Can the alliance initialize in the statement?

typedef union {
unsigned long U32;
 float f;
}U_U32_F;

U_U32_F u = 0xffffffff; // Does not work...is there a correct syntax for this?

U_U32_F u = {0xffffffff };

U_U32_F u = {.f = 42.0 };

For example, suppose we have a union

typedef union {
unsigned long U32;
float f;
}U_U32_F; 

Is there a way to set the initial value when declaring this union type variable?

U_U32_F u = 0xffffffff; // Does not work...is there a correct syntax for this?

Use the initialization list:

U_U32_F u = {0xffffffff };

You can set other members except the first one

U_U32_F u = {.f = 42.0 };