How to print in binary mode in gdb?

I tried b but seem not:

(gdb) p/b 0x00000000开发者_运维问答00400398
Size letters are meaningless in "print" command.

Is there such a switch?

You need the /t switch which works with both p and x:

(gdb) p /t 0x0000000000400398
$1 = 10000000000001110011000

See help x for more info on the FMT (format) switches.

x/4tb 0x0000000000400398 will let you analyze the Memory Address with Binary Values.