动态更改C中的结构数组

Question

I've been struggling to make a dynamically changing array of structures in C. My array size should change depending on how many times user decides to write data (city1 city2 and distance). I'm talking about addEdge function, which is supposed to make a bigger array every time user inputs data; it's also supposed to store structures in this array. I've used realloc function but it doesn't seem to work. In my main function I ask user to write his data twice and start my function after every input. It, for some reason, works after first function initialization, but it crashes after the second time. My intention is that it worked in a loop (user adds as much data as he wants). Here's the code:

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

typedef struct edge{
   char city1[30];
   char city2[30];
   int distance;
}edge;
void addEdge(edge **tab, char* city1, char* city2, int distance, int* n);
int main()
{
    edge *tab;
    tab=(edge*)malloc(0);
    char city1[30], city2[30];
    int distance, n=1;
        printf("\nType cities and distance in form: 'city1 city2 distance'\n");
    scanf("%s %s %d", city1, city2, &distance);
    addEdge(&tab, city1, city2, distance, &n);


        printf("\nType cities and distance in form: 'city1 city2 distance'\n");
    scanf("%s %s %d", city1, city2, &distance);
    addEdge(&tab, city1, city2, distance, &n);


    system("pause");
    return 0;
}


void addEdge(edge **tab, char* city1, char* city2, int distance, int* n)
{

    edge edgeN;
    strcpy(edgeN.city1, city1);
    strcpy(edgeN.city2, city2);
    edgeN.distance=distance;
    *tab=(edge*)realloc(*tab, *n * sizeof(edge));
    *tab[*n-1]=edgeN;
    *n=*n+1;

}

Answer 1

One of your mistake is that you never check any possible failure of your function call. scanf() return the number of field valid or an error, malloc() or realloc() can return an error.

edge *tab = NULL;
size_t n = 1; // should be size_t and not int
// ...
if (scanf("%29s %29s %d", city1, city2, &distance) != 3) {
    fprintf(stderr, "wrong input\n");
    exit(EXIT_FAILURE);
}
// ...
edge *tmp = realloc(*tab, *n * sizeof *edge);
if (!tmp) {
  exit(EXIT_FAILURE);
}
*tab = tmp;

Answer 2

You have fallen in an operator precedence trap. *tab[*n-1]=edgeN; is compiled as *(tab[*n - 1]) , when what you want is (*tab)[*n - 1]; .

When in doubt, use parens, they may be useless, but it would be harmless.

This is the real cause for your problems, but you should also follow Stargateur's advice and test the return values of all input and allocation functions.