Show HN: Gemini web client in 100 lines of C

by ir3k • 3 years ago

91 45 3 years ago

Gemini protocol documentation claims that it is possible to write basic web client in 100 lines of code proving protocol simplicity. Easy in modern scripting language but can it be done in ANSI C? Let the source code decide.

Someone suggested to share this silly project of mine with HN community so here it is. Enjoy

Narishma3• 3 years ago

What do you mean by web client? From the github it looks more like a command line program.

ir3k11• 3 years ago

Yea, you are right. Poor choose of words on my side. One can argue the definition of "web" but it would be much more precise to write "CLI client".

ploum• 3 years ago

There’s nothing to argue. The "World Wide Web" or "Web" is defined as servers and clients communicating using the HTTP protocol. (Usually, it means that content is using HTML but this is not mandatory).

"Gemini" is defined as servers and client communicating using the "Gemini" protocol (and content is usually in the gemtext format).

Both are part of the Internet, which is defined as an INTERconnection of NETworks, thus a physical worldwide network of computers. Any computer with a public IP (Internet Address) is thus part of the Internet (in your house, is usually your box which is part of the Internet and acting as a gateway for your computer).

So : Internet is mostly an hardware network. You then join different part of the Internet by running adhoc software: Web, Gemini, Gopher, Mail, FTP, Usenet, etc…

Those have very clear and crisp definitions, there absolutely nothing to argue about. A "Web Gemini client" means a browser capable of accessing both Gemini and the Web, which is not the case here. It is a "Gemini browser".

sjmulder3• 3 years ago

Web refers to Gemini here, it being a command-line client for the Gemini web.

These line-based browsers used to be more common, there were a few ones for the www but also ‘ftp’ has such a mode. As does my little ‘nostt’ Teletext reader.

6581• 3 years ago

Web refers to Gopher here, it being a command-line client for the Gopher web.

Gemini, not Gopher.

hollerith• 3 years ago

I've only ever heard it called "Gemini", not "Gemini web".

"Gemini web" is a bad name because Gemini's not being part of the web was the main motive in Gemini's creation.

"Geminisphere" or "Geminiverse" would be fine with me as a name for the totality of Gemini servers considered collectively.

lolinder3• 3 years ago

If you're going to nitpick you could nitpick on the use of "web" (though, as others note, that's not wholly inaccurate), but there are plenty of command-line web clients. For example, wget and curl.

slondr• 3 years ago

Yes, wget and curl are web clients. This is not.

sjmulder3• 3 years ago

Impressive, the code is dense but not too hard to follow. And you even managed to cram in history!

snvzz2• 3 years ago

Nice, except the dependency on openssl.

It would have been a great opportunity to use BearSSL[0].

0. https://bearssl.org/

ir3k11• 3 years ago

I found out about BearSSL after I had first version of client working. So I kept OpenSSL. I will remember to try BearSSL in next project.

cryo• 3 years ago

Is BearSSL still actively maintained? My approach is abstracting SSL away so the API can be exchanged wit OpenSSL, MbetTLS, etc.

snvzz2• 3 years ago

It is a slow-moving project[0], but not dead.

0. https://bearssl.org/gitweb/?p=BearSSL;a=summary

jeremycw• 3 years ago

https://github.com/ir33k/gmi100/blob/master/gmi100.c#L27 definitely threw me for a loop until I realized it was a line saving trick. It would be more readable to save lines elsewhere by exploiting the comma operator instead of essentially cramming irrelevant statements into a conditional.

For example:

  addr.sin_family = AF_INET;
  addr.sin_port = htons(1965);

Could become:

  addr.sin_family = AF_INET, addr.sin_port = htons(1965);

ir3k11• 3 years ago

Ah yes, there are couple of line saving tricks like that. Mostly in for loops.

Thanks for suggestion. I will go through code again to see if I can save more space with normal code.

Actually that was my workflow. Each time I managed to write something in simpler way I reverted few tricks.

pobrn• 3 years ago

It's a nice project, but - I guess as is tradition with the majority of C projects - it has resource leaks and buffer overflows. There is at least one resource leak, namely, `sfd` is not closed when certain `WARN()` invocations jump back to the `start` label; for example, when `gethostbyname()` fails (i.e. try a non-existent domain and observe that the sockets remain open with `lsof`). (It also seems to be leaked in the happy path, so presumably `SSL_set_fd()` does not take ownership.) And if the user simply presses enter, i.e. the input is an empty line, there is a buffer underflow in line 55 as `j` will be -1 initially.

