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:67Please respect copyright.ＰＥＮＡＮＡJfLLkW3XoM
----------http range request-------67Please respect copyright.ＰＥＮＡＮＡWGC6cNLoP9
GET /zen.iso HTTP/1.167Please respect copyright.ＰＥＮＡＮＡa5nWEMkXJO
Range: bytes=100-102367Please respect copyright.ＰＥＮＡＮＡoXObRabZs4
If-Match: "534231200_1732277116"67Please respect copyright.ＰＥＮＡＮＡ7oaPmQTyYB
......67Please respect copyright.ＰＥＮＡＮＡxSUF5pzJtQ
-----------------------------------67Please respect copyright.ＰＥＮＡＮＡTJzOPGnDwd
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:67Please respect copyright.ＰＥＮＡＮＡO5L39f6bmK
----------------html-----------------67Please respect copyright.ＰＥＮＡＮＡVDiBZCfqBy
<a href="/zen.iso">zen.iso</a>67Please respect copyright.ＰＥＮＡＮＡwuVdFBlzMe
-------------------------------------67Please respect copyright.ＰＥＮＡＮＡmZgYibV0tW
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:67Please respect copyright.ＰＥＮＡＮＡk16WY2am7A
----------get request------------67Please respect copyright.ＰＥＮＡＮＡyJQYXLoPzQ
GET /zen.iso HTTP/1.167Please respect copyright.ＰＥＮＡＮＡ8bwQyZozxX
......67Please respect copyright.ＰＥＮＡＮＡUbVBOxJRlA
---------------------------------
67Please respect copyright.ＰＥＮＡＮＡcUhQ9LSnHT
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:67Please respect copyright.ＰＥＮＡＮＡOfQVEkQORg
----------------server response--------------67Please respect copyright.ＰＥＮＡＮＡFHPZkMkKuR
HTTP/1.1 200 OK67Please respect copyright.ＰＥＮＡＮＡBMxKDRuUDm
Date: Sat, 23 Nov 2024 06:29:19 GMT67Please respect copyright.ＰＥＮＡＮＡR5MF5JBcNe
accept-ranges: bytes67Please respect copyright.ＰＥＮＡＮＡnQ12SnxxZZ
content-range: bytes 0-534231199/53423120067Please respect copyright.ＰＥＮＡＮＡ8YMLsLoZ2V
ETag: "534231200_1732277116"67Please respect copyright.ＰＥＮＡＮＡfyQ7rAoQ36
Content-Disposition: attachment; filename="zen.iso"67Please respect copyright.ＰＥＮＡＮＡ7bAnUU4Z6p
Content-Length: 53423120067Please respect copyright.ＰＥＮＡＮＡlkrEqqsHmB
Content-Type: application/octet-stream
(http response body is the whole file content)67Please respect copyright.ＰＥＮＡＮＡa5FCXtHHrS
-------------------------------------------
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:67Please respect copyright.ＰＥＮＡＮＡgkldHHDOn4
---------------http server send big file content by multiple times--------------67Please respect copyright.ＰＥＮＡＮＡ4rp2ClG2mg
const int buf_size=20000000; // buf size is about 20M67Please respect copyright.ＰＥＮＡＮＡOrJcMMRn7I
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers67Please respect copyright.ＰＥＮＡＮＡdGceHy8rFD
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 big67Please respect copyright.ＰＥＮＡＮＡZZAVkySKnU
while(there_is_remain_file_content_to_send)67Please respect copyright.ＰＥＮＡＮＡm0ZUqQpMIs
{ 67Please respect copyright.ＰＥＮＡＮＡvFgHFW0uA1
read_remain_file_content_to_send_buf(file, send_buf);67Please respect copyright.ＰＥＮＡＮＡoiio7QCcAh
send_result = send(client_socket, send_buf);67Please respect copyright.ＰＥＮＡＮＡcXUB7izmNk
if(send_result == fail) return; else continue;67Please respect copyright.ＰＥＮＡＮＡwPrCMAGoIp
}
// a way to make etag for a file using its file size and last modifed time67Please respect copyright.ＰＥＮＡＮＡYTN7VJClYa
const char *file_etag(const char *path) 67Please respect copyright.ＰＥＮＡＮＡAIVLSGGGil
{67Please respect copyright.ＰＥＮＡＮＡsRIAzd5mH0
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;67Please respect copyright.ＰＥＮＡＮＡ6PDWtU9emO
}67Please respect copyright.ＰＥＮＡＮＡDRIBF2xk6Z
-------------------------------------------------67Please respect copyright.ＰＥＮＡＮＡVUdFsGVVkE
"accept-ranges: bytes" tells client web browser that the http server support range.67Please respect copyright.ＰＥＮＡＮＡNknlGx4zS8
"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).67Please respect copyright.ＰＥＮＡＮＡdvp1ukmNVU
'ETag: "534231200_1732277116"' tells client web browser the file version.67Please respect copyright.ＰＥＮＡＮＡ3eNKT2Rhjp
