Simple Chat on TCP socket with full-duplex / multiplex communication

things that I marked with (?) indicate statements that I still have doubts about

Credits

Big thanks to sir @ammarfaizi2 for any support and help 😁

and other members on GNU/Weeb!

A few note

• So far, this program not stable and won't work properly on windows platform, known issue are:
  • [server] WSAPoll not notify or dispatch event on client disconnect - fixed on this commit
  • [server] excees padding on the reply packet from server to client - fixed on this commit
  • [client] WaitForMultipleObjects never response to recv from file descriptor tcp socket
• Well known bug on linux platform:
  • [client] need to figure out how to copy buffer from stdin to stdout when '\r' printed
• I/O Multiplexing
• TIL windows cannot connect to 0.0.0.0 while linux can do it, on windows, address 0.0.0.0 only useful for listen on all interface
• good article about make and Makefile script
• use SERVER_ADDR=0.0.0.0 make to override default server address, also
• sock_stream sock_dgram sock_raw is a part of network socket types
• setiap protokol seperti (IP_ICMP) mempunyai struktur data, fungsionalitas dan usecase masing-masing
• man 2 untuk syscall, man 3 untuk library, lihat lengkapnya pada command man man
• endian = bytes order, tidak digunakan untuk single byte
• network always use big endian, see network byte ordering rfc 1700, and the note

Useful tools and technique

• raw tcp client: nc program
• strace to debug syscall
• netstat to see network connection statistic, e.g list used port
• manage process in a single terminal session:
  • monitoring process
    • use command jobs to list related information
  • maintain process
    • use command & = move process to background
    • use command fg = move process to foreground
    • use command bg = move process to background, re-run freezed program

^C interrupt, this signal can be adjusted according to the way program is programmed
^Z stop / freeze program
^D EOF

see man sigaction for more info

New update

• handle multiple client
• broadcast from one to other client

New note

• flexible array member or dynamic data on variable need to be stored as pointer and cannot stored in stack therefore use malloc
• selain untuk optimasi, alignment memory needed in order to fit some structure data, and can be used properly
• jika tidak di-align dengan benar dapat menyebabkan buffer overflow
• salah satu cara meng-align nya adalah dengan menggunakan __attribute__((__packed__))
• buffering itu buat menghemat resource dan system call (?)

auto flush occurs when: new line/lf, get input: fgets | getchar | etc..., and exit

here's some example:

printf("> "); // by default line-buffered, so you need to add \n to automatic trigger flush
fflush(stdout); // or force flush fix the issue if the auto flush not performed
while (1);

FD - file descriptor

file descriptor is unique identifier that hold information related to I/O operation, for example tcp socket and some widely-known standard I/O stream and FD: stdin, stdout, and stderr and the preprocessor symbols or predefined constant in macro: STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO respectively

Diagram that shows variant of I/O device:

image from: What does "address space" means when talking about IO devices?

also see the File descriptor from wikipedia and the Linux_System_Programming-man7 book page 3-4

A few terms to note:

• buffer: some data that was held somewhere in memory
• I/O stream¹: wide-terms used for represents an input source or an output destination (?)
• flush: clear buffer and stream it to the specified FD, e.g printf tidak akan ditampilkan jika belum di-flush ke stdout dan masih disimpan dalam bentuk buffer
• buffering mode on stream¹: there's three types of buffering mode, unbuffered, line-buffered, and fullbuffered, each of mode have predefined constant in macro: _IONBF, _IOLBF, and _IOFBF respectively, see man setvbuf for the details of the behaviour
• also struct FILE * is refer to stream and doesn't literally refer to the actual file
• There's high-level and low-level interface²:
  • the use of FD is often only used at low levels, for e.g poll syscall³
  • some function from standard libraries like fread, fwrite, and fprintf are higher level interface⁴

Example syscall for asynchronus I/O operation (?)

• select (old but implemented on all system)
• poll posix-only (included bsd)
  • some event on a file descriptor: documented on man 2 poll
• epoll linux | queu bsd | iocp windows
• io_uring linux

writeup: my internal dialogue with myself

note: I want someone who reads this to validate and verify my statement, so feel free to open a pull request if you discover any misconcept, misinterpretation, or misleading information

• usually in the network domain, we often use dynamic allocated variable to hold flexible array, cuz it will be very wasteful if the variable are fixed-size since it will force the size to be fixed even if the data is small, and fill the rest with null char as a padding (?)
• on the network, you are passing the actual value or data, not the reference of memory address XD
• why always passing the size as additional information in the member struct? I think it's like a trade-off.
  • maybe the analogy is more or less like, Why bother to execute a bunch of instructions instead of passing additional information that is not so large (does this mean for the sake of efficiency?)
• sizeof means the size of a defined data structure on a variable, not the size of the variable's value itself. Why? You might ask, That's how data structure and memory work (?)
  • and not to confuse with strlen, which only applied to strings in order to count the amount of char.

Part 2

• How does the program interpret the network data? especially in terms of memory. I curious how to debug this process with gdb but didn't have any clue to get started :(
  • this curiousity comes from the buffer overflow vuln on the server
  • I think the highlighted thing in this context would be:
    • the way processors read/fetch data, how operator and operand are used at the machine-code (assembly) level
    • how could packed and aligned memory affect the program behavior?