3.2.4. Showing program internal values with GDB
Displaying the values of a program’s internal variables is important for understanding of what the program is doing. The GNU Debugger (GDB) offers multiple commands that you can use to inspect the internal variables. The following list describes the most useful of these commands.
p(print)Display the value of the argument given. Usually, the argument is the name of a variable of any complexity, from a simple single value to a structure. An argument can also be an expression valid in the current language, including the use of program variables and library functions, or functions defined in the program being tested.
It is possible to extend GDB with pretty-printer Python or Guile scripts for customized display of data structures (such as classes, structs) using the
printcommand.bt(backtrace)Display the chain of function calls used to reach the current execution point, or the chain of functions used up until execution was terminated. This is useful for investigating serious bugs (such as segmentation faults) with elusive causes.
Adding the
fulloption to thebacktracecommand displays local variables, too.It is possible to extend GDB with frame filter Python scripts for customized display of data displayed using the
btandinfo framecommands. The term frame refers to the data associated with a single function call.infoThe
infocommand is a generic command to provide information about various items. It takes an option specifying the item to describe.-
The
info argscommand displays options of the function call that is the currently selected frame. -
The
info localscommand displays local variables in the currently selected frame.
For a list of the possible items, run the command
help infoin a GDB session:(gdb) help info-
The
l(list)- Show program source code. When the program has been started, but is currently stopped, this command lists the source code where the program is currently stopped, along with a few lines of context. Before the program is started, this will list the main function. While not strictly a command to show internal state, list helps the user understand what changes to the internal state will happen in the next step of the program’s execution.
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