GDB Listing

without if:

   0xc0105f95 <+425>:	xchg   %bx,%bx
   0xc0105f98 <+428>:	mov    0xc010b018,%eax
   0xc0105f9d <+433>:	mov    %eax,-0xc(%ebp)
   0xc0105fa0 <+436>:	jmp    0xc0105fab <fork+447>
   0xc0105fa2 <+438>:	mov    -0xc(%ebp),%eax
   0xc0105fa5 <+441>:	mov    0x20(%eax),%eax
   0xc0105fa8 <+444>:	mov    %eax,-0xc(%ebp)
   0xc0105fab <+447>:	mov    -0xc(%ebp),%eax
   0xc0105fae <+450>:	mov    0x20(%eax),%eax
   0xc0105fb1 <+453>:	test   %eax,%eax
   0xc0105fb3 <+455>:	jne    0xc0105fa2 <fork+438>
   0xc0105fb5 <+457>:	mov    -0xc(%ebp),%eax
   0xc0105fb8 <+460>:	mov    -0x18(%ebp),%edx
   0xc0105fbb <+463>:	mov    %edx,0x20(%eax)
   0xc0105fbe <+466>:	call   0xc01061c0
   0xc0105fc3 <+471>:	mov    %eax,-0x1c(%ebp)
   0xc0105fc6 <+474>:	mov    0xc010b014,%eax
   0xc0105fcb <+479>:	cmp    -0x10(%ebp),%eax
   0xc0105fce <+482>:	jne    0xc0105ffc <fork+528>
   0xc0105fd0 <+484>:	mov    %esp,%ebx
   0xc0105fd2 <+486>:	mov    %ebx,-0x20(%ebp)
   0xc0105fd5 <+489>:	mov    %ebp,%ebx
   0xc0105fd7 <+491>:	mov    %ebx,-0x24(%ebp)
   0xc0105fda <+494>:	mov    -0x18(%ebp),%eax
   0xc0105fdd <+497>:	mov    -0x20(%ebp),%edx
   0xc0105fe0 <+500>:	mov    %edx,0x8(%eax)
   0xc0105fe3 <+503>:	mov    -0x18(%ebp),%eax
   0xc0105fe6 <+506>:	mov    -0x24(%ebp),%edx
   0xc0105fe9 <+509>:	mov    %edx,0xc(%eax)
   0xc0105fec <+512>:	mov    -0x18(%ebp),%eax
   0xc0105fef <+515>:	mov    -0x1c(%ebp),%edx
   0xc0105ff2 <+518>:	mov    %edx,0x10(%eax)
   0xc0105ff5 <+521>:	mov    -0x18(%ebp),%eax
   0xc0105ff8 <+524>:	mov    (%eax),%eax
   0xc0105ffa <+526>:	jmp    0xc0106001 <fork+533>
   0xc0105ffc <+528>:	mov    $0x0,%eax
   0xc0106001 <+533>:	add    $0x44,%esp
   0xc0106004 <+536>:	pop    %ebx
   0xc0106005 <+537>:	pop    %ebp
   0xc0106006 <+538>:	ret


with if:

