Get return value of function when last called, without calling it again-CodePudding

I am new to using gdb.

I have a function, which I will call a() that gives a different value every time it is called. Another function add_to_array uses a() as a parameter. Here is an example:

add_to_array(a())
add_to_array(a())
add_to_array(a())

How could I see what the return values of a() were each time it was called?

CodePudding user response：

How could I see what the return values of a() were each time it was called?

If you are currently stopped before e.g. the last call to add_to_array(), and are interested in what value the first call to a() returned, the only way to know that (with GDB) is to examine what's in the array (to which add_to_array() adds values. GDB can't "go back in time" and can't keep track of every value that ever changed.

But if you just want to observe these values on re-run of the program, simply set a breakpoint on add_to_array, and (if your program is built with debug info -- usually the -g flag), GDB will stop and print the parameter value every time.

You may also be interested in a replay debugger, which can go back in time and which can answer the question directly.

CodePudding user response：

In addition to Employed Russian's excellent answer, you could instrument the code this way:

#define VALUES_HISTORY_LEN  65536
int a_values_history[VALUES_HISTORY_LEN];
int a_values_index;
int add_values_history[VALUES_HISTORY_LEN];
int add_values_index;

// this will store up to VALUES_HISTORY_LEN values previously returned by a()
#define a() (a_values_history[a_values_index   % VALUES_HISTORY_LEN] = (a)())

// this will store up to VALUES_HISTORY_LEN values previously passed to add_to_array()
#define add_to_array(x)  (add_to_array)(add_values_history[add_values_index   % VALUES_HISTORY_LEN] = (x))