gdb - get variable name of register

Question

In GDB, info registers or info all-registers will show you all of the register symbol names and their values.

Question:

How do I get the variable name (i.e. from the source code) that is stored in that register? (or a line number in source code, or anything)

For example:

int my_reg = /* something */;
float another_reg = /* something else */;
...

Then perhaps, info all-registers will return:

R0             0x0      0
R1             0xfffbf0 16776176
R2             0x0      0
R3             0x0      0
R4             0x6      6

How do I determine which register (R0? R2? R4?) is "associated" with my_reg?

Answer 1

If you have access to the debug symbols (and understand how to read them - that is, you have some code that parses the debug symbols), it is possible to trace exactly which register corresponds to which register. However, this is quite possibly changing from one line to the next, as the compiler decides to move things around for one reason or another (e.g. some calculation starts with R1, and ends up with the result in R2, because that's better than trying to retain the value in R1 [or we need the original value in R1 too - think array[x++] - now we have the new value of x, hopefully in a register, and the value of the old x that we need to use for indexing, also needed to be in a register to add to the base-address of array.

Not all variables end up in registers (depending on processor, and "what registers are available").

The debugger WILL know where each variable is at any given time - but sometimes it can be a big confused, e.g:

int array[10000];
... 
for(int i = 0; i < 10000; i++)
{
   array[i] = rand();
}

may translate to something like this during optimization:

int array[10000]; 
int *ptr = array;
int *ptr2 = &array[10000];
while(ptr < ptr2)
{
   *ptr++ = rand();
}

Now try printing i... ;)

Answer 2

There might be one register, multiple registers, or even no registers associated with any given C variable at any given point in time. You'll have to inspect the disassembly to see what's going on.

Why not just print my_reg to see the value?

l *$pc will list the source code around the current instruction being executed.