Question

I have a struct called State:

typedef struct State{
    char alphabets[2][6]; 
    struct State *PREV; /*this points to the previous state it came from*/
    struct State *NEXT; /*this points to the next state in the linked list*/
    int cost; /*Number of moves done to get to this position*/
    int zero_index;/*this holds the index to the empty postion*/
} State;

Here’s my memAllocator() method:

memAllocator(){
struct State *p = (State*) malloc(sizeof(State));
if (p==NULL){
        printf("Malloc for a new position failed");
        exit(1);
}
return p;

}
Here’s my main method.

main(){
State *start_state_pointer=memAllocator();
State start_state;
start_state.zero_index=15;
start_state.PREV = NULL;
start_state.alphabets[0][0]='C';
start_state.alphabets[0][1]='A';
start_state.alphabets[0][2]='N';
start_state.alphabets[0][3]='A';
start_state.alphabets[0][4]='M';
start_state.alphabets[0][5]='A';
start_state.alphabets[1][0]='P';
start_state.alphabets[1][1]='A';
start_state.alphabets[1][2]='N';
start_state.alphabets[1][3]='A';
start_state.alphabets[1][4]='L';
start_state.alphabets[1][5]='_';
start_state_pointer=&(start_state);
/*start_state=*start_state_pointer;*/

}

I think the statement start_state_pointer=&(start_state); is just assigning the pointer start_state_pointer to to the small amount of temporary space created during State start_state, rather than to the space I allocated.
But when I try the commented out statement start_state=*start_state_pointer to deference the pointer and allocate the space to start state. It gives me a segmentation fault.

I am just starting out in C. Can some one help me with this?

Answer 1

Your memAllocator and main functions don’t have explicit return types. This style of code has been deprecated for over 10 years. Functions in C should always have a return type. For main, the return type should be int, and for your memAllocator function, it should be State *.

The second issue is that you allocate space for a State struct, but then fill a different State struct and overwrite the pointer to the previously allocated State struct using start_state_pointer = &(start_state);.

To use the memory that you just allocated, you want to use something like this:

State *start_state = memAllocator();
start_state->zero_index = 15;
start_state->PREV = NULL;
start_state->alphabets[0][0] = 'C';
// etc.

There is no need to create two State structs. When you use State start_start; in your original code, you are creating a struct with something called automatic storage. This means the space for this struct is allocated automatically and is deallocated automatically for you at the end of the scope it is declared in. If you take the address of this struct and pass it around other parts of your program, then you will be passing around a pointer to a deallocated struct, and this could be why your program is crashing.