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:197Please respect copyright.PENANABmObcuGsuN
----------http range request-------197Please respect copyright.PENANAxyzw2CLbkK
GET /zen.iso HTTP/1.1197Please respect copyright.PENANAAYg54JvfcY
Range: bytes=100-1023197Please respect copyright.PENANAVCoWTsYZlV
If-Match: "534231200_1732277116"197Please respect copyright.PENANAx9v06aj2Pm
......197Please respect copyright.PENANAWUKDzzPOIe
-----------------------------------197Please respect copyright.PENANABcsIUbLZjs
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:197Please respect copyright.PENANA7TxKrFyb48
----------------html-----------------197Please respect copyright.PENANArWSdYDRMjl
<a href="/zen.iso">zen.iso</a>197Please respect copyright.PENANAlJpDWgngyt
-------------------------------------197Please respect copyright.PENANAkPR5bKo3RD
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:197Please respect copyright.PENANAUy00Su01RM
----------get request------------197Please respect copyright.PENANARdicSEUami
GET /zen.iso HTTP/1.1197Please respect copyright.PENANAHusSS9WHqy
......197Please respect copyright.PENANA68wjeZL7jm
---------------------------------
197Please respect copyright.PENANAZi8O31K4YD
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:197Please respect copyright.PENANArpbejMCCCD
----------------server response--------------197Please respect copyright.PENANAhJyNDvh8eM
HTTP/1.1 200 OK197Please respect copyright.PENANAQXvzgiS2D8
Date: Sat, 23 Nov 2024 06:29:19 GMT197Please respect copyright.PENANAxXduK2WKgG
accept-ranges: bytes197Please respect copyright.PENANAO5Wg7epDin
content-range: bytes 0-534231199/534231200197Please respect copyright.PENANA4Y8rPAwLVh
ETag: "534231200_1732277116"197Please respect copyright.PENANAmiz0n01oIo
Content-Disposition: attachment; filename="zen.iso"197Please respect copyright.PENANAOJDSBgx3Qb
Content-Length: 534231200197Please respect copyright.PENANAUUWjqUTf5U
Content-Type: application/octet-stream
(http response body is the whole file content)197Please respect copyright.PENANAQyYaLnXQiw
-------------------------------------------
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:197Please respect copyright.PENANAh0RHSydMiQ
---------------http server send big file content by multiple times--------------197Please respect copyright.PENANARWy6A2kMyF
const int buf_size=20000000; // buf size is about 20M197Please respect copyright.PENANAOIKQF4VyDj
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers197Please respect copyright.PENANA8HeT7ZDEMw
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 big197Please respect copyright.PENANAr2gc7RruyW
while(there_is_remain_file_content_to_send)197Please respect copyright.PENANANjSWlt3Rbx
{ 197Please respect copyright.PENANAwzbfjswTml
read_remain_file_content_to_send_buf(file, send_buf);197Please respect copyright.PENANAKZFPbLkbUX
send_result = send(client_socket, send_buf);197Please respect copyright.PENANAJAQ5xqIgGq
if(send_result == fail) return; else continue;197Please respect copyright.PENANA0nlljIsurH
}
// a way to make etag for a file using its file size and last modifed time197Please respect copyright.PENANAeKPTrvP8HX
const char *file_etag(const char *path) 197Please respect copyright.PENANAp6ZCPqXT9B
{197Please respect copyright.PENANAdCogOzN66T
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;197Please respect copyright.PENANABw57JgHRwU
}197Please respect copyright.PENANAhYB7XLjc01
-------------------------------------------------197Please respect copyright.PENANADjQWN5BLt1
"accept-ranges: bytes" tells client web browser that the http server support range.197Please respect copyright.PENANAKL0cZ7YBWx
"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).197Please respect copyright.PENANABWxQKHDnDK
'ETag: "534231200_1732277116"' tells client web browser the file version.197Please respect copyright.PENANACjxCzEihk7
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.197Please respect copyright.PENANACyI0Hgea9d
"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.197Please respect copyright.PENANAnr16TWJFlY
"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.197Please respect copyright.PENANAdYUwEV848u
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.)197Please respect copyright.PENANAqk5Cln2GMz
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:197Please respect copyright.PENANA2PHKbh4EWE
---------resume range request---------197Please respect copyright.PENANA7ObehLNPds
GET /zen.iso HTTP/1.1197Please respect copyright.PENANA5KnTNax1p7
Range: bytes=243220480-197Please respect copyright.PENANAfQCDrGZK89
If-Match: "534231200_1732277116"197Please respect copyright.PENANA2OHsE5JJbF
......197Please respect copyright.PENANAffl0DERIQV
-------------------------------------197Please respect copyright.PENANA8BBklqyUvM
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.197Please respect copyright.PENANA6PxmENj9yA
'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:197Please respect copyright.PENANAbVr4J9cg9W
-------response content from resume point-------197Please respect copyright.PENANAng9KUD4DaA
HTTP/1.1 206 Partial Content197Please respect copyright.PENANAfq0JgL9afa
Date: Sat, 23 Nov 2024 06:56:17 GMT197Please respect copyright.PENANAJSWZHc4UuQ
accept-ranges: bytes197Please respect copyright.PENANAo4U6WRa1Xk
content-range: bytes 243220480-534231199/534231200197Please respect copyright.PENANAJnDWIItSBy
Content-Length: 291010720197Please respect copyright.PENANAtRiPIqCkCV
Content-Type: application/octet-stream197Please respect copyright.PENANAaolvAeVr8S
Content-Disposition: attachment; filename="zen.iso"197Please respect copyright.PENANAE0R20VZqJJ
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)197Please respect copyright.PENANAUYkv3OvOBH
------------------------------------------------197Please respect copyright.PENANAQ3Bjnr8ssu
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:197Please respect copyright.PENANAZRbg4tjAfk
-------response content from resume point-------197Please respect copyright.PENANA5jiAgXtLWR
HTTP/1.1 200 OK197Please respect copyright.PENANAv8iLuYVpKt
Date: Sat, 23 Nov 2024 06:29:19 GMT197Please respect copyright.PENANAayiwYoK8wI
accept-ranges: bytes197Please respect copyright.PENANAPlyBVXYjPt
content-range: bytes 0-634231201/634231202197Please respect copyright.PENANAaoqzp4nWFJ
ETag: "634231202_2732277118"197Please respect copyright.PENANAdxhLudIapr
Content-Disposition: attachment; filename="zen.iso"197Please respect copyright.PENANAkIOqfNc98j
Content-Length: 634231202197Please respect copyright.PENANACMCuQAOvrD
Content-Type: application/octet-stream
(http response body is the whole file content)197Please respect copyright.PENANA7xgxG7rNH8
------------------------------------------------197Please respect copyright.PENANAwcXsEbCdC3
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:197Please respect copyright.PENANAQJhSKxQQtb
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197Please respect copyright.PENANAGKayVzY14s
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197Please respect copyright.PENANAdSNxUCpuTD
https://gemini.google.com/
感谢互联网上无私奉献的参考资197Please respect copyright.PENANA3r1U3SWVRE
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests197Please respect copyright.PENANANXuK4gfFpY
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range197Please respect copyright.PENANAWBPVZWYry3
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag197Please respect copyright.PENANAGAj2qIlGKq
https://www.rfc-editor.org/rfc/rfc7233197Please respect copyright.PENANAU3zMWoHClY
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]197Please respect copyright.PENANA253S5cUhj2
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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