C kernel- printing string not working - c

Question

I am making a C kernel from scratch, and I literally just copied this code off of a website because my code wasn't working so I was confused.
void kmain(void)
{
const char *str = "my first kernel";
char *vidptr = (char*)0xb8000; //video mem begins here.
unsigned int i = 0;
unsigned int j = 0;
/* this loops clears the screen
* there are 25 lines each of 80 columns; each element takes 2 bytes */
while(j < 80 * 25 * 2) {
/* blank character */
vidptr[j] = ' ';
/* attribute-byte - light grey on black screen */
vidptr[j+1] = 0x07;
j = j + 2;
}
j = 0;
/* this loop writes the string to video memory */
while(str[j] != '\0') {
/* the character's ascii */
vidptr[i] = str[j];
/* attribute-byte: give character black bg and light grey fg */
vidptr[i+1] = 0x07;
++j;
i = i + 2;
}
return;
}
When I run my kernel, it prints an S to the screen and nothing else. I know my kernel is booting, because if I do
vidptr[0] = 'h';
vidptr[2] = 'e';
vidptr[4] = 'l';
vidptr[6] = 'l';
vidptr[8] = 'o';
it works as expected. What is happening?
EDIT: It might be something with my code that loads the kernel (might not have set up some of the registers) so I will just look into grub and other things.

Answer 1

Try using the volatile keyword with the variable
Ref page: http://wiki.osdev.org/Printing_To_Screen
// note this example will always write to the top
// line of the screen
void write_string( int colour, const char *string )
{
volatile char *video = (volatile char*)0xB8000;
while( *string != 0 )
{
*video++ = *string++;
*video++ = colour;
}
}

Answer 2

For some reason, if I use char str[] = "blabla"; instead of char *str = "blabla"; it works.

Answer 3

First, have you set the SP before jumping Kernel or before running the function? If you have not, please set them appropriately. In your starting function you should set the stack pointer DS, SS; you can use inline assembly at the beginning of the program. Also instead char * p = "Something" you should use char var [] = "Something", or
char var[10];
var[1] = 'S';
var[2] = 'o';
or
char var[] = { 'S', 'o', 'm', 'e', ..., '\0' } ;
My bootloader establishes the GDT and makes the switcthing to 32 bit mode and jumps (jmp 0x8:0x8000) Here 0x8 is my Code segment address in GDT, 0x10 for the DS, SS; so it jumps to kernel. Before I hadn't set the SS, SP, DS and I could not get it printed so I have written this small code:
[bits 32]
MOV AX, 0x10 ; 0x10 points at the new data selector
MOV DS, eax
MOV ES, AX
MOV FS, AX
MOV GS, AX
MOV SS, AX
MOV eax, 0x200000
MOV esp, eax
and compiled it with nasm and added to beginning of the kernel program manually by using a HEX editor program.