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:176Please respect copyright.ＰＥＮＡＮＡBgHaZlTeh0
----------http range request-------176Please respect copyright.ＰＥＮＡＮＡWX7QeLidmi
GET /zen.iso HTTP/1.1176Please respect copyright.ＰＥＮＡＮＡbxtuFWXBxO
Range: bytes=100-1023176Please respect copyright.ＰＥＮＡＮＡ8z8AZON1AF
If-Match: "534231200_1732277116"176Please respect copyright.ＰＥＮＡＮＡqc4hMW9nyK
......176Please respect copyright.ＰＥＮＡＮＡ4PhTD7L5r8
-----------------------------------176Please respect copyright.ＰＥＮＡＮＡTEYWfH228Q
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:176Please respect copyright.ＰＥＮＡＮＡl7vNFIvVAa
----------------html-----------------176Please respect copyright.ＰＥＮＡＮＡkrDij2G4fE
<a href="/zen.iso">zen.iso</a>176Please respect copyright.ＰＥＮＡＮＡiIGhaDlpIP
-------------------------------------176Please respect copyright.ＰＥＮＡＮＡbjGtC93O0x
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:176Please respect copyright.ＰＥＮＡＮＡ1zQearTswp
----------get request------------176Please respect copyright.ＰＥＮＡＮＡJLGDiGt6tw
GET /zen.iso HTTP/1.1176Please respect copyright.ＰＥＮＡＮＡ4NkxIBSaA7
......176Please respect copyright.ＰＥＮＡＮＡ6cWUKpCfGz
---------------------------------
176Please respect copyright.ＰＥＮＡＮＡWkCnj9bCeK
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:176Please respect copyright.ＰＥＮＡＮＡcMvDlRGwXh
----------------server response--------------176Please respect copyright.ＰＥＮＡＮＡw6rZQ4kIe5
HTTP/1.1 200 OK176Please respect copyright.ＰＥＮＡＮＡ7RHaAGx8dV
Date: Sat, 23 Nov 2024 06:29:19 GMT176Please respect copyright.ＰＥＮＡＮＡu5voQdJfBh
accept-ranges: bytes176Please respect copyright.ＰＥＮＡＮＡbE2lwjus8C
content-range: bytes 0-534231199/534231200176Please respect copyright.ＰＥＮＡＮＡUp8hXyHn5p
ETag: "534231200_1732277116"176Please respect copyright.ＰＥＮＡＮＡe2R985UMad
Content-Disposition: attachment; filename="zen.iso"176Please respect copyright.ＰＥＮＡＮＡbtmgj9kIo4
Content-Length: 534231200176Please respect copyright.ＰＥＮＡＮＡ4MYOXdZH0I
Content-Type: application/octet-stream
(http response body is the whole file content)176Please respect copyright.ＰＥＮＡＮＡ7NRsuRSXuu
-------------------------------------------
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:176Please respect copyright.ＰＥＮＡＮＡYzgVaO5miw
---------------http server send big file content by multiple times--------------176Please respect copyright.ＰＥＮＡＮＡnZiahLtWJb
const int buf_size=20000000; // buf size is about 20M176Please respect copyright.ＰＥＮＡＮＡnoKnbYKKCt
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers176Please respect copyright.ＰＥＮＡＮＡ5cJfDVLMls
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 big176Please respect copyright.ＰＥＮＡＮＡbQwSi0icqm
while(there_is_remain_file_content_to_send)176Please respect copyright.ＰＥＮＡＮＡ1Yr2yvvG24
{ 176Please respect copyright.ＰＥＮＡＮＡ9Pg14lSLSA
read_remain_file_content_to_send_buf(file, send_buf);176Please respect copyright.ＰＥＮＡＮＡKejyXETDJd
send_result = send(client_socket, send_buf);176Please respect copyright.ＰＥＮＡＮＡNHLVWIgAJP
if(send_result == fail) return; else continue;176Please respect copyright.ＰＥＮＡＮＡ3rVaC9RCTd
}
// a way to make etag for a file using its file size and last modifed time176Please respect copyright.ＰＥＮＡＮＡhTkss9QoB5
const char *file_etag(const char *path) 176Please respect copyright.ＰＥＮＡＮＡf2DAdAZEh0
{176Please respect copyright.ＰＥＮＡＮＡlaQQsKaZKc
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;176Please respect copyright.ＰＥＮＡＮＡ4aaoMCiVL7
}176Please respect copyright.ＰＥＮＡＮＡqjjciun0EX
-------------------------------------------------176Please respect copyright.ＰＥＮＡＮＡg2lzA1Tk5m
"accept-ranges: bytes" tells client web browser that the http server support range.176Please respect copyright.ＰＥＮＡＮＡ1grQItL5F7
"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).176Please respect copyright.ＰＥＮＡＮＡHbo1EPDVfj
'ETag: "534231200_1732277116"' tells client web browser the file version.176Please respect copyright.ＰＥＮＡＮＡjzHasP77TE
