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:72Please respect copyright.ＰＥＮＡＮＡRzQKzNo7aa
----------http range request-------72Please respect copyright.ＰＥＮＡＮＡGOSLC0mcnr
GET /zen.iso HTTP/1.172Please respect copyright.ＰＥＮＡＮＡxMGTBHhGXV
Range: bytes=100-102372Please respect copyright.ＰＥＮＡＮＡ4fATKEp6sb
If-Match: "534231200_1732277116"72Please respect copyright.ＰＥＮＡＮＡeYyp0sKpAF
......72Please respect copyright.ＰＥＮＡＮＡpEABb86nsv
-----------------------------------72Please respect copyright.ＰＥＮＡＮＡnRhlvQ4nQv
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:72Please respect copyright.ＰＥＮＡＮＡLhZQ0n6py5
----------------html-----------------72Please respect copyright.ＰＥＮＡＮＡnaSmhCs8YS
<a href="/zen.iso">zen.iso</a>72Please respect copyright.ＰＥＮＡＮＡgY5g76Lyb8
-------------------------------------72Please respect copyright.ＰＥＮＡＮＡNh8LWpG769
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:72Please respect copyright.ＰＥＮＡＮＡxkPOWbqJ1F
----------get request------------72Please respect copyright.ＰＥＮＡＮＡkSxM1kKQ5z
GET /zen.iso HTTP/1.172Please respect copyright.ＰＥＮＡＮＡPRJ0njFnCO
......72Please respect copyright.ＰＥＮＡＮＡ6ZzkM7QXPj
---------------------------------
72Please respect copyright.ＰＥＮＡＮＡOnut56z0PA
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:72Please respect copyright.ＰＥＮＡＮＡgxj0pt8xJ0
----------------server response--------------72Please respect copyright.ＰＥＮＡＮＡps2RtPi4Ib
HTTP/1.1 200 OK72Please respect copyright.ＰＥＮＡＮＡmh28BJXKXP
Date: Sat, 23 Nov 2024 06:29:19 GMT72Please respect copyright.ＰＥＮＡＮＡbUaho5tk3I
accept-ranges: bytes72Please respect copyright.ＰＥＮＡＮＡb9IWm3yJ3Z
content-range: bytes 0-534231199/53423120072Please respect copyright.ＰＥＮＡＮＡQO2FetSyX2
ETag: "534231200_1732277116"72Please respect copyright.ＰＥＮＡＮＡf3gdzFSbFw
Content-Disposition: attachment; filename="zen.iso"72Please respect copyright.ＰＥＮＡＮＡIAQXsCessL
Content-Length: 53423120072Please respect copyright.ＰＥＮＡＮＡ2EJIU3XcQS
Content-Type: application/octet-stream
(http response body is the whole file content)72Please respect copyright.ＰＥＮＡＮＡ2ctH2xEzt5
-------------------------------------------
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:72Please respect copyright.ＰＥＮＡＮＡszV9kzD651
---------------http server send big file content by multiple times--------------72Please respect copyright.ＰＥＮＡＮＡQS8EO2uejF
const int buf_size=20000000; // buf size is about 20M72Please respect copyright.ＰＥＮＡＮＡVofKDfzkc1
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers72Please respect copyright.ＰＥＮＡＮＡjt1AYbRvDk
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 big72Please respect copyright.ＰＥＮＡＮＡDVvqA9j2EZ
while(there_is_remain_file_content_to_send)72Please respect copyright.ＰＥＮＡＮＡAi2dAbGcRC
{ 72Please respect copyright.ＰＥＮＡＮＡbTUkT9Grdd
read_remain_file_content_to_send_buf(file, send_buf);72Please respect copyright.ＰＥＮＡＮＡlduCEoGbvn
send_result = send(client_socket, send_buf);72Please respect copyright.ＰＥＮＡＮＡ6bkKFgDHwj
if(send_result == fail) return; else continue;72Please respect copyright.ＰＥＮＡＮＡLLwQx10nC5
}
// a way to make etag for a file using its file size and last modifed time72Please respect copyright.ＰＥＮＡＮＡWRbIp3sM5l
const char *file_etag(const char *path) 72Please respect copyright.ＰＥＮＡＮＡtzvZrOa9Ge
{72Please respect copyright.ＰＥＮＡＮＡwNvKItW48n
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;72Please respect copyright.ＰＥＮＡＮＡV1SXL9sJue
}72Please respect copyright.ＰＥＮＡＮＡIpp3em5gPK
-------------------------------------------------72Please respect copyright.ＰＥＮＡＮＡVnf7roIdtS
"accept-ranges: bytes" tells client web browser that the http server support range.72Please respect copyright.ＰＥＮＡＮＡD6GTuTVaWP
"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).72Please respect copyright.ＰＥＮＡＮＡLO5oH3RAbd
'ETag: "534231200_1732277116"' tells client web browser the file version.72Please respect copyright.ＰＥＮＡＮＡtfcsVh4ub2
