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:166Please respect copyright.ＰＥＮＡＮＡSECakae65C
----------http range request-------166Please respect copyright.ＰＥＮＡＮＡyqMT6GT26W
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡ71uLXNGHED
Range: bytes=100-1023166Please respect copyright.ＰＥＮＡＮＡ7ARAKh9abk
If-Match: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡawxQMZoVMf
......166Please respect copyright.ＰＥＮＡＮＡxup5zHoqv7
-----------------------------------166Please respect copyright.ＰＥＮＡＮＡQ2GHX8GK8j
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:166Please respect copyright.ＰＥＮＡＮＡUnWz8dQF4X
----------------html-----------------166Please respect copyright.ＰＥＮＡＮＡu5C6kI92y7
<a href="/zen.iso">zen.iso</a>166Please respect copyright.ＰＥＮＡＮＡNe0JCXOG7a
-------------------------------------166Please respect copyright.ＰＥＮＡＮＡ73qUUbGWv8
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:166Please respect copyright.ＰＥＮＡＮＡqdOQqsSzIW
----------get request------------166Please respect copyright.ＰＥＮＡＮＡxnXckKIQ1m
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡx5OYbZ2Lqr
......166Please respect copyright.ＰＥＮＡＮＡogwi8coaPA
---------------------------------
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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:166Please respect copyright.ＰＥＮＡＮＡQdnrSi0ekP
----------------server response--------------166Please respect copyright.ＰＥＮＡＮＡ8Hoi8TOEpf
HTTP/1.1 200 OK166Please respect copyright.ＰＥＮＡＮＡtR7MF9WM82
Date: Sat, 23 Nov 2024 06:29:19 GMT166Please respect copyright.ＰＥＮＡＮＡiK1d6d45fU
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡgFSIGQzw0N
content-range: bytes 0-534231199/534231200166Please respect copyright.ＰＥＮＡＮＡNpsX6yRsu0
ETag: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡnYchWFsF3E
Content-Disposition: attachment; filename="zen.iso"166Please respect copyright.ＰＥＮＡＮＡuWu6678Jfw
Content-Length: 534231200166Please respect copyright.ＰＥＮＡＮＡhz7cKcPLHb
Content-Type: application/octet-stream
(http response body is the whole file content)166Please respect copyright.ＰＥＮＡＮＡWLElApMbCD
-------------------------------------------
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:166Please respect copyright.ＰＥＮＡＮＡO2vgkC7a83
---------------http server send big file content by multiple times--------------166Please respect copyright.ＰＥＮＡＮＡNHVg6bwxJa
const int buf_size=20000000; // buf size is about 20M166Please respect copyright.ＰＥＮＡＮＡ5GT4f8epT5
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers166Please respect copyright.ＰＥＮＡＮＡm3t1slrqu5
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 big166Please respect copyright.ＰＥＮＡＮＡHeX27IHkcM
while(there_is_remain_file_content_to_send)166Please respect copyright.ＰＥＮＡＮＡWPmSPiNEQQ
{ 166Please respect copyright.ＰＥＮＡＮＡpyF92oUuAA
read_remain_file_content_to_send_buf(file, send_buf);166Please respect copyright.ＰＥＮＡＮＡ3VqKdfqfnL
send_result = send(client_socket, send_buf);166Please respect copyright.ＰＥＮＡＮＡVHfFpTsxN0
if(send_result == fail) return; else continue;166Please respect copyright.ＰＥＮＡＮＡmTu0PE5A8o
}
// a way to make etag for a file using its file size and last modifed time166Please respect copyright.ＰＥＮＡＮＡk4oXQy5OKP
const char *file_etag(const char *path) 166Please respect copyright.ＰＥＮＡＮＡjn2sslPtu0
{166Please respect copyright.ＰＥＮＡＮＡrsbcJkYiyq
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;166Please respect copyright.ＰＥＮＡＮＡI86K8Bmb63
}166Please respect copyright.ＰＥＮＡＮＡ6Lqxs2Qzpm
-------------------------------------------------166Please respect copyright.ＰＥＮＡＮＡnoVb9zYysH
"accept-ranges: bytes" tells client web browser that the http server support range.166Please respect copyright.ＰＥＮＡＮＡGrK9IaHuHO
"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).166Please respect copyright.ＰＥＮＡＮＡewS3ea5C8I
'ETag: "534231200_1732277116"' tells client web browser the file version.166Please respect copyright.ＰＥＮＡＮＡpW0dKHeB3G
