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:161Please respect copyright.ＰＥＮＡＮＡMAE0RlyNs2
----------http range request-------161Please respect copyright.ＰＥＮＡＮＡIJ946PcJhv
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡaFdTx79Smx
Range: bytes=100-1023161Please respect copyright.ＰＥＮＡＮＡFKj9aA7Oqs
If-Match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡTbjlMVNe3U
......161Please respect copyright.ＰＥＮＡＮＡsxdkDqKIyS
-----------------------------------161Please respect copyright.ＰＥＮＡＮＡAx0hagweu5
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.
161Please respect copyright.ＰＥＮＡＮＡWypy6l2ioo
Step 1. make a link for file to download, for example:161Please respect copyright.ＰＥＮＡＮＡCrx1OHJhIN
----------------html-----------------161Please respect copyright.ＰＥＮＡＮＡPATz9bgkX2
<a href="/zen.iso">zen.iso</a>161Please respect copyright.ＰＥＮＡＮＡvYpkJA5COs
-------------------------------------161Please respect copyright.ＰＥＮＡＮＡeFtjcBR3Kk
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:161Please respect copyright.ＰＥＮＡＮＡQ9ygyBXO8P
----------get request------------161Please respect copyright.ＰＥＮＡＮＡw8GArQXLO7
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡZUvaz4gQRZ
......161Please respect copyright.ＰＥＮＡＮＡIlLTgwldQf
---------------------------------
161Please respect copyright.ＰＥＮＡＮＡJK7TM1xziM
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:161Please respect copyright.ＰＥＮＡＮＡexkBoAXcpq
----------------server response--------------161Please respect copyright.ＰＥＮＡＮＡ935RBlE5Bz
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡMwn8UDtIj6
Date: Sat, 23 Nov 2024 06:29:19 GMT161Please respect copyright.ＰＥＮＡＮＡXL5hLRPQkb
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡK0t6SnUTuf
content-range: bytes 0-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡzkfmtrHNkA
ETag: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡ2qbbhrvbTH
Content-Disposition: attachment; filename="zen.iso"161Please respect copyright.ＰＥＮＡＮＡm0wgj3f5Ck
Content-Length: 534231200161Please respect copyright.ＰＥＮＡＮＡ3fXSMbcOZu
Content-Type: application/octet-stream
(http response body is the whole file content)161Please respect copyright.ＰＥＮＡＮＡPgFXEhrR1b
-------------------------------------------
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:161Please respect copyright.ＰＥＮＡＮＡoj1v7dKhuO
---------------http server send big file content by multiple times--------------161Please respect copyright.ＰＥＮＡＮＡc9rWf26joK
const int buf_size=20000000; // buf size is about 20M161Please respect copyright.ＰＥＮＡＮＡjAxNLZFTsX
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers161Please respect copyright.ＰＥＮＡＮＡ2VulNOVh9A
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 big161Please respect copyright.ＰＥＮＡＮＡZbKkJJTlAs
while(there_is_remain_file_content_to_send)161Please respect copyright.ＰＥＮＡＮＡFZcdq9XI6X
{ 161Please respect copyright.ＰＥＮＡＮＡDoVDrOd6If
read_remain_file_content_to_send_buf(file, send_buf);161Please respect copyright.ＰＥＮＡＮＡ8Dr4uL0aOY
send_result = send(client_socket, send_buf);161Please respect copyright.ＰＥＮＡＮＡe00mdHqTNp
if(send_result == fail) return; else continue;161Please respect copyright.ＰＥＮＡＮＡw3oMvQrsIg
}
// a way to make etag for a file using its file size and last modifed time161Please respect copyright.ＰＥＮＡＮＡed2Iqmi91s
const char *file_etag(const char *path) 161Please respect copyright.ＰＥＮＡＮＡMwu2ZRDS2I
{161Please respect copyright.ＰＥＮＡＮＡaQ4Ms5g50w
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;161Please respect copyright.ＰＥＮＡＮＡCTJisXVc9v
}161Please respect copyright.ＰＥＮＡＮＡQjiwsVl2NU
-------------------------------------------------161Please respect copyright.ＰＥＮＡＮＡ1iSZDnJlmR
"accept-ranges: bytes" tells client web browser that the http server support range.161Please respect copyright.ＰＥＮＡＮＡJ9pHE0qObA
"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).161Please respect copyright.ＰＥＮＡＮＡQ1Hw4Qippt
'ETag: "534231200_1732277116"' tells client web browser the file version.161Please respect copyright.ＰＥＮＡＮＡWna1v6nxJP
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.161Please respect copyright.ＰＥＮＡＮＡnOPFevRejG
"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.161Please respect copyright.ＰＥＮＡＮＡpK8cMXcsbo
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
161Please respect copyright.ＰＥＮＡＮＡp9ATLbxl6T
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.161Please respect copyright.ＰＥＮＡＮＡeCFuRyVaiC
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.)161Please respect copyright.ＰＥＮＡＮＡAyfvGTG5gK
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.
