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/?
197Please respect copyright.ＰＥＮＡＮＡSbLrRpJoWq
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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