$ gcc -ansi -pedantic -Wall -Wextra foo.c -o foo
foo.c: In function ‘main’:
foo.c:15:2: warning: passing argument 1 of ‘foo’ from incompatible pointer type [enabled by default]
  foo(bar);
  ^
foo.c:8:6: note: expected ‘void * (*)(void)’ but argument is of type ‘void (*)(void)’
 void foo(void *callback(void))
      ^