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:130Please respect copyright.ＰＥＮＡＮＡgesXL6Zg9D
----------http range request-------130Please respect copyright.ＰＥＮＡＮＡr0bR7aHfbo
GET /zen.iso HTTP/1.1130Please respect copyright.ＰＥＮＡＮＡjp8DBiSTm4
Range: bytes=100-1023130Please respect copyright.ＰＥＮＡＮＡypVkXfmtjE
If-Match: "534231200_1732277116"130Please respect copyright.ＰＥＮＡＮＡGAmYmzgJx9
......130Please respect copyright.ＰＥＮＡＮＡTMjODWOmrv
-----------------------------------130Please respect copyright.ＰＥＮＡＮＡnWi0jrXeLq
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:130Please respect copyright.ＰＥＮＡＮＡPOiOq4L387
----------------html-----------------130Please respect copyright.ＰＥＮＡＮＡuxsUc67eRO
<a href="/zen.iso">zen.iso</a>130Please respect copyright.ＰＥＮＡＮＡ4ZpCXCbscd
-------------------------------------130Please respect copyright.ＰＥＮＡＮＡ5ehqmTTJ5Q
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:130Please respect copyright.ＰＥＮＡＮＡjUZOLTkFgx
----------get request------------130Please respect copyright.ＰＥＮＡＮＡLIAJOYkNUt
GET /zen.iso HTTP/1.1130Please respect copyright.ＰＥＮＡＮＡnWQryOnQgA
......130Please respect copyright.ＰＥＮＡＮＡ4qA11jP6if
---------------------------------
130Please respect copyright.ＰＥＮＡＮＡXKDNOsPopZ
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:130Please respect copyright.ＰＥＮＡＮＡhE3QgSeZ2P
----------------server response--------------130Please respect copyright.ＰＥＮＡＮＡFL2DILBy1q
HTTP/1.1 200 OK130Please respect copyright.ＰＥＮＡＮＡVPmDRoUYPh
Date: Sat, 23 Nov 2024 06:29:19 GMT130Please respect copyright.ＰＥＮＡＮＡjaZVfUIcNX
accept-ranges: bytes130Please respect copyright.ＰＥＮＡＮＡqsC0Bpvo8j
content-range: bytes 0-534231199/534231200130Please respect copyright.ＰＥＮＡＮＡl8e77TLN8D
ETag: "534231200_1732277116"130Please respect copyright.ＰＥＮＡＮＡ758Nd06giJ
Content-Disposition: attachment; filename="zen.iso"130Please respect copyright.ＰＥＮＡＮＡiNm2BwoQuH
Content-Length: 534231200130Please respect copyright.ＰＥＮＡＮＡQ9RSfW0Zkl
Content-Type: application/octet-stream
(http response body is the whole file content)130Please respect copyright.ＰＥＮＡＮＡPvoX3Fpdho
-------------------------------------------
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:130Please respect copyright.ＰＥＮＡＮＡS90FTew9Ho
---------------http server send big file content by multiple times--------------130Please respect copyright.ＰＥＮＡＮＡolR8WV6Ud5
const int buf_size=20000000; // buf size is about 20M130Please respect copyright.ＰＥＮＡＮＡUWlsTkHHbt
int client_socket; char send_buf[buf_size]; FILE *file;
// first send http response headers130Please respect copyright.ＰＥＮＡＮＡ7FqkOq8Csc
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 big130Please respect copyright.ＰＥＮＡＮＡy7Hf8WuLkk
while(there_is_remain_file_content_to_send)130Please respect copyright.ＰＥＮＡＮＡAnXZ9MeOEo
{ 130Please respect copyright.ＰＥＮＡＮＡT3g81vShAk
read_remain_file_content_to_send_buf(file, send_buf);130Please respect copyright.ＰＥＮＡＮＡV9AbAjKmA7
send_result = send(client_socket, send_buf);130Please respect copyright.ＰＥＮＡＮＡjcIF0XzeJO
if(send_result == fail) return; else continue;130Please respect copyright.ＰＥＮＡＮＡ4xQdAxE5TM
}
// a way to make etag for a file using its file size and last modifed time130Please respect copyright.ＰＥＮＡＮＡNAm4h2CvsC
const char *file_etag(const char *path) 130Please respect copyright.ＰＥＮＡＮＡFlXF1lqrls
{130Please respect copyright.ＰＥＮＡＮＡ5GOi1AAjOa
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;130Please respect copyright.ＰＥＮＡＮＡHnXJXO4EUC
}130Please respect copyright.ＰＥＮＡＮＡG7RkuAzbJ3
-------------------------------------------------130Please respect copyright.ＰＥＮＡＮＡhHvIgAMSwV
"accept-ranges: bytes" tells client web browser that the http server support range.130Please respect copyright.ＰＥＮＡＮＡ4QwBwuMEkA
"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).130Please respect copyright.ＰＥＮＡＮＡ1hj1ZowkMg
'ETag: "534231200_1732277116"' tells client web browser the file version.130Please respect copyright.ＰＥＮＡＮＡYP3BxuKhJk
'Content-Disposition: attachment; filename="zen.iso"' tells client web browser to download the file as attachment, and its file name.130Please respect copyright.ＰＥＮＡＮＡBO2iukOaGp
