extern int asm_main();

int main() {
  return asm_main();
}

section .text

extern printf
extern scanf

section .data
  scanf_string_a db '%d', 0
  printf_string_a db 'You entered the number %d!', 10, 0
  printf_string_b db '%d * 2 = %d', 10, 0
  printf_string_c db 'Enter a number: ', 0
section .text

global asm_main
asm_main:
	push ebp
	mov ebp, esp
	sub esp, 4 ; make room for dword int local variable

	push printf_string_c
	call printf
	add esp, 4

	mov eax, ebp
	sub eax, 4
	push eax
	push scanf_string_a
	call scanf
	add esp, 8

	push dword [ebp-4]
	push printf_string_a
	call printf
	add esp, 8

	mov eax, [ebp-4]
	shl eax, 1
	push eax
	push dword [ebp-4]
	push printf_string_b
	call printf
	add esp, 12

  mov eax, 0
  add esp, 4
	pop ebp
ret

$ ./a.out
Enter a number: 42
You entered the number 42!
42 * 2 = 84

$ echo 'Univat zber sha jvgu NFZ.
> 
> V znqr n cebtenz gung nccyvrf EBG13 gb fgqva naq cevagf vg gb fgqbhg, hfvat whfg Yvahk flfgrz pnyyf naq ab P yvoenel shapgvbaf.' | ./rot13
Having more fun with ASM.

I made a program that applies ROT13 to stdin and prints it to stdout, using just Linux system calls and no C library functions.