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:74Please respect copyright.ＰＥＮＡＮＡZUYQGnkgZ0
----------http range request-------74Please respect copyright.ＰＥＮＡＮＡGflVEymNJP
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡZ7x3bs2zXv
Range: bytes=100-102374Please respect copyright.ＰＥＮＡＮＡFVTXP5yWuN
If-Match: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡEufsOtT7Ql
......74Please respect copyright.ＰＥＮＡＮＡQcx0OuXHE6
-----------------------------------74Please respect copyright.ＰＥＮＡＮＡQK82I79qe0
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:74Please respect copyright.ＰＥＮＡＮＡ6EYv3nFhIi
----------------html-----------------74Please respect copyright.ＰＥＮＡＮＡ0CYsxgbNHO
<a href="/zen.iso">zen.iso</a>74Please respect copyright.ＰＥＮＡＮＡik7nYsoGoq
-------------------------------------74Please respect copyright.ＰＥＮＡＮＡFrgfVvtkdB
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:74Please respect copyright.ＰＥＮＡＮＡI5Pc6CZ8g0
----------get request------------74Please respect copyright.ＰＥＮＡＮＡ9i3fmv0pzE
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡH7Am4Xepkj
......74Please respect copyright.ＰＥＮＡＮＡvkoq2oCQn2
---------------------------------
74Please respect copyright.ＰＥＮＡＮＡzlieUVIUZV
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:74Please respect copyright.ＰＥＮＡＮＡvl47x1ccSu
----------------server response--------------74Please respect copyright.ＰＥＮＡＮＡH95c7ECtQ8
HTTP/1.1 200 OK74Please respect copyright.ＰＥＮＡＮＡ60eqMP8Rs6
Date: Sat, 23 Nov 2024 06:29:19 GMT74Please respect copyright.ＰＥＮＡＮＡsRlxLbnh8v
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡTFCQ8FlV5G
content-range: bytes 0-534231199/53423120074Please respect copyright.ＰＥＮＡＮＡblwGPL5uyA
ETag: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡkeqCZbvbYh
Content-Disposition: attachment; filename="zen.iso"74Please respect copyright.ＰＥＮＡＮＡEiP4yLbgcL
Content-Length: 53423120074Please respect copyright.ＰＥＮＡＮＡs9yizHC6W7
Content-Type: application/octet-stream
(http response body is the whole file content)74Please respect copyright.ＰＥＮＡＮＡQsc5DhrMx8
-------------------------------------------
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:74Please respect copyright.ＰＥＮＡＮＡnuhm1gaqXe
---------------http server send big file content by multiple times--------------74Please respect copyright.ＰＥＮＡＮＡASL91LsQsv
const int buf_size=20000000; // buf size is about 20M74Please respect copyright.ＰＥＮＡＮＡLHGhoZMfVx
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers74Please respect copyright.ＰＥＮＡＮＡnK3BBpM9Dr
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 big74Please respect copyright.ＰＥＮＡＮＡRb4pSlPPlC
while(there_is_remain_file_content_to_send)74Please respect copyright.ＰＥＮＡＮＡJjwAAWGfSH
{ 74Please respect copyright.ＰＥＮＡＮＡ8UryC4Ifpv
read_remain_file_content_to_send_buf(file, send_buf);74Please respect copyright.ＰＥＮＡＮＡ2415D78mfE
send_result = send(client_socket, send_buf);74Please respect copyright.ＰＥＮＡＮＡFihs6PAwAV
if(send_result == fail) return; else continue;74Please respect copyright.ＰＥＮＡＮＡZormx1ELEf
}
// a way to make etag for a file using its file size and last modifed time74Please respect copyright.ＰＥＮＡＮＡx01nhnknKG
const char *file_etag(const char *path) 74Please respect copyright.ＰＥＮＡＮＡEy2nu5SU9K
{74Please respect copyright.ＰＥＮＡＮＡ3GUcaJx1PV
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;74Please respect copyright.ＰＥＮＡＮＡjN2Esy5G8E
}74Please respect copyright.ＰＥＮＡＮＡ9HWxIphAft
-------------------------------------------------74Please respect copyright.ＰＥＮＡＮＡh2hOpNF6hZ
"accept-ranges: bytes" tells client web browser that the http server support range.74Please respect copyright.ＰＥＮＡＮＡzpCjPg6pXk
"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).74Please respect copyright.ＰＥＮＡＮＡl0mIBK8q27
'ETag: "534231200_1732277116"' tells client web browser the file version.74Please respect copyright.ＰＥＮＡＮＡBBvSI7ef9e
