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:189Please respect copyright.ＰＥＮＡＮＡqSe8fIr8B6
----------http range request-------189Please respect copyright.ＰＥＮＡＮＡ1nkdPtAtO3
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡGM50HY5HB4
Range: bytes=100-1023189Please respect copyright.ＰＥＮＡＮＡQBRVtIIh7d
If-Match: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡdqsufmoyce
......189Please respect copyright.ＰＥＮＡＮＡ34ePTKS3bp
-----------------------------------189Please respect copyright.ＰＥＮＡＮＡNSgYT2ZdJD
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:189Please respect copyright.ＰＥＮＡＮＡbppRt87Y57
----------------html-----------------189Please respect copyright.ＰＥＮＡＮＡosKKkaFyT6
<a href="/zen.iso">zen.iso</a>189Please respect copyright.ＰＥＮＡＮＡr6fyViopvu
-------------------------------------189Please respect copyright.ＰＥＮＡＮＡdYjFaMLzEN
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:189Please respect copyright.ＰＥＮＡＮＡ2I2v96ArrS
----------get request------------189Please respect copyright.ＰＥＮＡＮＡYxWPBfuSJ6
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡ5Z0im8KJdv
......189Please respect copyright.ＰＥＮＡＮＡjhcRUEvfrC
---------------------------------
189Please respect copyright.ＰＥＮＡＮＡnS8IDFiLoJ
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:189Please respect copyright.ＰＥＮＡＮＡqG1ktDXDnM
----------------server response--------------189Please respect copyright.ＰＥＮＡＮＡbbmL2mzAeI
HTTP/1.1 200 OK189Please respect copyright.ＰＥＮＡＮＡdJdC82RU7L
Date: Sat, 23 Nov 2024 06:29:19 GMT189Please respect copyright.ＰＥＮＡＮＡVfigJKwyXX
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡnLZk12Y1JD
content-range: bytes 0-534231199/534231200189Please respect copyright.ＰＥＮＡＮＡWPCghT4JlM
ETag: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡPndw46MSEl
Content-Disposition: attachment; filename="zen.iso"189Please respect copyright.ＰＥＮＡＮＡOfycVtgPFn
Content-Length: 534231200189Please respect copyright.ＰＥＮＡＮＡWUbKIyb8uF
Content-Type: application/octet-stream
(http response body is the whole file content)189Please respect copyright.ＰＥＮＡＮＡMLSN06n5UB
-------------------------------------------
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:189Please respect copyright.ＰＥＮＡＮＡOfYFhYxmLp
---------------http server send big file content by multiple times--------------189Please respect copyright.ＰＥＮＡＮＡ5TdwqNSQk6
const int buf_size=20000000; // buf size is about 20M189Please respect copyright.ＰＥＮＡＮＡqsE15cBsih
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers189Please respect copyright.ＰＥＮＡＮＡ0Prb83hDxx
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 big189Please respect copyright.ＰＥＮＡＮＡRG8t1KyXgE
while(there_is_remain_file_content_to_send)189Please respect copyright.ＰＥＮＡＮＡ9Ev2aSNtdk
{ 189Please respect copyright.ＰＥＮＡＮＡX1G9D2RGG5
read_remain_file_content_to_send_buf(file, send_buf);189Please respect copyright.ＰＥＮＡＮＡw46JboRlgX
send_result = send(client_socket, send_buf);189Please respect copyright.ＰＥＮＡＮＡnv0pZiegxP
if(send_result == fail) return; else continue;189Please respect copyright.ＰＥＮＡＮＡi9RijByhg5
}
// a way to make etag for a file using its file size and last modifed time189Please respect copyright.ＰＥＮＡＮＡZVvyiPBB8G
const char *file_etag(const char *path) 189Please respect copyright.ＰＥＮＡＮＡJXaBPQP0un
{189Please respect copyright.ＰＥＮＡＮＡ0eJXRDfa08
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;189Please respect copyright.ＰＥＮＡＮＡt9MbiCcWMV
}189Please respect copyright.ＰＥＮＡＮＡ258r63x0Wx
-------------------------------------------------189Please respect copyright.ＰＥＮＡＮＡXUyvAQVpES
"accept-ranges: bytes" tells client web browser that the http server support range.189Please respect copyright.ＰＥＮＡＮＡFesE1grk4l
"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).189Please respect copyright.ＰＥＮＡＮＡSwQtSvPwNG
'ETag: "534231200_1732277116"' tells client web browser the file version.189Please respect copyright.ＰＥＮＡＮＡ2hmNX8R7cH
