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:28Please respect copyright.ＰＥＮＡＮＡWMYrnZwILJ
----------http range request-------28Please respect copyright.ＰＥＮＡＮＡYRBJnDQjQW
GET /zen.iso HTTP/1.128Please respect copyright.ＰＥＮＡＮＡKbe36kx3px
Range: bytes=100-102328Please respect copyright.ＰＥＮＡＮＡSPFRbLCcX7
If-Match: "534231200_1732277116"28Please respect copyright.ＰＥＮＡＮＡVxDPzCrbXD
......28Please respect copyright.ＰＥＮＡＮＡbjmmbMoSCu
-----------------------------------28Please respect copyright.ＰＥＮＡＮＡIFN75WpD3B
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:28Please respect copyright.ＰＥＮＡＮＡhmvNpDNDYP
----------------html-----------------28Please respect copyright.ＰＥＮＡＮＡhfAhj41F9v
<a href="/zen.iso">zen.iso</a>28Please respect copyright.ＰＥＮＡＮＡsnlVtR7h6g
-------------------------------------28Please respect copyright.ＰＥＮＡＮＡv6xTNNNLSu
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:28Please respect copyright.ＰＥＮＡＮＡlsad8aD3jl
----------get request------------28Please respect copyright.ＰＥＮＡＮＡHQYn8zIxwa
GET /zen.iso HTTP/1.128Please respect copyright.ＰＥＮＡＮＡHFkOz1LO63
......28Please respect copyright.ＰＥＮＡＮＡfrl2MQlnRo
---------------------------------
28Please respect copyright.ＰＥＮＡＮＡ0Pm5nAL4ON
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:28Please respect copyright.ＰＥＮＡＮＡpDb0yJiICw
----------------server response--------------28Please respect copyright.ＰＥＮＡＮＡsJKoPLUzyE
HTTP/1.1 200 OK28Please respect copyright.ＰＥＮＡＮＡHAOh8sESls
Date: Sat, 23 Nov 2024 06:29:19 GMT28Please respect copyright.ＰＥＮＡＮＡPXXjBigz8V
accept-ranges: bytes28Please respect copyright.ＰＥＮＡＮＡv6vp53o1uB
content-range: bytes 0-534231199/53423120028Please respect copyright.ＰＥＮＡＮＡTWFjEAUz4E
ETag: "534231200_1732277116"28Please respect copyright.ＰＥＮＡＮＡVMRCCemZ0s
Content-Disposition: attachment; filename="zen.iso"28Please respect copyright.ＰＥＮＡＮＡXZD9OoZJeX
Content-Length: 53423120028Please respect copyright.ＰＥＮＡＮＡg68SI3cWrC
Content-Type: application/octet-stream
(http response body is the whole file content)28Please respect copyright.ＰＥＮＡＮＡBrUjJeNzNG
-------------------------------------------
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:28Please respect copyright.ＰＥＮＡＮＡD3p7abXADJ
---------------http server send big file content by multiple times--------------28Please respect copyright.ＰＥＮＡＮＡkU5wqX8Lqk
const int buf_size=20000000; // buf size is about 20M28Please respect copyright.ＰＥＮＡＮＡJXXgSpkqeF
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers28Please respect copyright.ＰＥＮＡＮＡkiUI9oCxE9
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 big28Please respect copyright.ＰＥＮＡＮＡIDZfROxI27
while(there_is_remain_file_content_to_send)28Please respect copyright.ＰＥＮＡＮＡnw1JcUuJgB
{ 28Please respect copyright.ＰＥＮＡＮＡYujG6kt63S
read_remain_file_content_to_send_buf(file, send_buf);28Please respect copyright.ＰＥＮＡＮＡld9vDbOHCi
send_result = send(client_socket, send_buf);28Please respect copyright.ＰＥＮＡＮＡwUNjJKVfK5
if(send_result == fail) return; else continue;28Please respect copyright.ＰＥＮＡＮＡlRxWoYsZI5
}
// a way to make etag for a file using its file size and last modifed time28Please respect copyright.ＰＥＮＡＮＡkxDUFYK3d6
const char *file_etag(const char *path) 28Please respect copyright.ＰＥＮＡＮＡntaosSAfe1
{28Please respect copyright.ＰＥＮＡＮＡQHT3muAEko
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;28Please respect copyright.ＰＥＮＡＮＡYErH54pZlW
}28Please respect copyright.ＰＥＮＡＮＡOoowaLEmEo
-------------------------------------------------28Please respect copyright.ＰＥＮＡＮＡCI49FkpKFO
"accept-ranges: bytes" tells client web browser that the http server support range.28Please respect copyright.ＰＥＮＡＮＡIM89HgDviy
"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).28Please respect copyright.ＰＥＮＡＮＡ9xY5tfgIwe
'ETag: "534231200_1732277116"' tells client web browser the file version.28Please respect copyright.ＰＥＮＡＮＡgkjo31riTy
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.28Please respect copyright.ＰＥＮＡＮＡgftIdd2oCA
