Given a pointer,how does gdb get its type?

Question

How does gdb know the pointer points to a int or struct or any other data types?

Answer 1

from: Examining the Symbol Table

whatis expr

Print the data type of expression expr. expr is not actually evaluated, and any side-effecting operations (such as assignments or function calls) inside it do not take place. See section Expressions.

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ptype expr

ptype

Print a description of the type of expression expr. ptype differs from whatis by printing a detailed description, instead of just the name of the type. For example, for this variable declaration:

struct complex {double real; double imag;} v;

the two commands give this output:

(gdb) whatis v
 type = struct complex
(gdb) ptype v
 type = struct complex {
    double real;
    double imag;
 }

Answer 2

gdb can't know, unless the pointer came from a variable or expression for which the type can be determined.

If gdb is given 0x4567789, it has no idea what that might point to. But if an int *p has that value, gdb can deference that and give you what that address contains.