c - why memcmp returns -1 although equal -

i'm comparing using memcmp() 2 variables of same struct (the struct has union in it). variables in 2 arrays , i'm running loop each iteration memcmp(&arr1[i], &arr2[i], sizeof(arrtype)).

when debugging see memcmp returns -1, looking @ 2 variables , values, see variables has equal values. these arrays zeroed memset @ beginning.

1. so know why memcmp returns -1 , not 0?
2. is there better way need (compare 2 memory blocks)?

code:

typedef struct type1 {     int version;     union {             option1_t opt1;             option2_t opt2;     } union_t; } type1_t;  typedef struct type0 {     type1_t member1;     type2_t member2;     type3_t member3;     type4_t member4;     type5_t member; } type0_t;   type0_t arr1[size]; type0_t arr2[size];  memset(arr1, 0, size * sizeof(type0_t)); memset(arr2, 0, size * sizeof(type0_t));  /* doing irrelevant stuff... */  /* values arr1, arr2 ... */   /* comparing both arrays in loop*/ value = memcmp(&arr1[i], &arr2[i], sizeof(type0_t)); 

you reading indeterminate values (unitialized memory, or memory overwritten contain unspecified data).

e.g. accessing member of union wasn't member last written. if don't, last-written member might smaller total extents of union, leading 'indeterminate' data beyond size.

struct x {      union {          char field1;          long long field2[10];     }; };  struct x a,b; a.field1 = 'a'; b.field1 = 'a'; 

you can't expect a , b compare equal bitwise because never initialized bits in first place (field2 has many more bits in excess of field1)

---depending on value of uninitialized memory invokes undefined behaviour.--- not true c11