gdb: breakpoint when register will have value 0xffaa

Yes in gdb you would set a watchpoint like so:

watch $eax == 0x0000ffaa

But it is dependent on watchpoint support being available for the target. You should note that this may slow down execution significantly.

If you would like to break in a certain location you can do so, by setting a conditional breakpoint:

break test.c:120 if $eax == 0x0000ffaa

To whit:

If you use $eax the condition is ignored and it becomes an unconditional watch/breakpoint.

(gdb) disass print_helloDump of assembler code for function print_hello:0x000000000040058c :     push   %rbp0x000000000040058d :     mov    %rsp,%rbp0x0000000000400590 :     sub    $0x20,%rsp0x0000000000400594 :     movl   $0x1,-0x4(%rbp)0x000000000040059b :    movl   $0x5,-0x4(%rbp)0x00000000004005a2 :    mov    -0x4(%rbp),%esi0x00000000004005a5 :    mov    $0x4006dc,%edi0x00000000004005aa :    mov    $0x0,%eax0x00000000004005af :    callq  0x400468 0x00000000004005b4 :    leaveq 0x00000000004005b5 :    retq
End of assembler dump.
(gdb) break *0x00000000004005af if $eax==0Breakpoint 1 at 0x4005af: file hello.c, line 7.(gdb) info breakNum     Type           Disp Enb Address            What1       breakpoint     keep y   0x00000000004005af in print_hello at hello.c:7        stop only if $eax==0(gdb) runStarting program: /home/dg/hello/hello hello world 2Error in testing breakpoint condition:Invalid type combination in equality test.
Breakpoint 1, 0x00000000004005af in print_hello () at hello.c:77               printf("hello %d\n", value);
(gdb) condition 1 $eax != 0(gdb) runThe program being debugged has been started already.Start it from the beginning? (y or n) yStarting program: /home/dg/hello/hello hello world 2Error in testing breakpoint condition:Invalid type combination in equality test.
Breakpoint 1, 0x00000000004005af in print_hello () at hello.c:77               printf("hello %d\n", value);(gdb) 

(gdb) condition 1 $rax != 0(gdb) info breakNum     Type           Disp Enb Address            What1       breakpoint     keep y   0x00000000004005af in print_hello at hello.c:7        stop only if $rax != 0        breakpoint already hit 1 time(gdb) runThe program being debugged has been started already.Start it from the beginning? (y or n) yStarting program: /home/dg/hello/hello hello world 2hello 5
Program exited normally.(gdb) condition 1 $rax == 0(gdb) runStarting program: /home/dg/hello/hello hello world 2
Breakpoint 1, 0x00000000004005af in print_hello () at hello.c:77               printf("hello %d\n", value);(gdb) 

This was all tested on gdb 6.8.50:GNU gdb (GDB; SUSE Linux Enterprise 11) 6.8.50.20081120-cvs

If you are on a 64 bit machine you have to watch $rax, not $eax.