memmove/cpy

When this line is executed I get an error that the memory is not accessable;

memmove(Text.string+ index + size,Text.string+index,Text.len - index);

Text.string is a pointer to a string. I want to make room to insert text into the string. Any one know why I get memory error messages?

Thanx in advance?

Pointer to what sort of string

char *bad = "this cannot be modified";

char good[100] = "this can be modified";

char *string;
string = (char *) calloc(length, 1);

I want to modify "string".