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:66Please respect copyright.ＰＥＮＡＮＡadNLVuoAuI
----------http range request-------66Please respect copyright.ＰＥＮＡＮＡL984iBaoVw
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡvf7SfbzfJl
Range: bytes=100-102366Please respect copyright.ＰＥＮＡＮＡNzkvkHsjUI
If-Match: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡkmCHmQZqKR
......66Please respect copyright.ＰＥＮＡＮＡquoPa6dYkQ
-----------------------------------66Please respect copyright.ＰＥＮＡＮＡxXR4YsSptF
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:66Please respect copyright.ＰＥＮＡＮＡwOCZq4G9wh
----------------html-----------------66Please respect copyright.ＰＥＮＡＮＡNGs4MPIoYx
<a href="/zen.iso">zen.iso</a>66Please respect copyright.ＰＥＮＡＮＡhzgvnXOjtL
-------------------------------------66Please respect copyright.ＰＥＮＡＮＡ4YNlEJGMFD
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:66Please respect copyright.ＰＥＮＡＮＡWemuyHmu7M
----------get request------------66Please respect copyright.ＰＥＮＡＮＡKiWqdnp0uk
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡJ7ujqLfLUU
......66Please respect copyright.ＰＥＮＡＮＡxRXNquxp9J
---------------------------------
66Please respect copyright.ＰＥＮＡＮＡGo65ACRe7d
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:66Please respect copyright.ＰＥＮＡＮＡvDm4TKhb75
----------------server response--------------66Please respect copyright.ＰＥＮＡＮＡvG2gxsWyIF
HTTP/1.1 200 OK66Please respect copyright.ＰＥＮＡＮＡ20Lu5rmdKI
Date: Sat, 23 Nov 2024 06:29:19 GMT66Please respect copyright.ＰＥＮＡＮＡSlPXIlRN4l
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡOnRCHrYjDb
content-range: bytes 0-534231199/53423120066Please respect copyright.ＰＥＮＡＮＡZl9lUyzkJM
ETag: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡLfwFEo8ebp
Content-Disposition: attachment; filename="zen.iso"66Please respect copyright.ＰＥＮＡＮＡP14g8L5KdY
Content-Length: 53423120066Please respect copyright.ＰＥＮＡＮＡhNwHBvpTqz
Content-Type: application/octet-stream
(http response body is the whole file content)66Please respect copyright.ＰＥＮＡＮＡLvOkawtrRg
-------------------------------------------
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:66Please respect copyright.ＰＥＮＡＮＡAHgjLbKrld
---------------http server send big file content by multiple times--------------66Please respect copyright.ＰＥＮＡＮＡhK0LzdfSoh
const int buf_size=20000000; // buf size is about 20M66Please respect copyright.ＰＥＮＡＮＡddHjtFTuz2
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers66Please respect copyright.ＰＥＮＡＮＡwxzIbPzyQX
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 big66Please respect copyright.ＰＥＮＡＮＡWmF8U0GIp1
while(there_is_remain_file_content_to_send)66Please respect copyright.ＰＥＮＡＮＡQgmXVMfZog
{ 66Please respect copyright.ＰＥＮＡＮＡW6gehooKXc
read_remain_file_content_to_send_buf(file, send_buf);66Please respect copyright.ＰＥＮＡＮＡANwAz5emix
send_result = send(client_socket, send_buf);66Please respect copyright.ＰＥＮＡＮＡPpyE7w5mzM
if(send_result == fail) return; else continue;66Please respect copyright.ＰＥＮＡＮＡ3sF9RIvC3M
}
// a way to make etag for a file using its file size and last modifed time66Please respect copyright.ＰＥＮＡＮＡ6FXNa3LgyW
const char *file_etag(const char *path) 66Please respect copyright.ＰＥＮＡＮＡI601oD4UCp
{66Please respect copyright.ＰＥＮＡＮＡKcbieueOWQ
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;66Please respect copyright.ＰＥＮＡＮＡfuk6bAy54s
}66Please respect copyright.ＰＥＮＡＮＡU5uotLUL2T
-------------------------------------------------66Please respect copyright.ＰＥＮＡＮＡMTgr4vBSdH
"accept-ranges: bytes" tells client web browser that the http server support range.66Please respect copyright.ＰＥＮＡＮＡj7JKTxOJ1y
"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).66Please respect copyright.ＰＥＮＡＮＡzao0qFoOD1
'ETag: "534231200_1732277116"' tells client web browser the file version.66Please respect copyright.ＰＥＮＡＮＡqxySzpQqHM
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.66Please respect copyright.ＰＥＮＡＮＡuutEqpyrjP
"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.66Please respect copyright.ＰＥＮＡＮＡcjJDIe1koc
"Content-Type: application/octet-stream" tells client web browser to treat the response body as binary stream.
