C - 结构中的结构数组在 dfa 程序中返回意外结果

Question

I am trying to make a program that creates a dfa based on user input. Each node and transition must be a struct and the dfa is also a struct. The dfa struct contains an array of node structs and a linked list of transition structs. Here is what I have so far:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define MAX_STATES 25
#define TRUE 1
#define FALSE 0
#define MAXCHAR 256

/* struct for a state of the DFA */
typedef struct state {
    char *id;
    int isInitial;
    int isFinal;
} STATE_T;

/* struct for a transition of the DFA*/
typedef struct transition {
    STATE_T *startState;
    char condition;
    STATE_T *endState;
    struct transistion *next;
} TRANSITION_T;

/* struct for the DFA */
typedef struct automaton {
    STATE_T state[MAX_STATES];
    TRANSITION_T *transition;
} AUTOMATON_T;

/* main function reads initial user inputs to build dfa */
int main() {
    AUTOMATON_T dfa;
    STATE_T stateA;
    TRANSITION_T *trans;

The first line of the text file is comma sperated nodes/states, for example: "a,b,c". But when I get to assigning each node as a state id into the dfa struct, the final state is assigned to the id for every single state. The for statement in the following code prints 0c 1c 2c where I would expect 0a 1b 2c.

    FILE *fp = fopen(fileName, "r");

    while (fgets(useIn, MAXCHAR, fp)) {
        if (lineNum == 0) {
            i = 0;
            char *token = strtok(useIn, ",");
            while (token != NULL) {
                strcpy(stateId, token);
                stateA.id = stateId;
                dfa.state[i] = stateA;
                i++;
                token = strtok(NULL, ",");
            }
            stateAmt = i;
            for (i = 0; i < stateAmt; i++) {
                printf("%d", i);
                printf("%s", dfa.state[i].id);
            }
        }
    fclose(fp);
    return 0;
}

Why would this be? Am I handling the array of states incorrectly?

Answer 1

The issue is these two lines:

strcpy(stateId, token);
stateA.id = stateId;

In the state structure, the id member is a pointer to a mutable string. The memory for storing that string was only allocated once, and its address has been stored in each state struct.

In order for each state to have its own ID, you need to allocate a new ID buffer for each state. You could do this dynamically using malloc at run-time, or you could simply include the storage for this buffer as part of the state structure like this:

typedef struct state {
    char id[MAX_ID_LENGTH+1];
    int isInitial;
    int isFinal;
} STATE_T;