This page shows some tips how to debug the queue in C++ example from cpp queue transaction page.

1. The project's main page shows a very simple example of using queues. There is no real challenge in debugging this small program, but to demonstrate how to view transactions ( smart pointer of a queue element), with gdb. As you will see printing an array of transactions is simple, but on my gdb version, it takes more than a single print to get to the output.


2. First one needs to make sure compile option is specified correctly in the makefile (-g or -ggdb is fine):
   
   #Pini
   #gdb -x br.txt run.exe
   DEBUG = -g
   #DEBUG = -ggdb
   
   


3. Next, I usually prepare a file (br.txt) with my current gdb commands. In this case it is not too complex. The file contains a single command.
   
   b my_queue_example.cpp:38
   


4. From the linux command line, gdb is fired using the following command:
   
   gdb -x br.txt run.exe
   
   Than hit r (run) and you get:
   
   Breakpoint 1, main () at my_queue_example.cpp:38
   38 c_trans c = myqueue.front();
   
   In my simple case I prepared an array. Each of the entries of the array is to be pushed to the queue.
   
   
   ...
     queue myqueue;
     
     unsigned da[BSZ]={1,2,3,4,5,6,7,8};
     const c_trans arr[2] = {c_trans(), c_trans(da, 1)};
     
     cout << "start" << endl;
     myqueue.push (arr[0]);
     myqueue.push (arr[1]);
   ...
   
   So first it is desired to see the content of the array:
   
   
   (gdb) p arr
   $1 = {
     seq_n = 0, data = {0, 0, 0, 0, 0, 0, 0, 0}},
     seq_n = 1, data = {1, 2, 3, 4, 5, 6, 7, 8}}
   }
   
   


5. Now we are ready to see what is in queue just before we are going to use it, by printing first myqueue:
   
   
   (gdb) p myqueue
   $2 = {c = {<std::_Deque_base<c_trans, std::allocator<c_trans> >> = {
     _M_impl = {<std::allocator<c_trans>> = {<__gnu_cxx::new_allocator<c_trans>> = {<No data fields>}, <No data fields>}, _M_map = 0x804b230,
     _M_map_size = 8, _M_start = {_M_cur = 0x804b258, _M_first = 0x804b258,
     _M_last = 0x804b450, _M_node = 0x804b23c}, _M_finish = {
   ...
   
   And some useful commands to inspect the queue:
   
   
   (gdb) p myqueue->c->_M_impl->_M_start->_M_first
   $3 = (c_trans *) 0x804b258
   (gdb) p *(c_trans *) 0x804b258
   $4 = {seq_n = 0, data = {0, 0, 0, 0, 0, 0, 0, 0}}
   
   (gdb) p *(c_trans *) 0x804b258@2
   $6 = {
     {seq_n = 0, data = {0, 0, 0, 0, 0, 0, 0, 0}},
     {seq_n = 1, data = {1, 2, 3, 4, 5, 6, 7, 8}}
   }
   

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