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:177Please respect copyright.ＰＥＮＡＮＡwXcPmJwOl5
----------http range request-------177Please respect copyright.ＰＥＮＡＮＡ3UPsj6YKdz
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡTwb4eGceJL
Range: bytes=100-1023177Please respect copyright.ＰＥＮＡＮＡ99ayM8ySwD
If-Match: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡzsikg7Zlkx
......177Please respect copyright.ＰＥＮＡＮＡZffphyRBl7
-----------------------------------177Please respect copyright.ＰＥＮＡＮＡjB2Z6eyphk
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:177Please respect copyright.ＰＥＮＡＮＡ4gWGMLFCdy
----------------html-----------------177Please respect copyright.ＰＥＮＡＮＡvHUuSkqna0
<a href="/zen.iso">zen.iso</a>177Please respect copyright.ＰＥＮＡＮＡPTFkkAW82m
-------------------------------------177Please respect copyright.ＰＥＮＡＮＡ3PuNObyObo
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:177Please respect copyright.ＰＥＮＡＮＡnNbhJxIwkv
----------get request------------177Please respect copyright.ＰＥＮＡＮＡOKUyYmemOS
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡIM5sGfiDs5
......177Please respect copyright.ＰＥＮＡＮＡ8xpCccDMs6
---------------------------------
177Please respect copyright.ＰＥＮＡＮＡTCo303xISC
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:177Please respect copyright.ＰＥＮＡＮＡaqk1jhCE1B
----------------server response--------------177Please respect copyright.ＰＥＮＡＮＡHEqYVZo3RR
HTTP/1.1 200 OK177Please respect copyright.ＰＥＮＡＮＡ84j6ibZkvw
Date: Sat, 23 Nov 2024 06:29:19 GMT177Please respect copyright.ＰＥＮＡＮＡQskNOUbTyX
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡbuzKbpt6uY
content-range: bytes 0-534231199/534231200177Please respect copyright.ＰＥＮＡＮＡ3VozBPVmo5
ETag: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡ9GWLhly8NR
Content-Disposition: attachment; filename="zen.iso"177Please respect copyright.ＰＥＮＡＮＡopqJ5xkisu
Content-Length: 534231200177Please respect copyright.ＰＥＮＡＮＡG4TEyDUXlw
Content-Type: application/octet-stream
(http response body is the whole file content)177Please respect copyright.ＰＥＮＡＮＡPWo1VUCmJz
-------------------------------------------
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:177Please respect copyright.ＰＥＮＡＮＡR45P8EfUk7
---------------http server send big file content by multiple times--------------177Please respect copyright.ＰＥＮＡＮＡ56FPmMviNR
const int buf_size=20000000; // buf size is about 20M177Please respect copyright.ＰＥＮＡＮＡ3cUA2IBS10
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers177Please respect copyright.ＰＥＮＡＮＡUJD7kzkMiB
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 big177Please respect copyright.ＰＥＮＡＮＡUjrcKYEscQ
while(there_is_remain_file_content_to_send)177Please respect copyright.ＰＥＮＡＮＡa4a9aDAiE1
{ 177Please respect copyright.ＰＥＮＡＮＡGsjrhlFCwe
read_remain_file_content_to_send_buf(file, send_buf);177Please respect copyright.ＰＥＮＡＮＡFVgWVVdvm0
send_result = send(client_socket, send_buf);177Please respect copyright.ＰＥＮＡＮＡ9CBCEs5cgc
if(send_result == fail) return; else continue;177Please respect copyright.ＰＥＮＡＮＡzD4Cr6wLp4
}
// a way to make etag for a file using its file size and last modifed time177Please respect copyright.ＰＥＮＡＮＡByxrHm5aaL
const char *file_etag(const char *path) 177Please respect copyright.ＰＥＮＡＮＡnoAwW70ehz
{177Please respect copyright.ＰＥＮＡＮＡtplzXv3rV3
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;177Please respect copyright.ＰＥＮＡＮＡjlnoCMs4fR
}177Please respect copyright.ＰＥＮＡＮＡfF7ZB8poXl
-------------------------------------------------177Please respect copyright.ＰＥＮＡＮＡVdKuO2FQfm
"accept-ranges: bytes" tells client web browser that the http server support range.177Please respect copyright.ＰＥＮＡＮＡha59k3RMEz
"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).177Please respect copyright.ＰＥＮＡＮＡwLvbikmYwf
'ETag: "534231200_1732277116"' tells client web browser the file version.177Please respect copyright.ＰＥＮＡＮＡNFiLfMOkzq
