[SOLVED] Different result of using gcc in way that source from pipe/stdin

Why is the output/result of using gcc in way that source from pipe/stdin different from the normal way, ie. from the text source file ?

Code:$ cat>u.c <<<'void main(){ int o=9;}'
$ gcc u.c -c

$ gcc -xc - <<<'void main(){ int o=9;}'

$ diff u.o a.out
Binary files u.o and a.out differ

$ ls -s u.o a.out
20 a.out 4 u.o