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.189Please respect copyright.ＰＥＮＡＮＡnVxsaTOJe8
"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.189Please respect copyright.ＰＥＮＡＮＡfxnKLQo3Pw
"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.189Please respect copyright.ＰＥＮＡＮＡ72e6Cs2NeG
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.)189Please respect copyright.ＰＥＮＡＮＡefJhFOe4c2
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:189Please respect copyright.ＰＥＮＡＮＡE96LN31Hdn
---------resume range request---------189Please respect copyright.ＰＥＮＡＮＡZYRtthOVui
GET /zen.iso HTTP/1.1189Please respect copyright.ＰＥＮＡＮＡzmfHLJFJqI
Range: bytes=243220480-189Please respect copyright.ＰＥＮＡＮＡFMnzwLi8wM
If-Match: "534231200_1732277116"189Please respect copyright.ＰＥＮＡＮＡkagugFangL
......189Please respect copyright.ＰＥＮＡＮＡSBIIeloKvX
-------------------------------------189Please respect copyright.ＰＥＮＡＮＡh7tgxzxKQj
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.189Please respect copyright.ＰＥＮＡＮＡHwpzZvktQy
'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:189Please respect copyright.ＰＥＮＡＮＡ5xQGCCDbI1
-------response content from resume point-------189Please respect copyright.ＰＥＮＡＮＡ0cIrzSxxDh
HTTP/1.1 206 Partial Content189Please respect copyright.ＰＥＮＡＮＡF7BUOZzczJ
Date: Sat, 23 Nov 2024 06:56:17 GMT189Please respect copyright.ＰＥＮＡＮＡBpSWjeOpft
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡOKotHM6ebP
content-range: bytes 243220480-534231199/534231200189Please respect copyright.ＰＥＮＡＮＡlJR7zsCIFF
Content-Length: 291010720189Please respect copyright.ＰＥＮＡＮＡZDyMFq6iTF
Content-Type: application/octet-stream189Please respect copyright.ＰＥＮＡＮＡbOxLEMGDqm
Content-Disposition: attachment; filename="zen.iso"189Please respect copyright.ＰＥＮＡＮＡuvdoU6tTFw
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)189Please respect copyright.ＰＥＮＡＮＡaLbnqfwas8
------------------------------------------------189Please respect copyright.ＰＥＮＡＮＡWzG8mvyXVg
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:189Please respect copyright.ＰＥＮＡＮＡfPlRKcBeo3
-------response content from resume point-------189Please respect copyright.ＰＥＮＡＮＡ0f15a8Rn2A
HTTP/1.1 200 OK189Please respect copyright.ＰＥＮＡＮＡK5W8D5VFC2
Date: Sat, 23 Nov 2024 06:29:19 GMT189Please respect copyright.ＰＥＮＡＮＡi9K1HRk7zk
accept-ranges: bytes189Please respect copyright.ＰＥＮＡＮＡZxLgIZvBDB
content-range: bytes 0-634231201/634231202189Please respect copyright.ＰＥＮＡＮＡ7V7SYNQ9eM
ETag: "634231202_2732277118"189Please respect copyright.ＰＥＮＡＮＡEBVQclSeft
Content-Disposition: attachment; filename="zen.iso"189Please respect copyright.ＰＥＮＡＮＡKEKE1rjhpL
Content-Length: 634231202189Please respect copyright.ＰＥＮＡＮＡTvfU7Z6489
Content-Type: application/octet-stream
(http response body is the whole file content)189Please respect copyright.ＰＥＮＡＮＡJnTuKZchNh
------------------------------------------------189Please respect copyright.ＰＥＮＡＮＡTRKdULJznb
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:189Please respect copyright.ＰＥＮＡＮＡqjVgGKlq5V
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
189Please respect copyright.ＰＥＮＡＮＡvRsxMhhJO2
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189Please respect copyright.ＰＥＮＡＮＡMwq77raUL6
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189Please respect copyright.ＰＥＮＡＮＡK9m5FWlx1q
https://gemini.google.com/
感谢互联网上无私奉献的参考资189Please respect copyright.ＰＥＮＡＮＡqKFl4KJ4rQ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests189Please respect copyright.ＰＥＮＡＮＡS3AqJG5vjl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range189Please respect copyright.ＰＥＮＡＮＡEOJDy46FYO
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag189Please respect copyright.ＰＥＮＡＮＡhCRXelvJo8
https://www.rfc-editor.org/rfc/rfc7233189Please respect copyright.ＰＥＮＡＮＡ5k7gsUtDYr
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]189Please respect copyright.ＰＥＮＡＮＡTmIiJSFIJe
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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