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:126Please respect copyright.ＰＥＮＡＮＡJaE40LND5A
----------http range request-------126Please respect copyright.ＰＥＮＡＮＡvCudwj4wH4
GET /zen.iso HTTP/1.1126Please respect copyright.ＰＥＮＡＮＡZvrfytExtK
Range: bytes=100-1023126Please respect copyright.ＰＥＮＡＮＡchNOvE78hq
If-Match: "534231200_1732277116"126Please respect copyright.ＰＥＮＡＮＡU9tmTHvsUM
......126Please respect copyright.ＰＥＮＡＮＡHcOXLnr1oR
-----------------------------------126Please respect copyright.ＰＥＮＡＮＡREmQPttlrT
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:126Please respect copyright.ＰＥＮＡＮＡxv5H52VinH
----------------html-----------------126Please respect copyright.ＰＥＮＡＮＡsTmTnfw03M
<a href="/zen.iso">zen.iso</a>126Please respect copyright.ＰＥＮＡＮＡj2lkqzReBW
-------------------------------------126Please respect copyright.ＰＥＮＡＮＡhXqcvrP0eS
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:126Please respect copyright.ＰＥＮＡＮＡSONtuxQJhm
----------get request------------126Please respect copyright.ＰＥＮＡＮＡbMEJ2Oqyp6
GET /zen.iso HTTP/1.1126Please respect copyright.ＰＥＮＡＮＡn9LXe2UOEA
......126Please respect copyright.ＰＥＮＡＮＡjUmkrAHWw2
---------------------------------
126Please respect copyright.ＰＥＮＡＮＡq9CHsZF63R
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:126Please respect copyright.ＰＥＮＡＮＡvpLRFUdp8L
----------------server response--------------126Please respect copyright.ＰＥＮＡＮＡtPOzgQvBDH
HTTP/1.1 200 OK126Please respect copyright.ＰＥＮＡＮＡNO8o070Dl5
Date: Sat, 23 Nov 2024 06:29:19 GMT126Please respect copyright.ＰＥＮＡＮＡQ03FRsx3Gf
accept-ranges: bytes126Please respect copyright.ＰＥＮＡＮＡP1DHUvJhcc
content-range: bytes 0-534231199/534231200126Please respect copyright.ＰＥＮＡＮＡT21DMU1jwS
ETag: "534231200_1732277116"126Please respect copyright.ＰＥＮＡＮＡUkFeyhnlQ7
Content-Disposition: attachment; filename="zen.iso"126Please respect copyright.ＰＥＮＡＮＡERHoOeQHjR
Content-Length: 534231200126Please respect copyright.ＰＥＮＡＮＡHW49BDvtH0
Content-Type: application/octet-stream
(http response body is the whole file content)126Please respect copyright.ＰＥＮＡＮＡLzEIyB1emp
-------------------------------------------
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:126Please respect copyright.ＰＥＮＡＮＡXHcXXHqS5q
---------------http server send big file content by multiple times--------------126Please respect copyright.ＰＥＮＡＮＡWmpfFeS3xw
const int buf_size=20000000; // buf size is about 20M126Please respect copyright.ＰＥＮＡＮＡgM0OsgS1pG
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers126Please respect copyright.ＰＥＮＡＮＡ4goNpv1H4T
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 big126Please respect copyright.ＰＥＮＡＮＡxIzQwOsvVL
while(there_is_remain_file_content_to_send)126Please respect copyright.ＰＥＮＡＮＡScM5NUK3hR
{ 126Please respect copyright.ＰＥＮＡＮＡ2NysQ04Xp4
read_remain_file_content_to_send_buf(file, send_buf);126Please respect copyright.ＰＥＮＡＮＡnc8z3U82lt
send_result = send(client_socket, send_buf);126Please respect copyright.ＰＥＮＡＮＡWyGbdsAB6u
if(send_result == fail) return; else continue;126Please respect copyright.ＰＥＮＡＮＡpDzylImHvf
}
// a way to make etag for a file using its file size and last modifed time126Please respect copyright.ＰＥＮＡＮＡgneBS29fh4
const char *file_etag(const char *path) 126Please respect copyright.ＰＥＮＡＮＡQt8HWovxsU
{126Please respect copyright.ＰＥＮＡＮＡXtkfKLEjfU
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;126Please respect copyright.ＰＥＮＡＮＡbA82LHtyXh
}126Please respect copyright.ＰＥＮＡＮＡAm5HURjNjP
-------------------------------------------------126Please respect copyright.ＰＥＮＡＮＡhpyGGmuMmR
"accept-ranges: bytes" tells client web browser that the http server support range.126Please respect copyright.ＰＥＮＡＮＡIUuGbWsJmm
"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).126Please respect copyright.ＰＥＮＡＮＡbphyCcd5bd
'ETag: "534231200_1732277116"' tells client web browser the file version.126Please respect copyright.ＰＥＮＡＮＡtLOd0HLdN9