161Please respect copyright.ＰＥＮＡＮＡSziQINpHzm
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:161Please respect copyright.ＰＥＮＡＮＡRnS0KChPnB
---------resume range request---------161Please respect copyright.ＰＥＮＡＮＡ0XlQZQzS8q
GET /zen.iso HTTP/1.1161Please respect copyright.ＰＥＮＡＮＡ1gQEWZejzP
Range: bytes=243220480-161Please respect copyright.ＰＥＮＡＮＡgX7UYUBbwX
If-Match: "534231200_1732277116"161Please respect copyright.ＰＥＮＡＮＡsOBwqVovzl
......161Please respect copyright.ＰＥＮＡＮＡL0QoXMU4ME
-------------------------------------161Please respect copyright.ＰＥＮＡＮＡQsRoTF2JDM
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.161Please respect copyright.ＰＥＮＡＮＡwudyjKC8CX
'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:161Please respect copyright.ＰＥＮＡＮＡVUDytn1Rf5
-------response content from resume point-------161Please respect copyright.ＰＥＮＡＮＡrPfRXJTbyo
HTTP/1.1 206 Partial Content161Please respect copyright.ＰＥＮＡＮＡFvULAdyIhk
Date: Sat, 23 Nov 2024 06:56:17 GMT161Please respect copyright.ＰＥＮＡＮＡwHawOdhbYF
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡwjwYvVnp92
content-range: bytes 243220480-534231199/534231200161Please respect copyright.ＰＥＮＡＮＡ8pfxlkjUbU
Content-Length: 291010720161Please respect copyright.ＰＥＮＡＮＡnCGM8D3zsc
Content-Type: application/octet-stream161Please respect copyright.ＰＥＮＡＮＡiUQWNyHLt4
Content-Disposition: attachment; filename="zen.iso"161Please respect copyright.ＰＥＮＡＮＡzrGdg3qrAi
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)161Please respect copyright.ＰＥＮＡＮＡeG7G6gymYf
------------------------------------------------161Please respect copyright.ＰＥＮＡＮＡ11qAH0RyaN
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:161Please respect copyright.ＰＥＮＡＮＡbTLq17uIxV
-------response content from resume point-------161Please respect copyright.ＰＥＮＡＮＡzzEa4eNti4
HTTP/1.1 200 OK161Please respect copyright.ＰＥＮＡＮＡtJM72jRfsh
Date: Sat, 23 Nov 2024 06:29:19 GMT161Please respect copyright.ＰＥＮＡＮＡn5tix0hEbi
accept-ranges: bytes161Please respect copyright.ＰＥＮＡＮＡ66soSBvJeM
content-range: bytes 0-634231201/634231202161Please respect copyright.ＰＥＮＡＮＡcWLxpYsyJq
ETag: "634231202_2732277118"161Please respect copyright.ＰＥＮＡＮＡLgBnSFIOzX
Content-Disposition: attachment; filename="zen.iso"161Please respect copyright.ＰＥＮＡＮＡmHiQ1ghdrj
Content-Length: 634231202161Please respect copyright.ＰＥＮＡＮＡA2vRJZ8Ie5
Content-Type: application/octet-stream
(http response body is the whole file content)161Please respect copyright.ＰＥＮＡＮＡQXTcyRxJR7
------------------------------------------------161Please respect copyright.ＰＥＮＡＮＡOBxZP9jUfS
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
161Please respect copyright.ＰＥＮＡＮＡ4eUWpkVL1d
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:161Please respect copyright.ＰＥＮＡＮＡKVzkTcgQFT
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
161Please respect copyright.ＰＥＮＡＮＡv03D53ldbo
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161Please respect copyright.ＰＥＮＡＮＡv00Nd0Qf9Q
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161Please respect copyright.ＰＥＮＡＮＡPyaJj7wCsL
https://gemini.google.com/
感谢互联网上无私奉献的参考资161Please respect copyright.ＰＥＮＡＮＡMGM2VNRF5q
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests161Please respect copyright.ＰＥＮＡＮＡBQFhdESPfW
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range161Please respect copyright.ＰＥＮＡＮＡfqUxWmb0jW
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag161Please respect copyright.ＰＥＮＡＮＡX2Ktjw782Y
https://www.rfc-editor.org/rfc/rfc7233161Please respect copyright.ＰＥＮＡＮＡT3fqBJtAXI
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/
161Please respect copyright.ＰＥＮＡＮＡiUXXDM4ZJV
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]161Please respect copyright.ＰＥＮＡＮＡ0Dc3AnAvyC
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]
如果您對C/C++ programming, Linux, website development, Vue, Git, vscode感興趣，邀請您加入「Linux/C/C++ Website Development」 微信群，請加我的微信（si_jinmin）以便拉您进群。
ns216.73.216.25da2
