Question

If I have a struct:

struct Rec
{
    uint16_t vals[500];
};


Rec * prec = malloc(sizeof(Rec));
//Rec * prec = (Rec *) malloc(sizeof(Rec)); This code was my original and is incorrect.
//                                          See Below for details.

// initialize everything in vals

Will this code suffice to free all memory used?

free(prec);

Or do I have to free the array separately?

Answer 1

This will suffice.
You did not allocate the array separately, so just freeing the allocated pointer shall suffice.

Follow the Rule:
You should only call free on address returned to you by malloc, anything else will result in Undefined Behavior.

References:
c99 Standard: 7.20.3.2 The free function

Synopsis
#include
void free(void *ptr);

Description:
The free function causes the space pointed to by ptr to be deallocated, that is, made
available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if
the argument does not match a pointer earlier returned by the calloc, malloc,or
realloc function, or if the space has been deallocated by a call to free or realloc,
the behavior is undefined.

Returns
The free function returns no value.