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.176Please respect copyright.ＰＥＮＡＮＡE4S8YDIOmc
"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.176Please respect copyright.ＰＥＮＡＮＡmrrjS2rgtd
"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.176Please respect copyright.ＰＥＮＡＮＡ4x318JBsLa
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.)176Please respect copyright.ＰＥＮＡＮＡ1od4JGzfbi
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:176Please respect copyright.ＰＥＮＡＮＡibtU7Qbjpn
---------resume range request---------176Please respect copyright.ＰＥＮＡＮＡv0ribyX8R8
GET /zen.iso HTTP/1.1176Please respect copyright.ＰＥＮＡＮＡw6Hd5h3eY9
Range: bytes=243220480-176Please respect copyright.ＰＥＮＡＮＡ87OgxNJ6lY
If-Match: "534231200_1732277116"176Please respect copyright.ＰＥＮＡＮＡKsjVpSsiiF
......176Please respect copyright.ＰＥＮＡＮＡFJUWD0Tfy0
-------------------------------------176Please respect copyright.ＰＥＮＡＮＡRHBfHfoffj
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.176Please respect copyright.ＰＥＮＡＮＡnJPuRCl9Wy
'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:176Please respect copyright.ＰＥＮＡＮＡHH1wqUiXPS
-------response content from resume point-------176Please respect copyright.ＰＥＮＡＮＡ8bADvwC2Js
HTTP/1.1 206 Partial Content176Please respect copyright.ＰＥＮＡＮＡk9rE90oYLI
Date: Sat, 23 Nov 2024 06:56:17 GMT176Please respect copyright.ＰＥＮＡＮＡOT2ho25yoX
accept-ranges: bytes176Please respect copyright.ＰＥＮＡＮＡdlCe17xhkj
content-range: bytes 243220480-534231199/534231200176Please respect copyright.ＰＥＮＡＮＡeCDKArQRGM
Content-Length: 291010720176Please respect copyright.ＰＥＮＡＮＡaUM7PiTgke
Content-Type: application/octet-stream176Please respect copyright.ＰＥＮＡＮＡFHpdUWsoOj
Content-Disposition: attachment; filename="zen.iso"176Please respect copyright.ＰＥＮＡＮＡjRVSQ4JU5p
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)176Please respect copyright.ＰＥＮＡＮＡLZ7ufJJLC6
------------------------------------------------176Please respect copyright.ＰＥＮＡＮＡP9iOORLqd0
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:176Please respect copyright.ＰＥＮＡＮＡobsCIcbMog
-------response content from resume point-------176Please respect copyright.ＰＥＮＡＮＡFW08cKHUmU
HTTP/1.1 200 OK176Please respect copyright.ＰＥＮＡＮＡaVdqfhf7mB
Date: Sat, 23 Nov 2024 06:29:19 GMT176Please respect copyright.ＰＥＮＡＮＡggqJtUeBqL
accept-ranges: bytes176Please respect copyright.ＰＥＮＡＮＡQkELBKqdci
content-range: bytes 0-634231201/634231202176Please respect copyright.ＰＥＮＡＮＡPygU4b6N57
ETag: "634231202_2732277118"176Please respect copyright.ＰＥＮＡＮＡjgvxuxoGuv
Content-Disposition: attachment; filename="zen.iso"176Please respect copyright.ＰＥＮＡＮＡVjXNkLqTtz
Content-Length: 634231202176Please respect copyright.ＰＥＮＡＮＡZ77vAwSBlh
Content-Type: application/octet-stream
(http response body is the whole file content)176Please respect copyright.ＰＥＮＡＮＡ2kBw1V7l4Q
------------------------------------------------176Please respect copyright.ＰＥＮＡＮＡYVRwtmIQx3
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:176Please respect copyright.ＰＥＮＡＮＡeLZ9DLv98I
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
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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176Please respect copyright.ＰＥＮＡＮＡ85qBybqeVA
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176Please respect copyright.ＰＥＮＡＮＡxwtZg4WAma
https://gemini.google.com/
感谢互联网上无私奉献的参考资176Please respect copyright.ＰＥＮＡＮＡ3dDgADzQth
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests176Please respect copyright.ＰＥＮＡＮＡsN6XXjzlQr
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range176Please respect copyright.ＰＥＮＡＮＡlLJ7Jggbgo
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag176Please respect copyright.ＰＥＮＡＮＡtTuKeSbcRo
https://www.rfc-editor.org/rfc/rfc7233176Please respect copyright.ＰＥＮＡＮＡAGmwyD6hFq
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]176Please respect copyright.ＰＥＮＡＮＡ4lXLMhko4P
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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