Problems using char strings

Hello,

I'm using Eclipse with cygwin.

I have an app with a Timer 0 FIQ and a UART0 IRQ. The timer 0 is used to
simulate a PWM. In the UART ISR I have implemented a echo. I also have a
print(char *str) function. The problem is that the only way which the
program runs is declaring a string like that:

char str[100];

str[0]='q';
str[0]='w';
str[0]='e';
str[0]='r';
...

Print(str);

If I do this, everything works fine. But if I try to initialize the
string like this:
str = "qwert"
all interrups don't work anymore.

I have tried declaring:

const char *str;
const char *str = "qwerty";
Print("qwerty");
const char *str = {'q','w','e','r','t','y'};
...

and the conclusion I have at this moment is that if I use in my code the
character "" to indicate a string, it doesn't work.

Do you have any suggestion?
Thatk you very much.