I can "strace -o -tt 123 a.out" before "A begin" but how could I
combine console output with the content of strace?

strace does output to stderr unless you specify the -o option. So by
using something like

$ strace -tt your_program

or

$ strace -tt your_program &> log.txt

you will have both outputs intermingled. Note that the console output
of your program will appear in the middle of the corresponding write()
system call.

Alex.