None of them would work. You need to allocate memory for s
. Something like:
void str_cpy(char *str_in, char *str_out);
int main(int argc, char *argv[])
{
char *s = calloc(sizeof(char), 100); //for 100 chars string
//OR char s[100] = { 0 };
//------v pass s not &s
str_cpy(s, "Hola Hola");
puts(s); //similary use s not &s
system("PAUSE");
return 0;
}
void str_cpy(char *str_in, char *str_out) {
strcat(str_in, str_out);
}