MOP2 programming intro - add printing in C and error codes

content/blog/MOP2/intro.adoc

@@ -55,8 +55,6 @@ writing something to a pipe.
 
 STRING:
   .string "Hello world!!!"
-STRING_LEN:
-  .quad . - STRING
 
 .section .text
 
@@ -64,7 +62,6 @@ STRING_LEN:
 _start: 
   movq $1, %rax     // select debugprint()
   lea STRING(%rip),     %rdi    // load STRING
-  lea STRING_LEN(%rip), %rsi    // load STRING_LEN
   int $0x80
 
   // quit
@@ -187,3 +184,62 @@ limitation of this approach is that the caller must ensure that enough space wit
 problematic in the future.
 
 After we've exited from `main()`, we just gracefully exit the application.
+
+=== "Hello world" but from C this time
+
+Now we can program our applications the "normal"/"human" way. We've gone over printing in assembly using the
+`debugprint()` syscall, so let's now try to use it from C. We'll also try to do some more advanced printing
+with (spoiler) `uprintf()`.
+
+.Calling `debugprint()` from C
+[source,c]
+----
+// Import `ulib`
+#include <ulib.h>
+
+void main(void) {
+  debugprint("hello world");
+}
+----
+
+That's it! We've just printed "hello world" to the terminal! How awesome is that?
+
+.`uprintf()` and formatted printing
+[source,c]
+----
+#include <ulib.h>
+
+void main(void) {
+  uprintf("Hello world %d %s %02X\n", 123, "this is a string literal", 0xBE);
+}
+----
+
+`uprintf()` is provided by Eyal Rozenberg (eyalroz), which originates from Macro Paland's printf. This printf
+library is super easily portable and doesn't require much in terms of standard C functions and headers. My main
+nitpick and a dealbreaker with other libraries was that they advertise themsevles as "freestanding" or "made for
+embedded" or something along those lines, but in reality they need so much of the C standard library, that you
+migh as well link with musl or glibc and use printf from there. And generally speaking, this is an issue with
+quite a bit of "freestanding" libraries that you can find online ;(.
+
+Printf rant over...
+
+=== Error codes in MOP2
+
+You might've noticed is that `main()` looks a little different from standard C `main()`. There's
+no return/error code, because MOP2 simply does not implement such feature. This is because MOP2 doesn't follow the
+UNIX philosophy.
+
+The UNIX workflow consists of combining many small/tiny programs into a one big commandline, which transforms text
+into some more text. For eg.:
+
+.Example bash command (Linux) to get a name of /proc/meminfo field
+[source,shell]
+----
+cat /proc/meminfo | awk 'NR==20 {print $1}' | rev | cut -c 2- | rev
+----
+
+Personally, I dislike this type of workflow. I prefer to have a few programs that perform tasks groupped by topic,
+so for eg. in MOP2, we have `$fs` for working with the filesystem or `$pctl` for working with processes. When we
+approach things the MOP2 way, it turns out error codes are kind of useless (or at least they wouldn't get much
+use), since we don't need to connect many programs together to get something done.
+