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:168Please respect copyright.ＰＥＮＡＮＡfQlHbwfzhP
----------http range request-------168Please respect copyright.ＰＥＮＡＮＡuQdjoLaNXw
GET /zen.iso HTTP/1.1168Please respect copyright.ＰＥＮＡＮＡKBGK3Cmdbj
Range: bytes=100-1023168Please respect copyright.ＰＥＮＡＮＡUnay6lHau8
If-Match: "534231200_1732277116"168Please respect copyright.ＰＥＮＡＮＡ0KDxuChAW1
......168Please respect copyright.ＰＥＮＡＮＡGyJa4vJC2M
-----------------------------------168Please respect copyright.ＰＥＮＡＮＡgSMOE4tdE9
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:168Please respect copyright.ＰＥＮＡＮＡErjwvdW4S8
----------------html-----------------168Please respect copyright.ＰＥＮＡＮＡslkpgTFAc3
<a href="/zen.iso">zen.iso</a>168Please respect copyright.ＰＥＮＡＮＡCJYn50LK0z
-------------------------------------168Please respect copyright.ＰＥＮＡＮＡHdrWECnuPT
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:168Please respect copyright.ＰＥＮＡＮＡXCKcDFhecw
----------get request------------168Please respect copyright.ＰＥＮＡＮＡyKfvGtFqZR
GET /zen.iso HTTP/1.1168Please respect copyright.ＰＥＮＡＮＡIeBvOi82JA
......168Please respect copyright.ＰＥＮＡＮＡjbNBtVj80j
---------------------------------
168Please respect copyright.ＰＥＮＡＮＡN28rTipRPa
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:168Please respect copyright.ＰＥＮＡＮＡZeXJlKtS2f
----------------server response--------------168Please respect copyright.ＰＥＮＡＮＡFQjVBfklke
HTTP/1.1 200 OK168Please respect copyright.ＰＥＮＡＮＡnKx2zhjEOl
Date: Sat, 23 Nov 2024 06:29:19 GMT168Please respect copyright.ＰＥＮＡＮＡOKE0e7qTPb
accept-ranges: bytes168Please respect copyright.ＰＥＮＡＮＡixw5umcOym
content-range: bytes 0-534231199/534231200168Please respect copyright.ＰＥＮＡＮＡiMI0Kgo9Gz
ETag: "534231200_1732277116"168Please respect copyright.ＰＥＮＡＮＡzv8j4YuI9J
Content-Disposition: attachment; filename="zen.iso"168Please respect copyright.ＰＥＮＡＮＡO23xRLHy3G
Content-Length: 534231200168Please respect copyright.ＰＥＮＡＮＡcRGPQ5pstO
Content-Type: application/octet-stream
(http response body is the whole file content)168Please respect copyright.ＰＥＮＡＮＡmL0bbDpFJH
-------------------------------------------
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:168Please respect copyright.ＰＥＮＡＮＡ2OkKZyvmUx
---------------http server send big file content by multiple times--------------168Please respect copyright.ＰＥＮＡＮＡIfXM3Y7kBT
const int buf_size=20000000; // buf size is about 20M168Please respect copyright.ＰＥＮＡＮＡP7HSzH3uYO
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers168Please respect copyright.ＰＥＮＡＮＡjqycMYaSwj
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 big168Please respect copyright.ＰＥＮＡＮＡCgdCK4KL0U
while(there_is_remain_file_content_to_send)168Please respect copyright.ＰＥＮＡＮＡgJDPBpQTCl
{ 168Please respect copyright.ＰＥＮＡＮＡVO0w7kwAIC
read_remain_file_content_to_send_buf(file, send_buf);168Please respect copyright.ＰＥＮＡＮＡKtqCGtrumH
send_result = send(client_socket, send_buf);168Please respect copyright.ＰＥＮＡＮＡrjWbFss59s
if(send_result == fail) return; else continue;168Please respect copyright.ＰＥＮＡＮＡzNE3NbsOzq
}
// a way to make etag for a file using its file size and last modifed time168Please respect copyright.ＰＥＮＡＮＡbdhdDwrhdk
const char *file_etag(const char *path) 168Please respect copyright.ＰＥＮＡＮＡO6K5eMyfWA
{168Please respect copyright.ＰＥＮＡＮＡuYcKlFepRC
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;168Please respect copyright.ＰＥＮＡＮＡnC6R4iu9mp
}168Please respect copyright.ＰＥＮＡＮＡcu1gaq2S7z
-------------------------------------------------168Please respect copyright.ＰＥＮＡＮＡmsr2W9Qbat
"accept-ranges: bytes" tells client web browser that the http server support range.168Please respect copyright.ＰＥＮＡＮＡKxyDYwixJz
"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).168Please respect copyright.ＰＥＮＡＮＡFgt5mnnYai
'ETag: "534231200_1732277116"' tells client web browser the file version.168Please respect copyright.ＰＥＮＡＮＡWqqiSK7vnt
