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:83Please respect copyright.ＰＥＮＡＮＡZsuBUuV4A8
----------http range request-------83Please respect copyright.ＰＥＮＡＮＡJcfo42G2Mn
GET /zen.iso HTTP/1.183Please respect copyright.ＰＥＮＡＮＡ1929rZbZxx
Range: bytes=100-102383Please respect copyright.ＰＥＮＡＮＡ0rI85DgeaF
If-Match: "534231200_1732277116"83Please respect copyright.ＰＥＮＡＮＡfljXg5f7hZ
......83Please respect copyright.ＰＥＮＡＮＡEzGXAHRhXc
-----------------------------------83Please respect copyright.ＰＥＮＡＮＡztOOi4qPkx
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:83Please respect copyright.ＰＥＮＡＮＡbYtNI1OXov
----------------html-----------------83Please respect copyright.ＰＥＮＡＮＡF8KgNf5JDY
<a href="/zen.iso">zen.iso</a>83Please respect copyright.ＰＥＮＡＮＡ6wxiwZVJ5j
-------------------------------------83Please respect copyright.ＰＥＮＡＮＡw4h1pNgXxk
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:83Please respect copyright.ＰＥＮＡＮＡWKeO9UE0kr
----------get request------------83Please respect copyright.ＰＥＮＡＮＡnegagoLvNS
GET /zen.iso HTTP/1.183Please respect copyright.ＰＥＮＡＮＡr28aGJLuxH
......83Please respect copyright.ＰＥＮＡＮＡ4d2GoqTpY3
---------------------------------
83Please respect copyright.ＰＥＮＡＮＡS6gVRGrKoq
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:83Please respect copyright.ＰＥＮＡＮＡNiX7pIqs2B
----------------server response--------------83Please respect copyright.ＰＥＮＡＮＡHBHnHSfgJU
HTTP/1.1 200 OK83Please respect copyright.ＰＥＮＡＮＡdxXHbTkprZ
Date: Sat, 23 Nov 2024 06:29:19 GMT83Please respect copyright.ＰＥＮＡＮＡ7zxMbldbZn
accept-ranges: bytes83Please respect copyright.ＰＥＮＡＮＡP5DKD2FiC4
content-range: bytes 0-534231199/53423120083Please respect copyright.ＰＥＮＡＮＡYbB6mV5mGd
ETag: "534231200_1732277116"83Please respect copyright.ＰＥＮＡＮＡ4vzfUkTAr5
Content-Disposition: attachment; filename="zen.iso"83Please respect copyright.ＰＥＮＡＮＡnoItDscEFI
Content-Length: 53423120083Please respect copyright.ＰＥＮＡＮＡnO1tspbygB
Content-Type: application/octet-stream
(http response body is the whole file content)83Please respect copyright.ＰＥＮＡＮＡVaNt9u9JYS
-------------------------------------------
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:83Please respect copyright.ＰＥＮＡＮＡEghgKtKYUc
---------------http server send big file content by multiple times--------------83Please respect copyright.ＰＥＮＡＮＡwHE27YgjGM
const int buf_size=20000000; // buf size is about 20M83Please respect copyright.ＰＥＮＡＮＡ8sAgI9AqUq
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers83Please respect copyright.ＰＥＮＡＮＡzO4gekcF4R
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 big83Please respect copyright.ＰＥＮＡＮＡK2YYJV4fAr
while(there_is_remain_file_content_to_send)83Please respect copyright.ＰＥＮＡＮＡVnlJR8zTod
{ 83Please respect copyright.ＰＥＮＡＮＡmxjLS14cla
read_remain_file_content_to_send_buf(file, send_buf);83Please respect copyright.ＰＥＮＡＮＡTT3LBOVGaH
send_result = send(client_socket, send_buf);83Please respect copyright.ＰＥＮＡＮＡe7lKItoYtv
if(send_result == fail) return; else continue;83Please respect copyright.ＰＥＮＡＮＡBcx5NMMB8g
}
// a way to make etag for a file using its file size and last modifed time83Please respect copyright.ＰＥＮＡＮＡytJxnqYmaX
const char *file_etag(const char *path) 83Please respect copyright.ＰＥＮＡＮＡ2yw5Vbz2ub
{83Please respect copyright.ＰＥＮＡＮＡad3NSFV41y
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;83Please respect copyright.ＰＥＮＡＮＡ6ndZQiCsT6
}83Please respect copyright.ＰＥＮＡＮＡRiXuW0QEgm
-------------------------------------------------83Please respect copyright.ＰＥＮＡＮＡ0d2hd0j4xO
"accept-ranges: bytes" tells client web browser that the http server support range.83Please respect copyright.ＰＥＮＡＮＡpUlLN0r6Be
"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).83Please respect copyright.ＰＥＮＡＮＡx26EY1HINb
'ETag: "534231200_1732277116"' tells client web browser the file version.83Please respect copyright.ＰＥＮＡＮＡbhIU8saNEa
