Question

typedef struct {

   struct {
      double i1, i2;
   } EXP;

   struct {
      double i1, i2;
   } SIN;

   struct {
      double i1, i2;
   } PULSE;

   struct {
      double *i1, *i2;
   } PWL;
} TRANS;

struct term {
   TRANS trans;
   struct term *nxt;
};

int main() {
   struct term *look;
}

I have the above structs and the pointer look to the struct term. Could someone tell me how to dereference pointer i1 inside struct PWL?

I’ve tried this:

*(look->trans.PWL.i1)

but it produces segmentation fault.

Thanks in advance!

Answer 1

The segmentation fault is because you allocated a pointer, but did not create memory for the pointer to point at. Once you do that, then *(look->trans.PWL.i1) is indeed how to access that field in the inner struct.

You need to allocate memory for the struct, and all references within.

struct term *look = malloc(sizeof(struct term));
look->trans.PWL.i1 = malloc(sizeof(double));
look->trans.PWL.i2 = malloc(sizeof(double));

And naturally you need to reverse the process with calls to free when you are done.

free(look->trans.PWL.i2);
free(look->trans.PWL.i1);
free(look);

Or, perhaps i1 and i2 are meant to point to values that are allocated elsewhere then it would look like this:

struct term *look = malloc(sizeof(struct term));
look->trans.PWL.i1 = &look->trans.EXP.i1;
look->trans.PWL.i2 = &look->trans.EXP.i2;

And to deallocate you just free look. Remember to pair each successful call to malloc with a call to free.