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.126Please respect copyright.ＰＥＮＡＮＡOwrF9BlORK
"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.126Please respect copyright.ＰＥＮＡＮＡlHUa7JIUDm
"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.126Please respect copyright.ＰＥＮＡＮＡMelGSBoNgE
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.)126Please respect copyright.ＰＥＮＡＮＡ1R1sp6ZQQV
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:126Please respect copyright.ＰＥＮＡＮＡY0YCaFQfqF
---------resume range request---------126Please respect copyright.ＰＥＮＡＮＡf42JBK7i28
GET /zen.iso HTTP/1.1126Please respect copyright.ＰＥＮＡＮＡquUj7ohsiC
Range: bytes=243220480-126Please respect copyright.ＰＥＮＡＮＡO2QN7a7PiA
If-Match: "534231200_1732277116"126Please respect copyright.ＰＥＮＡＮＡRPfngJpzFn
......126Please respect copyright.ＰＥＮＡＮＡf9Yryuj2Cg
-------------------------------------126Please respect copyright.ＰＥＮＡＮＡj7a3P4nrXZ
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.126Please respect copyright.ＰＥＮＡＮＡWtzFhjdy8D
'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:126Please respect copyright.ＰＥＮＡＮＡRK2AN8stg8
-------response content from resume point-------126Please respect copyright.ＰＥＮＡＮＡOK6rKlvILd
HTTP/1.1 206 Partial Content126Please respect copyright.ＰＥＮＡＮＡOjUUeKfsCY
Date: Sat, 23 Nov 2024 06:56:17 GMT126Please respect copyright.ＰＥＮＡＮＡmjYCzAQXMg
accept-ranges: bytes126Please respect copyright.ＰＥＮＡＮＡUs4mx9mb0B
content-range: bytes 243220480-534231199/534231200126Please respect copyright.ＰＥＮＡＮＡbKPcop2sz4
Content-Length: 291010720126Please respect copyright.ＰＥＮＡＮＡiQWXypwAtP
Content-Type: application/octet-stream126Please respect copyright.ＰＥＮＡＮＡvWn5tQ83nx
Content-Disposition: attachment; filename="zen.iso"126Please respect copyright.ＰＥＮＡＮＡ3ttXsigwzv
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)126Please respect copyright.ＰＥＮＡＮＡTJgnxOFbpY
------------------------------------------------126Please respect copyright.ＰＥＮＡＮＡ0UcxXjY0AG
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:126Please respect copyright.ＰＥＮＡＮＡrS7llYIIrJ
-------response content from resume point-------126Please respect copyright.ＰＥＮＡＮＡNua4sVHtFd
HTTP/1.1 200 OK126Please respect copyright.ＰＥＮＡＮＡFPfdFacb8T
Date: Sat, 23 Nov 2024 06:29:19 GMT126Please respect copyright.ＰＥＮＡＮＡtw4pvTlr2Z
accept-ranges: bytes126Please respect copyright.ＰＥＮＡＮＡvl3TanKIPO
content-range: bytes 0-634231201/634231202126Please respect copyright.ＰＥＮＡＮＡLGrjJ9Lecw
ETag: "634231202_2732277118"126Please respect copyright.ＰＥＮＡＮＡ7XxsFvVIgE
Content-Disposition: attachment; filename="zen.iso"126Please respect copyright.ＰＥＮＡＮＡ9Pbuwxn00y
Content-Length: 634231202126Please respect copyright.ＰＥＮＡＮＡLGbb25AL3r
Content-Type: application/octet-stream
(http response body is the whole file content)126Please respect copyright.ＰＥＮＡＮＡ2wyISioIQO
------------------------------------------------126Please respect copyright.ＰＥＮＡＮＡ0MMmn10lss
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:126Please respect copyright.ＰＥＮＡＮＡkGzBF14EyN
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
126Please respect copyright.ＰＥＮＡＮＡ4YkXKQgoDw
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126Please respect copyright.ＰＥＮＡＮＡG6pEa16yM5
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126Please respect copyright.ＰＥＮＡＮＡUmbIz2vDXR
https://gemini.google.com/
感谢互联网上无私奉献的参考资126Please respect copyright.ＰＥＮＡＮＡkFTPp2Fw1B
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests126Please respect copyright.ＰＥＮＡＮＡtG9oMDQsUl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range126Please respect copyright.ＰＥＮＡＮＡSqZcQApnkx
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag126Please respect copyright.ＰＥＮＡＮＡtQXeXFdQHa
https://www.rfc-editor.org/rfc/rfc7233126Please respect copyright.ＰＥＮＡＮＡT9QgPK33Se
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]126Please respect copyright.ＰＥＮＡＮＡliY0qjLPOp
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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