8QRktB
{132Please respect copyright.ＰＥＮＡＮＡkv9enmMu3T
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;132Please respect copyright.ＰＥＮＡＮＡq73HVESbXy
}132Please respect copyright.ＰＥＮＡＮＡboy8K2n7Gw
-------------------------------------------------132Please respect copyright.ＰＥＮＡＮＡwbc5b7t80e
"accept-ranges: bytes" tells client web browser that the http server support range.132Please respect copyright.ＰＥＮＡＮＡwgT1T6fFh5
"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).132Please respect copyright.ＰＥＮＡＮＡr7njeK3L4x
'ETag: "534231200_1732277116"' tells client web browser the file version.132Please respect copyright.ＰＥＮＡＮＡnuRwtjLMGS
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.132Please respect copyright.ＰＥＮＡＮＡaaebyY7iUn
"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.132Please respect copyright.ＰＥＮＡＮＡaU8vlM0l6X
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
132Please respect copyright.ＰＥＮＡＮＡxFV9DPmtxA
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.132Please respect copyright.ＰＥＮＡＮＡliWWys8ZMb
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.)132Please respect copyright.ＰＥＮＡＮＡ2eQT5F2314
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:132Please respect copyright.ＰＥＮＡＮＡPk9nkBu70S
---------resume range request---------132Please respect copyright.ＰＥＮＡＮＡ0D3rtkWOx3
GET /zen.iso HTTP/1.1132Please respect copyright.ＰＥＮＡＮＡobbVn0PUU4
Range: bytes=243220480-132Please respect copyright.ＰＥＮＡＮＡLYhBdKOmru
If-Match: "534231200_1732277116"132Please respect copyright.ＰＥＮＡＮＡrmjNFC5dJP
......132Please respect copyright.ＰＥＮＡＮＡUdVYte5pOx
-------------------------------------132Please respect copyright.ＰＥＮＡＮＡDYpE0mVdam
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.132Please respect copyright.ＰＥＮＡＮＡEk038q4hiM
'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:132Please respect copyright.ＰＥＮＡＮＡJzysafQJxp
-------response content from resume point-------132Please respect copyright.ＰＥＮＡＮＡJJwW3xXbCP
HTTP/1.1 206 Partial Content132Please respect copyright.ＰＥＮＡＮＡmtTES3PEXH
Date: Sat, 23 Nov 2024 06:56:17 GMT132Please respect copyright.ＰＥＮＡＮＡSHGW3E3Q4K
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡPCkFo8xTzh
content-range: bytes 243220480-534231199/534231200132Please respect copyright.ＰＥＮＡＮＡG9Vk1CKSs2
Content-Length: 291010720132Please respect copyright.ＰＥＮＡＮＡ2hRMSOq9VI
Content-Type: application/octet-stream132Please respect copyright.ＰＥＮＡＮＡvBtY56qvd2
Content-Disposition: attachment; filename="zen.iso"132Please respect copyright.ＰＥＮＡＮＡWrDcOoaDoh
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)132Please respect copyright.ＰＥＮＡＮＡGyVtSJVAwZ
------------------------------------------------132Please respect copyright.ＰＥＮＡＮＡ7Exea6j3ww
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:132Please respect copyright.ＰＥＮＡＮＡTQZzTKaO62
-------response content from resume point-------132Please respect copyright.ＰＥＮＡＮＡTxHH2ZWWAC
HTTP/1.1 200 OK132Please respect copyright.ＰＥＮＡＮＡQHWE15VS6R
Date: Sat, 23 Nov 2024 06:29:19 GMT132Please respect copyright.ＰＥＮＡＮＡsB5sKERN4d
accept-ranges: bytes132Please respect copyright.ＰＥＮＡＮＡiBzzUeEajj
content-range: bytes 0-634231201/634231202132Please respect copyright.ＰＥＮＡＮＡMPeR5d0Evq
ETag: "634231202_2732277118"132Please respect copyright.ＰＥＮＡＮＡzQPCDc3GJV
Content-Disposition: attachment; filename="zen.iso"132Please respect copyright.ＰＥＮＡＮＡ2TbV2TBFxD
Content-Length: 634231202132Please respect copyright.ＰＥＮＡＮＡahYEbTd40c
Content-Type: application/octet-stream
(http response body is the whole file content)132Please respect copyright.ＰＥＮＡＮＡczLCto5uMK
------------------------------------------------132Please respect copyright.ＰＥＮＡＮＡ2K3migAuee
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
132Please respect copyright.ＰＥＮＡＮＡHBoZ1jo15q
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:132Please respect copyright.ＰＥＮＡＮＡNApRI3pyhj
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
132Please respect copyright.ＰＥＮＡＮＡCMpjpy3hCT
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132Please respect copyright.ＰＥＮＡＮＡwBta2AzELu
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132Please respect copyright.ＰＥＮＡＮＡDXtvdld7Je
https://gemini.google.com/
感谢互联网上无私奉献的参考资132Please respect copyright.ＰＥＮＡＮＡe8JBhmtIOu
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests132Please respect copyright.ＰＥＮＡＮＡNLIUiDKcjM
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range132Please respect copyright.ＰＥＮＡＮＡYTiJwkNfYl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag132Please respect copyright.ＰＥＮＡＮＡ79LljyhaGD
https://www.rfc-editor.org/rfc/rfc7233132Please respect copyright.ＰＥＮＡＮＡDU6WXCvxYE
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]132Please respect copyright.ＰＥＮＡＮＡXkeiTbySXp
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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