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.168Please respect copyright.ＰＥＮＡＮＡGlGYjL3gwY
"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.168Please respect copyright.ＰＥＮＡＮＡIyQwPqjJhc
"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.168Please respect copyright.ＰＥＮＡＮＡubbrAe8Tad
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.)168Please respect copyright.ＰＥＮＡＮＡIXd8aJg0t3
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:168Please respect copyright.ＰＥＮＡＮＡtETDdvuAPW
---------resume range request---------168Please respect copyright.ＰＥＮＡＮＡWF5qQ1tMOz
GET /zen.iso HTTP/1.1168Please respect copyright.ＰＥＮＡＮＡAjkTd9jDN1
Range: bytes=243220480-168Please respect copyright.ＰＥＮＡＮＡcY6qCkZE4D
If-Match: "534231200_1732277116"168Please respect copyright.ＰＥＮＡＮＡrgdnyJwifY
......168Please respect copyright.ＰＥＮＡＮＡbXuEVxGNOv
-------------------------------------168Please respect copyright.ＰＥＮＡＮＡRsEpKliU8b
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.168Please respect copyright.ＰＥＮＡＮＡgoCD0u3F6T
'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:168Please respect copyright.ＰＥＮＡＮＡcArkBrPEvS
-------response content from resume point-------168Please respect copyright.ＰＥＮＡＮＡI9m0CFOiXr
HTTP/1.1 206 Partial Content168Please respect copyright.ＰＥＮＡＮＡg1vZsy436r
Date: Sat, 23 Nov 2024 06:56:17 GMT168Please respect copyright.ＰＥＮＡＮＡgLxbZbJKX2
accept-ranges: bytes168Please respect copyright.ＰＥＮＡＮＡNk9rK23WAU
content-range: bytes 243220480-534231199/534231200168Please respect copyright.ＰＥＮＡＮＡFrqeYzgq8G
Content-Length: 291010720168Please respect copyright.ＰＥＮＡＮＡ9jrPvxKBn0
Content-Type: application/octet-stream168Please respect copyright.ＰＥＮＡＮＡnCIQnFGnlr
Content-Disposition: attachment; filename="zen.iso"168Please respect copyright.ＰＥＮＡＮＡrMhZGwcvKd
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)168Please respect copyright.ＰＥＮＡＮＡZ4WST3IrPX
------------------------------------------------168Please respect copyright.ＰＥＮＡＮＡLiKwIKSlHB
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:168Please respect copyright.ＰＥＮＡＮＡpzGHV3aOuY
-------response content from resume point-------168Please respect copyright.ＰＥＮＡＮＡJNmSMCkTX3
HTTP/1.1 200 OK168Please respect copyright.ＰＥＮＡＮＡkD2KovYvCw
Date: Sat, 23 Nov 2024 06:29:19 GMT168Please respect copyright.ＰＥＮＡＮＡ5XyTkZU49i
accept-ranges: bytes168Please respect copyright.ＰＥＮＡＮＡNdcQsFIcdC
content-range: bytes 0-634231201/634231202168Please respect copyright.ＰＥＮＡＮＡAhKxEJMkDE
ETag: "634231202_2732277118"168Please respect copyright.ＰＥＮＡＮＡ3l4NOv7aQG
Content-Disposition: attachment; filename="zen.iso"168Please respect copyright.ＰＥＮＡＮＡhCAs2QrWHE
Content-Length: 634231202168Please respect copyright.ＰＥＮＡＮＡg1u4vPnu8U
Content-Type: application/octet-stream
(http response body is the whole file content)168Please respect copyright.ＰＥＮＡＮＡS01eMG7mc4
------------------------------------------------168Please respect copyright.ＰＥＮＡＮＡLw1mzAmzKI
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:168Please respect copyright.ＰＥＮＡＮＡ3ON7ZGvdp4
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168Please respect copyright.ＰＥＮＡＮＡdY8CZskqlQ
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168Please respect copyright.ＰＥＮＡＮＡoOPQiFiLum
https://gemini.google.com/
感谢互联网上无私奉献的参考资168Please respect copyright.ＰＥＮＡＮＡDQt9CesuN7
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests168Please respect copyright.ＰＥＮＡＮＡLjsz0nKs5Y
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range168Please respect copyright.ＰＥＮＡＮＡbyBu5s7iLR
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag168Please respect copyright.ＰＥＮＡＮＡA6BcNNWEnx
https://www.rfc-editor.org/rfc/rfc7233168Please respect copyright.ＰＥＮＡＮＡOCzyrmJpm0
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]168Please respect copyright.ＰＥＮＡＮＡttsGqlmdHO
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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