"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.28Please respect copyright.ＰＥＮＡＮＡYcxiWZijPr
"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.28Please respect copyright.ＰＥＮＡＮＡZz7G4JIXZJ
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.)28Please respect copyright.ＰＥＮＡＮＡ33LdYf8uV9
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:28Please respect copyright.ＰＥＮＡＮＡDwnabm8t95
---------resume range request---------28Please respect copyright.ＰＥＮＡＮＡPkspWYNGgg
GET /zen.iso HTTP/1.128Please respect copyright.ＰＥＮＡＮＡSTZQtp2v7M
Range: bytes=243220480-28Please respect copyright.ＰＥＮＡＮＡdlGqXwyA0f
If-Match: "534231200_1732277116"28Please respect copyright.ＰＥＮＡＮＡVFTl2gUiIp
......28Please respect copyright.ＰＥＮＡＮＡM5fGdE0FRP
-------------------------------------28Please respect copyright.ＰＥＮＡＮＡPvl8ypAvOM
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.28Please respect copyright.ＰＥＮＡＮＡQXqnWrK5Ys
'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:28Please respect copyright.ＰＥＮＡＮＡT4knjCwY89
-------response content from resume point-------28Please respect copyright.ＰＥＮＡＮＡhtlYob8PQb
HTTP/1.1 206 Partial Content28Please respect copyright.ＰＥＮＡＮＡwadB1bP9rG
Date: Sat, 23 Nov 2024 06:56:17 GMT28Please respect copyright.ＰＥＮＡＮＡg8wJZqgDjC
accept-ranges: bytes28Please respect copyright.ＰＥＮＡＮＡNsEe0tY3Ek
content-range: bytes 243220480-534231199/53423120028Please respect copyright.ＰＥＮＡＮＡavGRofda7s
Content-Length: 29101072028Please respect copyright.ＰＥＮＡＮＡRMRyGVgmks
Content-Type: application/octet-stream28Please respect copyright.ＰＥＮＡＮＡZAaqfNnacs
Content-Disposition: attachment; filename="zen.iso"28Please respect copyright.ＰＥＮＡＮＡZCpFjEcNOL
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)28Please respect copyright.ＰＥＮＡＮＡ0xdrHylrgV
------------------------------------------------28Please respect copyright.ＰＥＮＡＮＡHH88SfX8B7
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:28Please respect copyright.ＰＥＮＡＮＡw7404Q0jSD
-------response content from resume point-------28Please respect copyright.ＰＥＮＡＮＡ1L0RGJKSSP
HTTP/1.1 200 OK28Please respect copyright.ＰＥＮＡＮＡFDIg7XYqw1
Date: Sat, 23 Nov 2024 06:29:19 GMT28Please respect copyright.ＰＥＮＡＮＡAn5fLdT5hU
accept-ranges: bytes28Please respect copyright.ＰＥＮＡＮＡD9SAWMMuwZ
content-range: bytes 0-634231201/63423120228Please respect copyright.ＰＥＮＡＮＡ7WDVojcmNU
ETag: "634231202_2732277118"28Please respect copyright.ＰＥＮＡＮＡ7tKwCOo1eX
Content-Disposition: attachment; filename="zen.iso"28Please respect copyright.ＰＥＮＡＮＡU0Ma3aitke
Content-Length: 63423120228Please respect copyright.ＰＥＮＡＮＡFl9MVdu4M6
Content-Type: application/octet-stream
(http response body is the whole file content)28Please respect copyright.ＰＥＮＡＮＡcIIh0DHQYK
------------------------------------------------28Please respect copyright.ＰＥＮＡＮＡk6uMj2eY14
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:28Please respect copyright.ＰＥＮＡＮＡCXBYm5v6rd
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
28Please respect copyright.ＰＥＮＡＮＡ6P4O23Ene5
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28Please respect copyright.ＰＥＮＡＮＡgkie1GgKzz
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28Please respect copyright.ＰＥＮＡＮＡ0n133bprmR
https://gemini.google.com/
感谢互联网上无私奉献的参考资28Please respect copyright.ＰＥＮＡＮＡLP1tmRGTzh
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests28Please respect copyright.ＰＥＮＡＮＡaaGOKyRGQU
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range28Please respect copyright.ＰＥＮＡＮＡOQA7jkcmuf
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag28Please respect copyright.ＰＥＮＡＮＡWT92MGhUsF
https://www.rfc-editor.org/rfc/rfc723328Please respect copyright.ＰＥＮＡＮＡlHEweQuuc8
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: si.jinmin@gmail.com28Please respect copyright.ＰＥＮＡＮＡE97gXCe4H3
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: si.jinmin@gmail.com
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