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.72Please respect copyright.ＰＥＮＡＮＡatHHauiOm3
"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.72Please respect copyright.ＰＥＮＡＮＡS9GsL4Azhy
"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.72Please respect copyright.ＰＥＮＡＮＡnEKmyLntZE
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.)72Please respect copyright.ＰＥＮＡＮＡcs5HCutF2s
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:72Please respect copyright.ＰＥＮＡＮＡgEKB7Y1yps
---------resume range request---------72Please respect copyright.ＰＥＮＡＮＡaOvFJ57f7N
GET /zen.iso HTTP/1.172Please respect copyright.ＰＥＮＡＮＡSKoi2QK5aH
Range: bytes=243220480-72Please respect copyright.ＰＥＮＡＮＡqnFJ6zuEma
If-Match: "534231200_1732277116"72Please respect copyright.ＰＥＮＡＮＡtULwCyLpTN
......72Please respect copyright.ＰＥＮＡＮＡXZlXw2omV7
-------------------------------------72Please respect copyright.ＰＥＮＡＮＡroSQUgGMaj
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.72Please respect copyright.ＰＥＮＡＮＡwnG0JK88l7
'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:72Please respect copyright.ＰＥＮＡＮＡngKQ7qu6MS
-------response content from resume point-------72Please respect copyright.ＰＥＮＡＮＡayVlwDZZ4X
HTTP/1.1 206 Partial Content72Please respect copyright.ＰＥＮＡＮＡDFchGmmvld
Date: Sat, 23 Nov 2024 06:56:17 GMT72Please respect copyright.ＰＥＮＡＮＡBVyLF05yGj
accept-ranges: bytes72Please respect copyright.ＰＥＮＡＮＡb9vadoXw4u
content-range: bytes 243220480-534231199/53423120072Please respect copyright.ＰＥＮＡＮＡ9rGTSXQc3c
Content-Length: 29101072072Please respect copyright.ＰＥＮＡＮＡN8sEr65gVu
Content-Type: application/octet-stream72Please respect copyright.ＰＥＮＡＮＡpinbEOfI2v
Content-Disposition: attachment; filename="zen.iso"72Please respect copyright.ＰＥＮＡＮＡIVKodPHeLq
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)72Please respect copyright.ＰＥＮＡＮＡfAnPbemvha
------------------------------------------------72Please respect copyright.ＰＥＮＡＮＡizfYcNSdL2
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:72Please respect copyright.ＰＥＮＡＮＡGrRWMEhL11
-------response content from resume point-------72Please respect copyright.ＰＥＮＡＮＡdqXiLUQ6uL
HTTP/1.1 200 OK72Please respect copyright.ＰＥＮＡＮＡBo1GqKKizm
Date: Sat, 23 Nov 2024 06:29:19 GMT72Please respect copyright.ＰＥＮＡＮＡNkeepVrmOr
accept-ranges: bytes72Please respect copyright.ＰＥＮＡＮＡ3RZjG1JQ7S
content-range: bytes 0-634231201/63423120272Please respect copyright.ＰＥＮＡＮＡiixIGKH2wv
ETag: "634231202_2732277118"72Please respect copyright.ＰＥＮＡＮＡOnMBiCUk0N
Content-Disposition: attachment; filename="zen.iso"72Please respect copyright.ＰＥＮＡＮＡPVtzcvECwE
Content-Length: 63423120272Please respect copyright.ＰＥＮＡＮＡZzDtsko1yU
Content-Type: application/octet-stream
(http response body is the whole file content)72Please respect copyright.ＰＥＮＡＮＡkmhesNsXE3
------------------------------------------------72Please respect copyright.ＰＥＮＡＮＡeUGrBB7JtG
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
72Please respect copyright.ＰＥＮＡＮＡJCag3hUok2
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:72Please respect copyright.ＰＥＮＡＮＡfN0CJa6yPR
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72Please respect copyright.ＰＥＮＡＮＡIm3ZMt8GZF
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72Please respect copyright.ＰＥＮＡＮＡHPTdzoUKUu
https://gemini.google.com/
感谢互联网上无私奉献的参考资72Please respect copyright.ＰＥＮＡＮＡHgncKUWWjf
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests72Please respect copyright.ＰＥＮＡＮＡ29ucXErgr0
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range72Please respect copyright.ＰＥＮＡＮＡktaKRGtVV8
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag72Please respect copyright.ＰＥＮＡＮＡOcVmfEb6fn
https://www.rfc-editor.org/rfc/rfc723372Please respect copyright.ＰＥＮＡＮＡBpn3mpnced
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]72Please respect copyright.ＰＥＮＡＮＡrVRy7McuOU
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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