struct node{
        int value;
        node* next;
};

int main(){
        node* b;
        b->value=1;
        linkedList a;
         ...

Wed Mar 29; 15:59:29; marton;~/documents/github/fmi_summer_2017/daa/bfs ;  $  gdb a.out
GNU gdb (GDB; openSUSE Leap 42.1) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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and "show warranty" for details.
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Type "show configuration" for configuration details.
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Reading symbols from a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x4008f5: file main.cpp, line 5.
Starting program: /home/marton/documents/github/fmi_summer_2017/daa/bfs/a.out
step
Temporary breakpoint 1, main () at main.cpp:5
5               b->value=1;
(gdb) step

Program received signal SIGSEGV, Segmentation fault.
0x00000000004008f9 in main () at main.cpp:5
5               b->value=1;
(gdb)

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
(gdb)