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:80Please respect copyright.ＰＥＮＡＮＡS5rsSTUbEH
----------http range request-------80Please respect copyright.ＰＥＮＡＮＡzcheIkpXH9
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡ4rm4aCLlhV
Range: bytes=100-102380Please respect copyright.ＰＥＮＡＮＡOAYhqjtCwD
If-Match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡIWlpfJ2Kh4
......80Please respect copyright.ＰＥＮＡＮＡfRSFyFqumB
-----------------------------------80Please respect copyright.ＰＥＮＡＮＡqDxaEaiybe
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:80Please respect copyright.ＰＥＮＡＮＡCYOiwNFceL
----------------html-----------------80Please respect copyright.ＰＥＮＡＮＡaN3bDQb05I
<a href="/zen.iso">zen.iso</a>80Please respect copyright.ＰＥＮＡＮＡuezgZsObYq
-------------------------------------80Please respect copyright.ＰＥＮＡＮＡ8hANvlpKTD
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:80Please respect copyright.ＰＥＮＡＮＡBKLOBCDOyp
----------get request------------80Please respect copyright.ＰＥＮＡＮＡR31ZNzmaWU
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡ1QcIaghh14
......80Please respect copyright.ＰＥＮＡＮＡeoMekE1oe3
---------------------------------
80Please respect copyright.ＰＥＮＡＮＡSPMLkgwRay
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:80Please respect copyright.ＰＥＮＡＮＡF2C5O4lQoR
----------------server response--------------80Please respect copyright.ＰＥＮＡＮＡkb3FULNdvA
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡ07DPi5fb7O
Date: Sat, 23 Nov 2024 06:29:19 GMT80Please respect copyright.ＰＥＮＡＮＡ7TqE60KrbW
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡqebEmteRIA
content-range: bytes 0-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡIbpuk3LSnX
ETag: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡuKLkGUAaJS
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡY4JOTlQb0y
Content-Length: 53423120080Please respect copyright.ＰＥＮＡＮＡhKKJ3aD1GX
Content-Type: application/octet-stream
(http response body is the whole file content)80Please respect copyright.ＰＥＮＡＮＡXHhKNOCesy
-------------------------------------------
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:80Please respect copyright.ＰＥＮＡＮＡE4rRCcP8vI
---------------http server send big file content by multiple times--------------80Please respect copyright.ＰＥＮＡＮＡbywmKPWkVo
const int buf_size=20000000; // buf size is about 20M80Please respect copyright.ＰＥＮＡＮＡPMUz6nqXsX
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers80Please respect copyright.ＰＥＮＡＮＡ93KjBmwiLp
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 big80Please respect copyright.ＰＥＮＡＮＡfTgSqqpEeo
while(there_is_remain_file_content_to_send)80Please respect copyright.ＰＥＮＡＮＡu9tTTJD8sr
{ 80Please respect copyright.ＰＥＮＡＮＡ1NUCwORHCP
read_remain_file_content_to_send_buf(file, send_buf);80Please respect copyright.ＰＥＮＡＮＡSblrJyXpS1
send_result = send(client_socket, send_buf);80Please respect copyright.ＰＥＮＡＮＡhXO8my6uYi
if(send_result == fail) return; else continue;80Please respect copyright.ＰＥＮＡＮＡ9Sbutv0ERd
}
// a way to make etag for a file using its file size and last modifed time80Please respect copyright.ＰＥＮＡＮＡQQeUnp4DOG
const char *file_etag(const char *path) 80Please respect copyright.ＰＥＮＡＮＡ5wZqPkGdQh
{80Please respect copyright.ＰＥＮＡＮＡc4JnH2Njos
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;80Please respect copyright.ＰＥＮＡＮＡ1pNCx6jP6V
}80Please respect copyright.ＰＥＮＡＮＡoi77yjxvvo
-------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡNDlaOzrWGU
"accept-ranges: bytes" tells client web browser that the http server support range.80Please respect copyright.ＰＥＮＡＮＡcAlUrawlbH
"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).80Please respect copyright.ＰＥＮＡＮＡO1nEVBE10W
'ETag: "534231200_1732277116"' tells client web browser the file version.80Please respect copyright.ＰＥＮＡＮＡrw3ykEOTeJ
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.80Please respect copyright.ＰＥＮＡＮＡ2Vljrg14a6