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.177Please respect copyright.ＰＥＮＡＮＡOAP3RrR6d8
"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.177Please respect copyright.ＰＥＮＡＮＡqZubhbnUUz
"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.177Please respect copyright.ＰＥＮＡＮＡMaQxid1vq8
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.)177Please respect copyright.ＰＥＮＡＮＡwuMfhwRzWl
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:177Please respect copyright.ＰＥＮＡＮＡ7rg16XfDZM
---------resume range request---------177Please respect copyright.ＰＥＮＡＮＡbzgn20pPth
GET /zen.iso HTTP/1.1177Please respect copyright.ＰＥＮＡＮＡgRwS5a1fZ7
Range: bytes=243220480-177Please respect copyright.ＰＥＮＡＮＡ9gHgSTYxQK
If-Match: "534231200_1732277116"177Please respect copyright.ＰＥＮＡＮＡWSuzXHO253
......177Please respect copyright.ＰＥＮＡＮＡ54h2bBWuiJ
-------------------------------------177Please respect copyright.ＰＥＮＡＮＡ9qDrBO0N9q
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.177Please respect copyright.ＰＥＮＡＮＡYb8fAHQBF1
'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:177Please respect copyright.ＰＥＮＡＮＡexYECN1rcx
-------response content from resume point-------177Please respect copyright.ＰＥＮＡＮＡ7uIO5KAsvI
HTTP/1.1 206 Partial Content177Please respect copyright.ＰＥＮＡＮＡNIIR7ItgAK
Date: Sat, 23 Nov 2024 06:56:17 GMT177Please respect copyright.ＰＥＮＡＮＡMRddjDu8Tb
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡzkRucpTfGS
content-range: bytes 243220480-534231199/534231200177Please respect copyright.ＰＥＮＡＮＡTrMyejjTmU
Content-Length: 291010720177Please respect copyright.ＰＥＮＡＮＡE0EUyLu411
Content-Type: application/octet-stream177Please respect copyright.ＰＥＮＡＮＡJuZTbFAIbY
Content-Disposition: attachment; filename="zen.iso"177Please respect copyright.ＰＥＮＡＮＡhxbQ1wHG7y
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)177Please respect copyright.ＰＥＮＡＮＡkIXfEKHz74
------------------------------------------------177Please respect copyright.ＰＥＮＡＮＡNFXoPglRdB
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:177Please respect copyright.ＰＥＮＡＮＡQFVb73qVOK
-------response content from resume point-------177Please respect copyright.ＰＥＮＡＮＡ81PTB989vD
HTTP/1.1 200 OK177Please respect copyright.ＰＥＮＡＮＡX5fs2KXQpc
Date: Sat, 23 Nov 2024 06:29:19 GMT177Please respect copyright.ＰＥＮＡＮＡ7W4QDDu1bl
accept-ranges: bytes177Please respect copyright.ＰＥＮＡＮＡifegV40Yx3
content-range: bytes 0-634231201/634231202177Please respect copyright.ＰＥＮＡＮＡ4pxCl5CRM4
ETag: "634231202_2732277118"177Please respect copyright.ＰＥＮＡＮＡ3bi9q10PVn
Content-Disposition: attachment; filename="zen.iso"177Please respect copyright.ＰＥＮＡＮＡ6FyA7ZuzR5
Content-Length: 634231202177Please respect copyright.ＰＥＮＡＮＡoNfF0Av6xi
Content-Type: application/octet-stream
(http response body is the whole file content)177Please respect copyright.ＰＥＮＡＮＡy0Kf4v3LgR
------------------------------------------------177Please respect copyright.ＰＥＮＡＮＡ3wL5yVDkoS
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:177Please respect copyright.ＰＥＮＡＮＡUsvSBZxYS6
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
177Please respect copyright.ＰＥＮＡＮＡIW99eTYw24
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177Please respect copyright.ＰＥＮＡＮＡbGqqVeGBb5
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177Please respect copyright.ＰＥＮＡＮＡsLd2B5YnoP
https://gemini.google.com/
感谢互联网上无私奉献的参考资177Please respect copyright.ＰＥＮＡＮＡPHRZto0Yam
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests177Please respect copyright.ＰＥＮＡＮＡsNLVM2Hpdc
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range177Please respect copyright.ＰＥＮＡＮＡFnOWpFLlS1
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag177Please respect copyright.ＰＥＮＡＮＡjU0NcbapgC
https://www.rfc-editor.org/rfc/rfc7233177Please respect copyright.ＰＥＮＡＮＡL4XXYtO234
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]177Please respect copyright.ＰＥＮＡＮＡuAYDvGI6cR
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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