Also

    addr.sin_addr.s_addr = *((unsigned long*)he->h_addr_list[i]);

is a potential buffer overflow where `long` is 64 bits since only the first four bytes of `h_addr_list[i]` can be accessed, and also potentially misaligned for `unsigned long`; and it will also not work correctly on big endian platforms where `long` is 64 bits. Using `memcpy()` would have avoided all these problems. I am really confused as to how you arrived at the conclusion that "yes, this is the way to copy 4 bytes from A to B".

This sounds rude, I know, and I apologize; I don't want to single you/this project out personally. I am just frustrated that even today there are people who work on C/C++ projects seemingly without having made GCC's -fanalyzer/asan/ubsan/valgrind/etc. an important part of their development workflow.

ir3k11• 3 years ago

This is such a great feedback. Thank you a lot. OFC I will try to correct my mistakes.

Not to make any excuses, just for the context. I'm a beginner in C programming and I wrote only few small programs. My tooling is basically non existent. I will try to improve tho.

anthk3• 3 years ago

Try "valgrind" to check potential memory leaks and also compile your C projects with -Wall and -Wextra, -pedantic it's fine too.

lelanthran• 3 years ago

There's a page on the net somewhere, which was on HN front-page a while ago too, that listed all the useful warning flags for gcc, with an explanation.

I'd look for it, as a C programmer. On phone now, so sadly don't have it handy.

ir3k11• 3 years ago

I looked into those points and:

1. `sfd` is now closed. This cost me an extra line of code tho so I had to improvise.

2. `j` was not initially -1 as even empty input contains one "\n" character. But it could go pass value of 0 underflowing the buffer when buffer contained only white space characters (like in case of single "\n"). This was corrected.

3. `h_addr_list[i]` about this one. I'm quite sure I took it from example in one of man pages. Anyway, I replaced it with `memcpy`.

I also tried to use valgrind. This is OFC my first step but I will continue my studies on detecting memory leaks in future projects.

marginalia_nu• 3 years ago

Arthur Whitney, is that you? [1]

[1] https://code.jsoftware.com/wiki/Essays/Incunabulum

dvorak_typist• 3 years ago

The way he seems to have written 'C' is to use the C preprocessor and Macros to create a domain specific language and then write the program in that! Mind Blown!

sbjs• 3 years ago

This is the most unreadable code I may have ever seen. Congrats on successfully writing it and getting it to work.

ir3k11• 3 years ago

Yea it's very hard to read. I'm surprised that people are actually can follow the flow and can understand dirty tricks.

The most unreadable code would be the code that was intentionally obfuscated. Here I at least try to pretend that it is readable in some parts.

Thanks for kind words.

eesmith• 3 years ago

Try the J source code, like https://github.com/jsoftware/jsource/blob/master/jsrc/v.c

  F1(jttable){A z,zz;I r,wr;
   F1PREFIP;ARGCHK1(w);
   // We accept the pristine calculations from ravel
   wr=AR(w); r=(RANKT)jt->ranks; r=wr<r?wr:r;  // r=rank to use
   //  obsolete  RZ(IRSIP1(w,0L,r-1<0?0:r-1,jtravel,z));  // perform ravel on items
   RZ(IRSIP1(w,0L,r-((UI)r>0),jtravel,z));  // perform ravel on items
   R r?z:IRSIP1(z,0L,0L,jtravel,zz);  // If we are raveling atoms, do it one more time on atoms
  } // ,."r y

Quoting https://corecursive.com/065-competitive-coding-with-conor-ho... :

Yeah. I started a project a couple months ago. I’m slowly porting that code base, the J source code to C++ 20. And yeah, I think there should be a name for these types of code bases, because it’s not C, it’s like a macro variant of C, where it’s a CDSL, where 80% of your “library” is macros. I did a search, there’s 10,000 macros in the source code, and those macros are used like functions.

