Why the Access violation here ?

[Dhafir]

Hi all,
why I am getting "access violation" in line 1 below in foo(...)?

void foo( char* str )
{
if( str != NULL && strlen(str) > 0 )
{
str[0] = '9'; // line 1
}
}


int main(int argc, char* argv[])
{

char* str2 = "123456789";

foo( str2);

return 0;
}