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Please see the modified version of file tty.c below from MEATY SKELETON.
This question concerns 1) the statement at the beginning:
extern char getkb();
and 2) the third function from the bottom: void terminal_getkb(void).

Label getkb is the entry point to an assembly routine.
The function line: c = getkb()
gets the error: undefined reference to `getkb'
as though the statement: extern char getkb() has no effect.
It should not get that error even if the assembly routine is deleted.

If the compile or link lines in the makefile were wrong I would expect
a different message like: file foo.bar not found. I suspect I cannot
use the line: extern char getkb(void) in this code because it violates
something about these OS programming files.
MY QUESTION: why the error: undefined reference to getkb

TIA Bill S.

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <kernel/vga.h>

extern char getkb(void);
char c='A';

size_t terminal_row;
size_t terminal_column;
uint8_t terminal_color;
uint16_t* terminal_buffer;

void terminal_initialize(void)
{
terminal_row = 0;
terminal_column = 0;
terminal_color = make_color(COLOR_MAGENTA, COLOR_BLACK);
terminal_buffer = VGA_MEMORY;
for ( size_t y = 0; y < VGA_HEIGHT; y++ )
{
for ( size_t x = 0; x < VGA_WIDTH; x++ )
{
const size_t index = y * VGA_WIDTH + x;
terminal_buffer[index] = make_vgaentry(' ', terminal_color);
}
}
}

void scrolldn(){
for (size_t i = 0; i<VGA_WIDTH*VGA_HEIGHT; i++)
terminal_buffer = terminal_buffer[i+VGA_WIDTH];
}

void terminal_setcolor(uint8_t color)
{
terminal_color = color;
}

void terminal_putentryat(char c, uint8_t color, size_t x, size_t y)
{
const size_t index = y * VGA_WIDTH + x;
terminal_buffer[index] = make_vgaentry(c, color);
}

void terminal_putchar(char c) {
if (c != '\n'){
terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
if (++terminal_column == VGA_WIDTH) {
terminal_column = 0;
if (++terminal_row == VGA_HEIGHT)
scrolldn();
}
}
else
{
terminal_column = 0;
terminal_row++;
if (terminal_row == VGA_HEIGHT)
scrolldn();
}
}

void terminal_getkb(void)
{
while (1)
{ c = getkb();
terminal_putchar(c);
}
}

void terminal_write(const char* data, size_t size)
{
for ( size_t i = 0; i < size; i++ )
terminal_putchar(data);
}

//void terminal_writestring(const char* data)
//{
// terminal_write(data, strlen(data));
//}

"undefined reference" is a linker error, not a compiler error. It has nothing to do with your C code directly. It means that there is an unresolved symbol during linking.

The assembly file is as follows: And the pertinent lines from the makefile are:
The added line results in a newer tty.o than the version of tty.c therefore the newer tty.o does not get recompiled (thus not wiped out) by the next rule - as desired.

I do not see why the build gets the error: 'undefined reference to getkb'

What is your OBJ_LINK_LIST?

OBJ_LINK_LIST:=\

$(CRTI_OBJ) \
$(CRTBEGIN_OBJ) \
$(OBJS) \
$(CRTEND_OBJ) \
$(CRTN_OBJ) \

which is:

myos/kernel/arch/i386/crti.o \
i686-elf-gcc $(CFLAGS) $(LDFLAGS) -print-file-name=crtbegin.o) \
myos/kernel/arch/i386/boot.o \
myos/kernel/arch/i386/tty.o \
myos/kernel/kernel/kernel.o \
i686-elf-gcc $(CFLAGS) $(LDFLAGS) -print-file-name=crtend.o) \
myos/kernel/arch/i386/crtn.o

Obviously, you're missing kb1.o. By the way, your assembly code won't work, because BIOS interrupts are not available in 32-bit protected mode.

Ok. adding kb1.o works. Thanks. One more question: I'm not in protected mode yet so I expect this (still) real mode system to get my keyboard input and display it on the screen - just for the hell of it. But it doesn't do it.

If you are writing C code, you are in protected mode. If you aren't in protected mode, you can't use C.

- Monk

P.S. There are ways to write real mode C, all of which are out of date, broken, or just not recommended.

The "Meaty skeleton" tutorial uses GRUB, which enables protected mode before booting a multiboot-compliant kernel. Although, you still must setup your own GDT, because the GDT setup by GRUB is unreliable.