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.67Please respect copyright.ＰＥＮＡＮＡF3gsOVOPRt
"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.67Please respect copyright.ＰＥＮＡＮＡG9Wc2YFCwB
"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.67Please respect copyright.ＰＥＮＡＮＡsidkl1xEUb
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.)67Please respect copyright.ＰＥＮＡＮＡElfGms6W1M
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:67Please respect copyright.ＰＥＮＡＮＡ6dJvflZqWc
---------resume range request---------67Please respect copyright.ＰＥＮＡＮＡxpURoZCyLo
GET /zen.iso HTTP/1.167Please respect copyright.ＰＥＮＡＮＡV5PdwgeKzk
Range: bytes=243220480-67Please respect copyright.ＰＥＮＡＮＡUSSrGfRvKh
If-Match: "534231200_1732277116"67Please respect copyright.ＰＥＮＡＮＡP8dAfCBUbZ
......67Please respect copyright.ＰＥＮＡＮＡig6GecYNd0
-------------------------------------67Please respect copyright.ＰＥＮＡＮＡu7vuLGv71c
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.67Please respect copyright.ＰＥＮＡＮＡqf5p798cAD
'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:67Please respect copyright.ＰＥＮＡＮＡxVYR278xno
-------response content from resume point-------67Please respect copyright.ＰＥＮＡＮＡH2p9vae3Ij
HTTP/1.1 206 Partial Content67Please respect copyright.ＰＥＮＡＮＡMzjox8VdMH
Date: Sat, 23 Nov 2024 06:56:17 GMT67Please respect copyright.ＰＥＮＡＮＡchbZqtEjnO
accept-ranges: bytes67Please respect copyright.ＰＥＮＡＮＡoYIpa89Uyb
content-range: bytes 243220480-534231199/53423120067Please respect copyright.ＰＥＮＡＮＡdZrH9lOP0s
Content-Length: 29101072067Please respect copyright.ＰＥＮＡＮＡT8azDYiIOn
Content-Type: application/octet-stream67Please respect copyright.ＰＥＮＡＮＡymbN9rB8YT
Content-Disposition: attachment; filename="zen.iso"67Please respect copyright.ＰＥＮＡＮＡu4QtXnV9No
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)67Please respect copyright.ＰＥＮＡＮＡsg0NLK40xk
------------------------------------------------67Please respect copyright.ＰＥＮＡＮＡaXDWF59II4
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:67Please respect copyright.ＰＥＮＡＮＡtsp10PdtxQ
-------response content from resume point-------67Please respect copyright.ＰＥＮＡＮＡikdT2N5EIz
HTTP/1.1 200 OK67Please respect copyright.ＰＥＮＡＮＡARKhey4XLG
Date: Sat, 23 Nov 2024 06:29:19 GMT67Please respect copyright.ＰＥＮＡＮＡw0sUyZgD44
accept-ranges: bytes67Please respect copyright.ＰＥＮＡＮＡtPsJKHSdQC
content-range: bytes 0-634231201/63423120267Please respect copyright.ＰＥＮＡＮＡXvPWgsSm4Q
ETag: "634231202_2732277118"67Please respect copyright.ＰＥＮＡＮＡwLXkui6Y68
Content-Disposition: attachment; filename="zen.iso"67Please respect copyright.ＰＥＮＡＮＡoFbItcEWHt
Content-Length: 63423120267Please respect copyright.ＰＥＮＡＮＡuhLVH4SSEN
Content-Type: application/octet-stream
(http response body is the whole file content)67Please respect copyright.ＰＥＮＡＮＡZFPmQlFHSQ
------------------------------------------------67Please respect copyright.ＰＥＮＡＮＡig7kK0AdBE
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
67Please respect copyright.ＰＥＮＡＮＡJvLZtMCEnT
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:67Please respect copyright.ＰＥＮＡＮＡY5905h3z6k
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67Please respect copyright.ＰＥＮＡＮＡLY1SieuXxj
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67Please respect copyright.ＰＥＮＡＮＡlrD7C4IRwF
https://gemini.google.com/
感谢互联网上无私奉献的参考资67Please respect copyright.ＰＥＮＡＮＡskOBvHdj3J
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests67Please respect copyright.ＰＥＮＡＮＡz7VOmyYp7x
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range67Please respect copyright.ＰＥＮＡＮＡ0RXYTtm4XJ
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag67Please respect copyright.ＰＥＮＡＮＡz7Evu9COaJ
https://www.rfc-editor.org/rfc/rfc723367Please respect copyright.ＰＥＮＡＮＡnELnSxpcvs
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.com67Please respect copyright.ＰＥＮＡＮＡLtpAcUyvR2
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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