__s• 3 years ago

Nice to see someone else fall upon what I call funnel-based control flow, as opposed to stack-based control flow of most programs using functions. I used the same pattern in nobox: https://github.com/serprex/nobox/blob/master/nobox.c

Idea is that using goto your program can be mostly a loop where you make forward jumps to anywhere in the code & everything converges or jumps back to the start. It ends up making for very terse code since you're no longer passing values back & forth as much through structs/params/returns. You align variables & jump to code. Each label has a kind of ABI where the variables are the registers

anthk3• 3 years ago

I remember gplaces, but it doesn't compile under OpenBSD. It needs lots of fixes to be built on systems beside GNU/Linux.

On golang, Bombadillo today supports "images" with Unicode-art and searching inside the "pages" a la ctrl-f in browsers, so it's better than Amfora and one of the best TUI clients ever. Also, it does gopher and https thru external tools like lynx.

ir3k11• 3 years ago

Besides GNU/Linux I compiled on MacOS. I made no effort to test on BSD systems or Windows. I included only few std libs so I wonder - what exactly makes it not compatible with OpenBSD?

anthk3• 3 years ago

No, gplaces didn't work. Yours' work fine with a just an extra #include for inet.

kazinator• 3 years ago

That program has made me realize there exists a single stylistic justification for 8 space tabs: you can fit up to six character goto labels into the indentation margins; you can put a label on any statement without adding a line.

ir3k11• 3 years ago

Yes, that was my reasoning. Also there is non-written rule in this project that forbids me from going wider than 80 columns. With that I initially had indentation set to 2 spaces but later I realized that code is indented at most 2 times and it's better to make space for goto labels on the left.

keepamovin• 3 years ago

This is cool. I asked ChatGPT4 to explain to me what it does, and it gives a useful overview:

This is a minimal Gemini protocol client written in C. The Gemini protocol is an application-level internet protocol for serving hypertext documents over secure connections.

The code includes various standard C libraries and the OpenSSL library for secure connections. The `WARN` macro definition is used to print warning messages to `stderr` and jump to the `start:` label.

In the `main` function, it defines variables and initializes certain structs, like `struct sockaddr_in addr` that is used for internet socket addresses. It then opens a connection with the Gemini server on port 1965 (the standard port for the Gemini protocol). The connection is encrypted using the OpenSSL library functions. It also opens two file streams, one for history (`.gmi100`) and another for a temporary file. The file names and locations are generated by the `tmpnam()` function.

The main loop reads commands from `stdin`. Depending on the command, it can execute system commands (when the input starts with '!'), navigate through the history (when input starts with a digit), process a variety of single-letter commands (like 'q' for quit, 'r' for refresh, 'c' to print current URI, 'u' for up a level, 'b' for back in history), or interpret the input as a URL or a part of a URL to be navigated to.

For URL navigation, it creates a TCP socket connection with the Gemini server, sets up an SSL connection over that socket, and sends a request to the server for the resource identified by the URL. The client handles the server response depending on the status code. For example, if the status code is '1' it's a request for input and if it's '3' it's a redirect. It prints the response to `stdout` and also stores it in the temporary file.

For `text/*` type responses, it additionally parses any lines that start with '=>' as links and assigns them a number, storing this in the temporary file for later navigation. If the line starts with '=>' but doesn't have a description, it goes back to reading the rest of the description.

After processing the server response, it uses the `less` command to display the content of the temporary file, allowing the user to scroll through it if it's longer than the screen. Before each new iteration of the main loop, it flushes the temporary file and goes back to the `start:` label. If the user quits, the function returns 0 and the program terminates.

Overall, this program is a command-line based Gemini client that allows a user to navigate the geminispace, issue queries, and view text documents served over the Gemini protocol. It also keeps a history of visited URIs in the `.gmi100` file.

source: https://sharegpt.com/c/xscNWg6

ir3k11• 3 years ago

Wow, this is actually very good description of program flow. There are only few small mistakes. For example:

navigate through the history (when input starts with a digit)

Should be more like: navigate to link from currently displayed page (when input starts with a digit).

It prints the response to `stdout` and also stores it in the temporary file.

Response is not printed to `stdout` at all. It is only stored in temporary file.

Also the possibility to use first program argument to overwrite the `less` command was ignored.

But overall very good job.

Show HN: Gemini web client in 100 lines of C

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