C Turing Machine infinite loop - c

Question

I'm trying to code a Turing machine in C.
But my program doesn't work, it gets stuck in an endless loop.
Here's my code with some explanations:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 3 //number of different states for the cells
#define K 20 //length of the tape
typedef struct
{
int state;
int head;
char tape[];
}mt; //machine
void init_mt(mt* machine, char val[], int n)
{
machine->state=1; //edited mistake
machine->head=0; // edited mistake
int i;
for(i=0;i<n;i++)
{
machine->tape[i]=val[i];
}
}; //initialization of a machine
typedef struct
{
char write;
char direction;
int state;
}actions; //actions composed of three instructions
typedef struct
{
actions exec01;
actions exec02;
actions exec11;
actions exec12;
}program; //program composed of four actions
void execute(actions exec, mt mach)
{
mach.tape[mach.head] = exec.write;
mach.state = exec.state;
if(exec.direction == 'R')
{
mach.head++;
}
else
{
mach.head--;
}
} //class that follows the instructions from the actions
void execute2(mt mach, program p)
{ do{
printf("%c %d %d \n", mach.tape[mach.head], mach.head, mach.state );
if(mach.tape[mach.head] == 0)
{
if(mach.state == 1)
{
execute(p.exec01, mach);
}
else if(mach.state == 2)
{
execute(p.exec02,mach);
}
}
else if(mach.tape[mach.head] == 1)
{
if(mach.state == 1)
{
execute(p.exec11,mach);
}
else if(mach.state == 2)
{
execute(p.exec12,mach);
}
}
}while( (mach.head<K) && (mach.state != 3));
} // class that read the program and act according to the states of the cells,
//keeps going until the machine is at the third state or if it reaches the end of the tape
int main(){
mt machine;
char t[10]={'1','1','1','0','0','1','0','1','0','1'};
init_mt(&machine, t, 10);
program p ={ {'0','R',1}, {'0','R',1}, {'1','R',2}, {'0','L',3} };
execute2(machine, p);
return 0;
} //main with a tape composed of 10 cells and a program composed of four actions
This program keeps displaying "0,0,1" indefinitely and I can't find the error.
Thanks for the help and sorry if this unclear.

Answer 1

There are a few problems here:
In some instances you're passing your structures as arguments, rather than pointers to them. This creates local copies of the entire structures in the called function. Any changes made to those copies will be lost when the functions return. It is also inefficient. Just pass structure pointers.
You aren't declaring any space for tape in your structures, so it's basically a zero-length array. Any kind of access whatsoever will corrupt memory and result in undefined behavior. You have a couple choices here. You can either choose some fixed size for it, and use that for the array size, or you can change it to a pointer and dynamically allocate storage for it. One way or another, you have to allocate the storage.
In execute2, it's comparing mach.tape[mach.head] to the integers 0 and 1. But the tape doesn't contain those values. It contains the characters '0' and '1'. So surround those constants with single quotes. It would also be a good idea to print an error if an unexpected value is encountered. That would have caught this problem instantly.

Answer 2

In function execute, you pass the structure mach by value. In that function you also perform
mach.head++
This, value presumably should be given back to the function execute2. So you will have to pass the structure mach by reference to this function.