C: Reading property of struct pointer causes property to change? - c

Question

I have a program that stores two different structs (tri & quad) in an array of a special struct defined below:
struct inst_ptr
{
void* p;
unsigned type;
};
The structs tri & quad have a superbase property
The following code displays how the structs are defined:
struct inst
{
char name[50];
int id;
};
struct tri
{
struct inst superbase;
};
struct quad
{
struct inst superbase;
};
Function for creating new instances (all located outside of main):
struct inst_ptr insts[100]; //instance container
int cinsts=0; //instance count
int inst_new(int type)
{
switch(type)
{
case TYPE_TRI:
{
struct tri i;
insts[cinsts].p=&i;
}
case TYPE_QUAD:
{
struct quad i;
insts[cinsts].p=&i;
}
}
insts[cinsts].type=type;
cinsts++;
return cinsts-1; //return index of instance in array
}
In my main function I create a sample tri, writing and reading it's values:
struct tri* t1=(struct tri*)insts[inst_new(TYPE_TRI)].p;
strcpy(t1->superbase.name, "tri1");
t1->superbase.id=70;
printf("id: %d\n", t1->superbase.id);
printf("id: %d\n", t1->superbase.id); <- problem occurs here and onward
Printing the id the first time works, printing 70 in the output. However, the second time it prints a long random integer.
Output:
id: 69
id: 1973802146
id: 1973802146
As you shown above, after the second read it keeps printing 1973802146. I'm not really sure what I'm doing wrong but I have a feeling I'm missing an extremely basic concept relating to pointers and structs.

Answer 1

You are saving the address of local variables in the array. That's bad because those variables doesn't exist once the function returns. You should use dynamic allocation instead.
Besides that you switch need break statements. And perhaps also a default case.
Like:
int inst_new(int type)
{
switch(type)
{
case TYPE_TRI:
{
struct tri *p = malloc(sizeof *p);
if (p == NULL) exit(1);
insts[cinsts].p=p;
break;
}
case TYPE_QUAD:
{
struct quad *p = malloc(sizeof *p);
if (p == NULL) exit(1);
insts[cinsts].p=p;
break;
}
default:
{
// Unknown type.. add error handling here
exit(1);
}
}
insts[cinsts].type=type;
cinsts++;
return cinsts-1; //return index of instance in array
}
As an alternative you can do:
int inst_new(int type)
{
insts[cinsts].p = NULL
switch(type)
{
case TYPE_TRI:
{
insts[cinsts].p=malloc(sizeof(struct tri));
break;
}
case TYPE_QUAD:
{
insts[cinsts].p=malloc(sizeof(struct quad));
break;
}
}
if (insts[cinsts].p == NULL) exit(1); // malloc failed
insts[cinsts].type=type;
cinsts++;
return cinsts-1; //return index of instance in array
}