Created crt (markdown)

explain/crt.md

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+## The C runtime
+
+Even though C is a relatively low-level language, some assembly code is needed
+to get the process in an acceptable state to the language standard.
+
+First of all, `main` isn't the standard entry point, `_start` is. This function
+is provided by your compiler, typically from a file called `crt*.o`. This
+function:
+
+1. aligns the stack
+2. reads `argc`, `argv`, `environ` and [the auxiliary vector
+   ](https://www.youtube.com/watch?v=j72GExsDnDU&t=1015) from the stack
+3. sets up TLS, locales, and other crap
+4. calls `__libc_start_user`, which calls `main`, and `exit` automatically as
+   well.
+
+Of course, it's huge, but we can provide our own: step 3 can be skipped, 1 and 2
+are really small to implement in assembly, and 4 is replaced by calling `main`.
+Of course, the second step can be skipped as well when you don't need
+args/environ.
+
+However, when external libs need to read from the environment (eg. X11 needs
+`$DISPLAY`), you still have to call `__libc_start_user`; it's a portable
+interface that asks for `argc`, `argv`, `envp` and `main`. With smol, this
+isn't exactly a hassle anymore.
+
+For exiting the program when not using `__libc_start_main`, you can just use a
+bare syscall, or `int3`.