How do I show what fields a struct has in GDB?

In GDB (GNU Debugger), you can use the ptype command to view the fields of a structure. The ptype command prints information about types, including detailed information for structures, unions, enums, and other composite types. Specifically for structures, ptype displays all fields and their types.

Specific Steps:

1. Load GDB and the Program: First, load your C or C++ program in GDB. Assuming the executable is named example, start GDB in the terminal using:

   shell
   

gdb example

shell

2. **Set a Breakpoint**: To view structure details, set a breakpoint at an appropriate location so the program pauses there. For example, to inspect the structure at the start of the `main` function, use:

break main

shell

3. **Run the Program**: Execute the program until it reaches the breakpoint:

run

shell

4. **Use the `ptype` Command**: When the program is paused at the breakpoint, use the `ptype` command to view the structure definition. For example, if you have a structure type named `MyStruct`, input:

ptype MyStruct

shell

### Example:

Assume you have the following C code defining a structure:
```c
typedef struct {
 int id;
 char name[50];
 float salary;
} Employee;

In GDB, use ptype Employee to view the structure definition. The output may appear as:

shell
type = struct Employee {
    int id;
    char name[50];
    float salary;
}

This shows that the Employee structure contains three fields: id (integer), name (character array), and salary (floating-point).

Notes:

• Ensure GDB has loaded the source code containing the structure definition before using ptype.
• If the structure is defined within a specific scope (e.g., inside a function), you must be in that scope's context to correctly view it with ptype.

Using the ptype command is a direct and effective method for examining the composition of various data structures in your program, which is invaluable for debugging and understanding internal program structure.