66Please respect copyright.ＰＥＮＡＮＡ7qx4sbuplx
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.66Please respect copyright.ＰＥＮＡＮＡsVNymJo4M5
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.)66Please respect copyright.ＰＥＮＡＮＡkyXjo12ZXD
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.
66Please respect copyright.ＰＥＮＡＮＡnOsSeoTdMA
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:66Please respect copyright.ＰＥＮＡＮＡ4mK5rVLMB2
---------resume range request---------66Please respect copyright.ＰＥＮＡＮＡgS9ylcMIPy
GET /zen.iso HTTP/1.166Please respect copyright.ＰＥＮＡＮＡmu936M1uML
Range: bytes=243220480-66Please respect copyright.ＰＥＮＡＮＡxC9alW7Hgj
If-Match: "534231200_1732277116"66Please respect copyright.ＰＥＮＡＮＡ5en0goWrr6
......66Please respect copyright.ＰＥＮＡＮＡUvfBZke1a6
-------------------------------------66Please respect copyright.ＰＥＮＡＮＡ4txOCtgkso
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.66Please respect copyright.ＰＥＮＡＮＡFdSzygo0n4
'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:66Please respect copyright.ＰＥＮＡＮＡFJ5oaSdwWi
-------response content from resume point-------66Please respect copyright.ＰＥＮＡＮＡDPg5v5rAFS
HTTP/1.1 206 Partial Content66Please respect copyright.ＰＥＮＡＮＡSecPLg1smO
Date: Sat, 23 Nov 2024 06:56:17 GMT66Please respect copyright.ＰＥＮＡＮＡgqjeaiATIx
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡ0SExxHJz6j
content-range: bytes 243220480-534231199/53423120066Please respect copyright.ＰＥＮＡＮＡ0VzKc6D2XR
Content-Length: 29101072066Please respect copyright.ＰＥＮＡＮＡjp2Sb8wC0b
Content-Type: application/octet-stream66Please respect copyright.ＰＥＮＡＮＡ1iHhVoq4Ro
Content-Disposition: attachment; filename="zen.iso"66Please respect copyright.ＰＥＮＡＮＡbZqP0Q5Nfr
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)66Please respect copyright.ＰＥＮＡＮＡxrmaVpjLyI
------------------------------------------------66Please respect copyright.ＰＥＮＡＮＡYs2N7wDxNA
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:66Please respect copyright.ＰＥＮＡＮＡu8D0ZGBI6T
-------response content from resume point-------66Please respect copyright.ＰＥＮＡＮＡVwZzXmsOOe
HTTP/1.1 200 OK66Please respect copyright.ＰＥＮＡＮＡPOS8ICV7qO
Date: Sat, 23 Nov 2024 06:29:19 GMT66Please respect copyright.ＰＥＮＡＮＡjPvMucZvU8
accept-ranges: bytes66Please respect copyright.ＰＥＮＡＮＡpZdPtAqmXX
content-range: bytes 0-634231201/63423120266Please respect copyright.ＰＥＮＡＮＡhxAKucpP9B
ETag: "634231202_2732277118"66Please respect copyright.ＰＥＮＡＮＡpeOtyIG9B3
Content-Disposition: attachment; filename="zen.iso"66Please respect copyright.ＰＥＮＡＮＡBugexuqdJf
Content-Length: 63423120266Please respect copyright.ＰＥＮＡＮＡWL1udOwQI4
Content-Type: application/octet-stream
(http response body is the whole file content)66Please respect copyright.ＰＥＮＡＮＡbWMYItUHmk
------------------------------------------------66Please respect copyright.ＰＥＮＡＮＡnaxnElGNvM
When client download manager receives this response, it will clear the already downloaded content and start download from beginning.
66Please respect copyright.ＰＥＮＡＮＡLkLPdydVCC
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:66Please respect copyright.ＰＥＮＡＮＡyeFovtdAtj
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
66Please respect copyright.ＰＥＮＡＮＡr1NoR0Bve6
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66Please respect copyright.ＰＥＮＡＮＡz28Oks3FFN
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66Please respect copyright.ＰＥＮＡＮＡwOnEqL0In3
https://gemini.google.com/
感谢互联网上无私奉献的参考资66Please respect copyright.ＰＥＮＡＮＡKJFCRCtwV6
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests66Please respect copyright.ＰＥＮＡＮＡYlNziFwyKH
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range66Please respect copyright.ＰＥＮＡＮＡFsIogJb7AV
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag66Please respect copyright.ＰＥＮＡＮＡRRFekRGzFA
https://www.rfc-editor.org/rfc/rfc723366Please respect copyright.ＰＥＮＡＮＡWlzY9YweoY
https://mirrors.tuna.tsinghua.edu.cn/ubuntu-releases/24.10/
66Please respect copyright.ＰＥＮＡＮＡsWw607buok
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]66Please respect copyright.ＰＥＮＡＮＡZtThB4BZFG
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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