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.83Please respect copyright.ＰＥＮＡＮＡjiQzeLwrbn
"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.83Please respect copyright.ＰＥＮＡＮＡmn4wxJQcp2
"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.83Please respect copyright.ＰＥＮＡＮＡZMb5GVC3Cv
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.)83Please respect copyright.ＰＥＮＡＮＡW2c7MHH30B
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:83Please respect copyright.ＰＥＮＡＮＡ3icYFajf5O
---------resume range request---------83Please respect copyright.ＰＥＮＡＮＡOkfKHtDPXJ
GET /zen.iso HTTP/1.183Please respect copyright.ＰＥＮＡＮＡoBgbk3foiU
Range: bytes=243220480-83Please respect copyright.ＰＥＮＡＮＡ24OSY3XOMM
If-Match: "534231200_1732277116"83Please respect copyright.ＰＥＮＡＮＡL3AnUIwioH
......83Please respect copyright.ＰＥＮＡＮＡ0spShuPbe7
-------------------------------------83Please respect copyright.ＰＥＮＡＮＡqXZH4PDnYR
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.83Please respect copyright.ＰＥＮＡＮＡ8bFOs0cW2r
'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:83Please respect copyright.ＰＥＮＡＮＡNqfZQ5RJRv
-------response content from resume point-------83Please respect copyright.ＰＥＮＡＮＡ3HkJBQoozl
HTTP/1.1 206 Partial Content83Please respect copyright.ＰＥＮＡＮＡRrD596ppxk
Date: Sat, 23 Nov 2024 06:56:17 GMT83Please respect copyright.ＰＥＮＡＮＡn7U0rCuJJQ
accept-ranges: bytes83Please respect copyright.ＰＥＮＡＮＡTNqSzv3ePT
content-range: bytes 243220480-534231199/53423120083Please respect copyright.ＰＥＮＡＮＡtpCuLthW0k
Content-Length: 29101072083Please respect copyright.ＰＥＮＡＮＡ2CqAMRx8Y8
Content-Type: application/octet-stream83Please respect copyright.ＰＥＮＡＮＡWQts1Rm1HK
Content-Disposition: attachment; filename="zen.iso"83Please respect copyright.ＰＥＮＡＮＡiwSgzeIAMc
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)83Please respect copyright.ＰＥＮＡＮＡfDMJdTX2Yu
------------------------------------------------83Please respect copyright.ＰＥＮＡＮＡV1CcfcxIz6
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:83Please respect copyright.ＰＥＮＡＮＡW2RZKoW2D3
-------response content from resume point-------83Please respect copyright.ＰＥＮＡＮＡ8mOhyqxrVA
HTTP/1.1 200 OK83Please respect copyright.ＰＥＮＡＮＡhw34usvVpB
Date: Sat, 23 Nov 2024 06:29:19 GMT83Please respect copyright.ＰＥＮＡＮＡVgwRn5AaSL
accept-ranges: bytes83Please respect copyright.ＰＥＮＡＮＡO0YuEqHhx5
content-range: bytes 0-634231201/63423120283Please respect copyright.ＰＥＮＡＮＡFNN2bYlxTG
ETag: "634231202_2732277118"83Please respect copyright.ＰＥＮＡＮＡNwNqK1gO68
Content-Disposition: attachment; filename="zen.iso"83Please respect copyright.ＰＥＮＡＮＡE7cbZLVaxg
Content-Length: 63423120283Please respect copyright.ＰＥＮＡＮＡ6IjILynKVk
Content-Type: application/octet-stream
(http response body is the whole file content)83Please respect copyright.ＰＥＮＡＮＡMadYmlngMV
------------------------------------------------83Please respect copyright.ＰＥＮＡＮＡrJOdnWxuCv
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:83Please respect copyright.ＰＥＮＡＮＡVeHPU7Jmjo
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
83Please respect copyright.ＰＥＮＡＮＡTAn2AMyaAi
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83Please respect copyright.ＰＥＮＡＮＡ4mQIXTMbaR
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83Please respect copyright.ＰＥＮＡＮＡY0Ry8HBPPf
https://gemini.google.com/
感谢互联网上无私奉献的参考资83Please respect copyright.ＰＥＮＡＮＡYtdG73Wyyk
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests83Please respect copyright.ＰＥＮＡＮＡRGlqFRNwH8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range83Please respect copyright.ＰＥＮＡＮＡ4eRddiMOFi
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag83Please respect copyright.ＰＥＮＡＮＡ2jhg7YqjGE
https://www.rfc-editor.org/rfc/rfc723383Please respect copyright.ＰＥＮＡＮＡfZkvb0w6lQ
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]83Please respect copyright.ＰＥＮＡＮＡExvnbH9jYC
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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