"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.130Please respect copyright.ＰＥＮＡＮＡLTagqiJxLP
"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.130Please respect copyright.ＰＥＮＡＮＡEu4OLxNLfP
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.)130Please respect copyright.ＰＥＮＡＮＡeMunavtNmN
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:130Please respect copyright.ＰＥＮＡＮＡMqlYygmXl0
---------resume range request---------130Please respect copyright.ＰＥＮＡＮＡBt2XnotMat
GET /zen.iso HTTP/1.1130Please respect copyright.ＰＥＮＡＮＡoWpMNJQztg
Range: bytes=243220480-130Please respect copyright.ＰＥＮＡＮＡ4nqaj8jODD
If-Match: "534231200_1732277116"130Please respect copyright.ＰＥＮＡＮＡIPQUse9uHS
......130Please respect copyright.ＰＥＮＡＮＡmm9lQcnS13
-------------------------------------130Please respect copyright.ＰＥＮＡＮＡtIQ42BJ2Kd
"Range: bytes=243220480-" tells the http server to send content from byte 243220480 to end.130Please respect copyright.ＰＥＮＡＮＡZ9HIh11aF9
'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:130Please respect copyright.ＰＥＮＡＮＡNPidxvvAzC
-------response content from resume point-------130Please respect copyright.ＰＥＮＡＮＡchHI3VPInh
HTTP/1.1 206 Partial Content130Please respect copyright.ＰＥＮＡＮＡIApnPmxryu
Date: Sat, 23 Nov 2024 06:56:17 GMT130Please respect copyright.ＰＥＮＡＮＡUjR6HYNLXn
accept-ranges: bytes130Please respect copyright.ＰＥＮＡＮＡarcdwIgkJo
content-range: bytes 243220480-534231199/534231200130Please respect copyright.ＰＥＮＡＮＡAyDoNuIhgD
Content-Length: 291010720130Please respect copyright.ＰＥＮＡＮＡNe4yOakCym
Content-Type: application/octet-stream130Please respect copyright.ＰＥＮＡＮＡHQmgsin8FJ
Content-Disposition: attachment; filename="zen.iso"130Please respect copyright.ＰＥＮＡＮＡ1M4XnAPw01
ETag: "534231200_1732277116"
(http response body is the file content from resume point to end)130Please respect copyright.ＰＥＮＡＮＡhhidr8Y4rH
------------------------------------------------130Please respect copyright.ＰＥＮＡＮＡM3VKzYr3jF
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:130Please respect copyright.ＰＥＮＡＮＡvWDyFWIrGh
-------response content from resume point-------130Please respect copyright.ＰＥＮＡＮＡDwB4pKixsg
HTTP/1.1 200 OK130Please respect copyright.ＰＥＮＡＮＡ00nUkcQ7Th
Date: Sat, 23 Nov 2024 06:29:19 GMT130Please respect copyright.ＰＥＮＡＮＡrvgPDuBD3Z
accept-ranges: bytes130Please respect copyright.ＰＥＮＡＮＡSAeEMvWIAn
content-range: bytes 0-634231201/634231202130Please respect copyright.ＰＥＮＡＮＡeamRREJ3Ku
ETag: "634231202_2732277118"130Please respect copyright.ＰＥＮＡＮＡuz8b92IOv9
Content-Disposition: attachment; filename="zen.iso"130Please respect copyright.ＰＥＮＡＮＡWlFaBZz5HR
Content-Length: 634231202130Please respect copyright.ＰＥＮＡＮＡGEvHVIbmSY
Content-Type: application/octet-stream
(http response body is the whole file content)130Please respect copyright.ＰＥＮＡＮＡOrX3OSpVdP
------------------------------------------------130Please respect copyright.ＰＥＮＡＮＡ49qvf32Twv
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:130Please respect copyright.ＰＥＮＡＮＡkpwdlkGteO
https://www.zeng.dev/post/2023-http-range-and-play-mp4-in-browser/?
130Please respect copyright.ＰＥＮＡＮＡ8AcdF1RBTe
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130Please respect copyright.ＰＥＮＡＮＡP2gIDRAeWB
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130Please respect copyright.ＰＥＮＡＮＡgaU1T8sT4x
https://gemini.google.com/
感谢互联网上无私奉献的参考资130Please respect copyright.ＰＥＮＡＮＡYtnaKsO85T
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests130Please respect copyright.ＰＥＮＡＮＡwmWKfscgk1
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Range130Please respect copyright.ＰＥＮＡＮＡySWRi2oTvb
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag130Please respect copyright.ＰＥＮＡＮＡ27l5o2vl80
https://www.rfc-editor.org/rfc/rfc7233130Please respect copyright.ＰＥＮＡＮＡ38wG0zZx4M
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]130Please respect copyright.ＰＥＮＡＮＡ3OUaYllGgA
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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