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.166Please respect copyright.ＰＥＮＡＮＡ1L8aQWsp3S
"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.166Please respect copyright.ＰＥＮＡＮＡerg0LdVir2
"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.166Please respect copyright.ＰＥＮＡＮＡ7HnUtsJepG
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.)166Please respect copyright.ＰＥＮＡＮＡ9brRrYsGc6
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:166Please respect copyright.ＰＥＮＡＮＡgGrgIstVR8
---------resume range request---------166Please respect copyright.ＰＥＮＡＮＡ01XxZOiUPe
GET /zen.iso HTTP/1.1166Please respect copyright.ＰＥＮＡＮＡeggaRdc2Cr
Range: bytes=243220480-166Please respect copyright.ＰＥＮＡＮＡ74Vif1SF0X
If-Match: "534231200_1732277116"166Please respect copyright.ＰＥＮＡＮＡp4JDeKDbxN
......166Please respect copyright.ＰＥＮＡＮＡDS12NqqDuf
-------------------------------------166Please respect copyright.ＰＥＮＡＮＡtLBrWuZONO
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.166Please respect copyright.ＰＥＮＡＮＡatsEeSY9wq
'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:166Please respect copyright.ＰＥＮＡＮＡmorrFQ8ZIt
-------response content from resume point-------166Please respect copyright.ＰＥＮＡＮＡ4YJH60ly9a
HTTP/1.1 206 Partial Content166Please respect copyright.ＰＥＮＡＮＡwnmc8FFU71
Date: Sat, 23 Nov 2024 06:56:17 GMT166Please respect copyright.ＰＥＮＡＮＡAdvQYPVopf
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡrOEHarr3wb
content-range: bytes 243220480-534231199/534231200166Please respect copyright.ＰＥＮＡＮＡUL3AlI6plQ
Content-Length: 291010720166Please respect copyright.ＰＥＮＡＮＡlza8jSCKyc
Content-Type: application/octet-stream166Please respect copyright.ＰＥＮＡＮＡPo2S0hG6Vb
Content-Disposition: attachment; filename="zen.iso"166Please respect copyright.ＰＥＮＡＮＡNVkljCRCHG
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)166Please respect copyright.ＰＥＮＡＮＡKEA67xQBcj
------------------------------------------------166Please respect copyright.ＰＥＮＡＮＡVmeDCgz3ka
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:166Please respect copyright.ＰＥＮＡＮＡjPqreOd99N
-------response content from resume point-------166Please respect copyright.ＰＥＮＡＮＡhqwQXhfOk8
HTTP/1.1 200 OK166Please respect copyright.ＰＥＮＡＮＡIIGNv6IsQ9
Date: Sat, 23 Nov 2024 06:29:19 GMT166Please respect copyright.ＰＥＮＡＮＡalk41KkBF3
accept-ranges: bytes166Please respect copyright.ＰＥＮＡＮＡA4JRSCm9xe
content-range: bytes 0-634231201/634231202166Please respect copyright.ＰＥＮＡＮＡ3mKZ9RixHw
ETag: "634231202_2732277118"166Please respect copyright.ＰＥＮＡＮＡTOc45Ee9uC
Content-Disposition: attachment; filename="zen.iso"166Please respect copyright.ＰＥＮＡＮＡbwvh19h4rP
Content-Length: 634231202166Please respect copyright.ＰＥＮＡＮＡ8wwOmgRYpc
Content-Type: application/octet-stream
(http response body is the whole file content)166Please respect copyright.ＰＥＮＡＮＡqjZpHMKc2b
------------------------------------------------166Please respect copyright.ＰＥＮＡＮＡZAkDfNwtnV
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:166Please respect copyright.ＰＥＮＡＮＡ8IE82OKUDh
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
166Please respect copyright.ＰＥＮＡＮＡAxpLS9KBX6
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166Please respect copyright.ＰＥＮＡＮＡrSLeNP1zXM
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166Please respect copyright.ＰＥＮＡＮＡppFceTtf6n
https://gemini.google.com/
感谢互联网上无私奉献的参考资166Please respect copyright.ＰＥＮＡＮＡE5WfifxQX9
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests166Please respect copyright.ＰＥＮＡＮＡNIA3RcVP2w
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range166Please respect copyright.ＰＥＮＡＮＡOtnN6REgtE
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag166Please respect copyright.ＰＥＮＡＮＡXkfDf2aLQO
https://www.rfc-editor.org/rfc/rfc7233166Please respect copyright.ＰＥＮＡＮＡ2WfnZ2hbAE
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]166Please respect copyright.ＰＥＮＡＮＡhbmVzdJLfN
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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