"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.80Please respect copyright.ＰＥＮＡＮＡKKPHEOh6MH
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
80Please respect copyright.ＰＥＮＡＮＡk7uxWClOx6
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.80Please respect copyright.ＰＥＮＡＮＡYikhfCUonq
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.)80Please respect copyright.ＰＥＮＡＮＡzTMCXHDPs6
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:80Please respect copyright.ＰＥＮＡＮＡ7H2R9qJRxh
---------resume range request---------80Please respect copyright.ＰＥＮＡＮＡcEuDKfjYKc
GET /zen.iso HTTP/1.180Please respect copyright.ＰＥＮＡＮＡNuLZHuZFVM
Range: bytes=243220480-80Please respect copyright.ＰＥＮＡＮＡlaWfAY4NLd
If-Match: "534231200_1732277116"80Please respect copyright.ＰＥＮＡＮＡTUz0C6xHrm
......80Please respect copyright.ＰＥＮＡＮＡwqsTBHSbX3
-------------------------------------80Please respect copyright.ＰＥＮＡＮＡBd6StZLfFb
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.80Please respect copyright.ＰＥＮＡＮＡ6qfFIDQBQX
'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:80Please respect copyright.ＰＥＮＡＮＡYIijU8iTl9
-------response content from resume point-------80Please respect copyright.ＰＥＮＡＮＡZHHy6uilYW
HTTP/1.1 206 Partial Content80Please respect copyright.ＰＥＮＡＮＡbR7eC5aGLm
Date: Sat, 23 Nov 2024 06:56:17 GMT80Please respect copyright.ＰＥＮＡＮＡIybEscPXny
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡlomwIOYahJ
content-range: bytes 243220480-534231199/53423120080Please respect copyright.ＰＥＮＡＮＡw3AJqFLu6d
Content-Length: 29101072080Please respect copyright.ＰＥＮＡＮＡk4GNUkvvSM
Content-Type: application/octet-stream80Please respect copyright.ＰＥＮＡＮＡKnsRceoMrg
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡxmJVAYEO4C
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)80Please respect copyright.ＰＥＮＡＮＡOV2EFxmPnX
------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡXNNEFthWl8
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:80Please respect copyright.ＰＥＮＡＮＡKMfaVQBW7k
-------response content from resume point-------80Please respect copyright.ＰＥＮＡＮＡRNEaqLnGhl
HTTP/1.1 200 OK80Please respect copyright.ＰＥＮＡＮＡn7L8baQzzd
Date: Sat, 23 Nov 2024 06:29:19 GMT80Please respect copyright.ＰＥＮＡＮＡQv09v1SMCR
accept-ranges: bytes80Please respect copyright.ＰＥＮＡＮＡHc2QBtFG4S
content-range: bytes 0-634231201/63423120280Please respect copyright.ＰＥＮＡＮＡ9ftjrPClvI
ETag: "634231202_2732277118"80Please respect copyright.ＰＥＮＡＮＡtMNJscrnqQ
Content-Disposition: attachment; filename="zen.iso"80Please respect copyright.ＰＥＮＡＮＡM5rfRUFYmS
Content-Length: 63423120280Please respect copyright.ＰＥＮＡＮＡvWJ8YL2rhP
Content-Type: application/octet-stream
(http response body is the whole file content)80Please respect copyright.ＰＥＮＡＮＡdvSakqEgs2
------------------------------------------------80Please respect copyright.ＰＥＮＡＮＡU1NlxtcCl9
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
80Please respect copyright.ＰＥＮＡＮＡGEozp35IS6
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:80Please respect copyright.ＰＥＮＡＮＡmVGUvSNtds
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
80Please respect copyright.ＰＥＮＡＮＡ1O8bX6Juce
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80Please respect copyright.ＰＥＮＡＮＡKu4PiCEg9L
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80Please respect copyright.ＰＥＮＡＮＡzrjZ7ne2Eh
https://gemini.google.com/
感谢互联网上无私奉献的参考资80Please respect copyright.ＰＥＮＡＮＡVtn9swAuL2
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests80Please respect copyright.ＰＥＮＡＮＡrhd1mdSUmW
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range80Please respect copyright.ＰＥＮＡＮＡImuLx0Pxp5
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag80Please respect copyright.ＰＥＮＡＮＡetP6QrYyhZ
https://www.rfc-editor.org/rfc/rfc723380Please respect copyright.ＰＥＮＡＮＡXvBQ6Rt1Xa
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]80Please respect copyright.ＰＥＮＡＮＡF4BTZfGEA2
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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