I am having following union

union data { uint64_t val; struct{ .... }
};

and I have a function

func(union data mydata[])
{ printf("%llu",(uint64_t)mydata[0]); // Here is the error
}

When i compile this code it is giving following error

error: aggregate value used where an integer was expected

2 Answers

You are failing to access a field of the indexed union array: mydata[0] is a value of type union data, and can't be cast to uint64_t.

You need to access the proper union member:

printf("%" PRIu64, mydata[0].val);

to select the uint64_t value. No need for the cast.

Also: Use PRIu64 to portably print 64-bit values, you can't assume that %llu is the right format specifier.

3

The "dirty" solution to access the first member of the n^th element of the array of unions pointed to by mydata as an int64_t without knowing its name is:

#include <inttypes.h>
#include <stdio.h>
union data
{ uint64_t val; ...
};
func(union data mydata[])
{ size_t n = 3; printf("%"PRIu64, *((uint64_t *)(mydata + n));
}

This works as the first member's address of a union or struct is guaranteed to be the same as the address of the union or struct itself.

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