 0xc0105f95 <+425>:	xchg   %bx,%bx				;BOCHS_BREAKPOINT
   0xc0105f98 <+428>:	mov    -0x14(%ebp),%eax
   0xc0105f9b <+431>:	mov    (%eax),%eax
   0xc0105f9d <+433>:	test   %eax,%eax
   0xc0105f9f <+435>:	je     0xc0105fc9 <fork+477>
   0xc0105fa1 <+437>:	mov    0xc010b018,%eax
   0xc0105fa6 <+442>:	mov    %eax,-0xc(%ebp)
   0xc0105fa9 <+445>:	jmp    0xc0105fb4 <fork+456>
   0xc0105fab <+447>:	mov    -0xc(%ebp),%eax
   0xc0105fae <+450>:	mov    0x20(%eax),%eax
   0xc0105fb1 <+453>:	mov    %eax,-0xc(%ebp)
   0xc0105fb4 <+456>:	mov    -0xc(%ebp),%eax
   0xc0105fb7 <+459>:	mov    0x20(%eax),%eax
   0xc0105fba <+462>:	test   %eax,%eax
   0xc0105fbc <+464>:	jne    0xc0105fab <fork+447>
   0xc0105fbe <+466>:	mov    -0xc(%ebp),%eax
   0xc0105fc1 <+469>:	mov    -0x1c(%ebp),%edx
   0xc0105fc4 <+472>:	mov    %edx,0x20(%eax)
   0xc0105fc7 <+475>:	jmp    0xc0105fef <fork+515>
   0xc0105fc9 <+477>:	mov    0xc010b018,%eax
   0xc0105fce <+482>:	mov    %eax,-0x10(%ebp)
   0xc0105fd1 <+485>:	jmp    0xc0105fdc <fork+496>
   0xc0105fd3 <+487>:	mov    -0x10(%ebp),%eax
   0xc0105fd6 <+490>:	mov    0x20(%eax),%eax
   0xc0105fd9 <+493>:	mov    %eax,-0x10(%ebp)
   0xc0105fdc <+496>:	mov    -0x10(%ebp),%eax
   0xc0105fdf <+499>:	mov    0x20(%eax),%eax
   0xc0105fe2 <+502>:	test   %eax,%eax
   0xc0105fe4 <+504>:	jne    0xc0105fd3 <fork+487>
   0xc0105fe6 <+506>:	mov    -0x10(%ebp),%eax
   0xc0105fe9 <+509>:	mov    -0x1c(%ebp),%edx
   0xc0105fec <+512>:	mov    %edx,0x20(%eax)
   0xc0105fef <+515>:	call   0xc01061f0
   0xc0105ff4 <+520>:	mov    %eax,-0x20(%ebp)
   0xc0105ff7 <+523>:	mov    0xc010b014,%eax
   0xc0105ffc <+528>:	cmp    -0x14(%ebp),%eax
   0xc0105fff <+531>:	jne    0xc010602d <fork+577>
   0xc0106001 <+533>:	mov    %esp,%ebx
   0xc0106003 <+535>:	mov    %ebx,-0x24(%ebp)
   0xc0106006 <+538>:	mov    %ebp,%ebx
   0xc0106008 <+540>:	mov    %ebx,-0x28(%ebp)
   0xc010600b <+543>:	mov    -0x1c(%ebp),%eax
   0xc010600e <+546>:	mov    -0x24(%ebp),%edx
   0xc0106011 <+549>:	mov    %edx,0x8(%eax)
   0xc0106014 <+552>:	mov    -0x1c(%ebp),%eax
   0xc0106017 <+555>:	mov    -0x28(%ebp),%edx
   0xc010601a <+558>:	mov    %edx,0xc(%eax)
   0xc010601d <+561>:	mov    -0x1c(%ebp),%eax
   0xc0106020 <+564>:	mov    -0x20(%ebp),%edx
   0xc0106023 <+567>:	mov    %edx,0x10(%eax)
   0xc0106026 <+570>:	mov    -0x1c(%ebp),%eax
   0xc0106029 <+573>:	mov    (%eax),%eax
   0xc010602b <+575>:	jmp    0xc0106032 <fork+582>
   0xc010602d <+577>:	mov    $0x0,%eax
   0xc0106032 <+582>:	add    $0x44,%esp
   0xc0106035 <+585>:	pop    %ebx
   0xc0106036 <+586>:	pop    %ebp
   0xc0106037 <+587>:	ret

test2:
#include "syscall.h"

int main() {

	int fildes = syscall_open("/dev/fb0", 16);

	if(fildes >= 0) {
		char buffer[20] = "yaaaayyy\n";
		syscall_write(fildes, buffer, 9);
	}

	int ret = syscall_fork();
	if(!ret) {
		char bufferca[20] = "CHILD_A\n";
		int fildesca = syscall_open("/dev/fb0", 16);
		if(fildesca >= 0) {
			syscall_write(fildesca, bufferca, 8);
		}
	} else {
		char buffercb[20] = "CHILD_B\n";
		int fildescb = syscall_open("/dev/fb0", 16);
		if(fildescb >= 0) {
			syscall_write(fildescb, buffercb, 8);
		}
	}

	syscall_exit();

	return 0;
}