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.74Please respect copyright.ＰＥＮＡＮＡIDMNVNIjvi
"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.74Please respect copyright.ＰＥＮＡＮＡu5dutmV0KF
"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.74Please respect copyright.ＰＥＮＡＮＡznVCWPRE0A
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.)74Please respect copyright.ＰＥＮＡＮＡlOio4o19r7
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:74Please respect copyright.ＰＥＮＡＮＡW1mCkWGvPo
---------resume range request---------74Please respect copyright.ＰＥＮＡＮＡo2D1dzOr88
GET /zen.iso HTTP/1.174Please respect copyright.ＰＥＮＡＮＡXtaYODLOI0
Range: bytes=243220480-74Please respect copyright.ＰＥＮＡＮＡ6cPStHLya4
If-Match: "534231200_1732277116"74Please respect copyright.ＰＥＮＡＮＡheKRqZX32P
......74Please respect copyright.ＰＥＮＡＮＡF2qD7McSjG
-------------------------------------74Please respect copyright.ＰＥＮＡＮＡvEYgbLvWoR
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.74Please respect copyright.ＰＥＮＡＮＡ7VJl45o0XM
'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:74Please respect copyright.ＰＥＮＡＮＡxwuh7NXHA9
-------response content from resume point-------74Please respect copyright.ＰＥＮＡＮＡc4k4qpkhyv
HTTP/1.1 206 Partial Content74Please respect copyright.ＰＥＮＡＮＡ0hyN36woUi
Date: Sat, 23 Nov 2024 06:56:17 GMT74Please respect copyright.ＰＥＮＡＮＡNZzO7BKy89
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡ9tAiSk7nLT
content-range: bytes 243220480-534231199/53423120074Please respect copyright.ＰＥＮＡＮＡ1rNff7essE
Content-Length: 29101072074Please respect copyright.ＰＥＮＡＮＡQwVzZ46uAU
Content-Type: application/octet-stream74Please respect copyright.ＰＥＮＡＮＡlbXXG1HIWE
Content-Disposition: attachment; filename="zen.iso"74Please respect copyright.ＰＥＮＡＮＡebco6XTJ9i
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)74Please respect copyright.ＰＥＮＡＮＡ2ldXLSCzDh
------------------------------------------------74Please respect copyright.ＰＥＮＡＮＡ0okf1ULz83
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:74Please respect copyright.ＰＥＮＡＮＡyb3jNiq5YO
-------response content from resume point-------74Please respect copyright.ＰＥＮＡＮＡJJicUkpL4m
HTTP/1.1 200 OK74Please respect copyright.ＰＥＮＡＮＡC6Rm8du0Jv
Date: Sat, 23 Nov 2024 06:29:19 GMT74Please respect copyright.ＰＥＮＡＮＡCK0hg4jvWt
accept-ranges: bytes74Please respect copyright.ＰＥＮＡＮＡgn1fwEQqS2
content-range: bytes 0-634231201/63423120274Please respect copyright.ＰＥＮＡＮＡaFQqeupkud
ETag: "634231202_2732277118"74Please respect copyright.ＰＥＮＡＮＡAt9oEh0RpT
Content-Disposition: attachment; filename="zen.iso"74Please respect copyright.ＰＥＮＡＮＡ6ypwvXNbRT
Content-Length: 63423120274Please respect copyright.ＰＥＮＡＮＡ4IgOl5jfnR
Content-Type: application/octet-stream
(http response body is the whole file content)74Please respect copyright.ＰＥＮＡＮＡJMg8y8yVaN
------------------------------------------------74Please respect copyright.ＰＥＮＡＮＡiWuu87jSFQ
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:74Please respect copyright.ＰＥＮＡＮＡ3IDsXBHXhe
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
74Please respect copyright.ＰＥＮＡＮＡrIgTKBIag5
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74Please respect copyright.ＰＥＮＡＮＡW9PJ1pjrkZ
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74Please respect copyright.ＰＥＮＡＮＡox3x7uaKuo
https://gemini.google.com/
感谢互联网上无私奉献的参考资74Please respect copyright.ＰＥＮＡＮＡzv4pPMuQGE
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests74Please respect copyright.ＰＥＮＡＮＡ1AZxIgPAeE
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range74Please respect copyright.ＰＥＮＡＮＡCbXKOFwoER
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag74Please respect copyright.ＰＥＮＡＮＡSeDqERY7KK
https://www.rfc-editor.org/rfc/rfc723374Please respect copyright.ＰＥＮＡＮＡjTIiqKyj4K
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]74Please respect copyright.ＰＥＮＡＮＡzYMnCnvHPg
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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