Warning: X may be used uninitialized in this function

Question

I am writing a custom \"vector\" struct. I do not understand why I\'m getting a Warning: \"one\" may be used uninitialized here.

This is my vector.h fil

Answer 1

When you use Vector *one you are merely creating a pointer to the structure but there is no memory allocated to it.

Simply use one = (Vector *)malloc(sizeof(Vector)); to declare memory and instantiate it.

Answer 2

You get the warning because you did not assign a value to one, which is a pointer. This is undefined behavior.

You should declare it like this:

Vector* one = malloc(sizeof(Vector));

or like this:

Vector one;

in which case you need to replace -> operator with . like this:

one.a = 12;
one.b = 13;
one.c = -11;

Finally, in C99 and later you can use designated initializers:

Vector one = {
   .a = 12
,  .b = 13
,  .c = -11
};

Answer 3

one has not been assigned so points to an unpredictable location. You should either place it on the stack:

Vector one;
one.a = 12;
one.b = 13;
one.c = -11

or dynamically allocate memory for it:

Vector* one = malloc(sizeof(*one))
one->a = 12;
one->b = 13;
one->c = -11
free(one);

Note the use of free in this case. In general, you'll need exactly one call to free for each call made to malloc.