Appendix: GDB Commands and FAQ

Command: info locals

Prints the value of all of the local variables in the current stack frame

Command: command

Executes a list of commands every time a break point is reached. For example:

Set a breakpoint:

b 73

Type commands followed by the breakpoint number:

commands 1

Type the list of commands that you want to execute separated by a new line. After your list of commands, type end and hit Enter.

p var1
p var2
end

Command: delete

Deletes the specified breakpoint. See the reference card for more info.

FAQ

What is a null terminator?

A null terminator is a character used to denote the end of a string in C. The null terminator is written as '\0'. The ASCII value of the null terminator is 0. When you make a character array, you should terminate the array with a null terminator like this

char my_str[] = {'e', 'x', 'a', 'm', 'p', 'l', 'e', '\0'};

If you are using double quotes to create a string, the null terminator is implicitly added, so you should not add it yourself. For example:

char *my_str = "example";

What is an executable?

An executable is a file composed of binary that can be executed on your computer. Executables are created by compiling source code.

What is strlen?

See the man pages for a full description. Type the following into your terminal

man strlen

To exit the man pages, press q.

What is a macro?

A macro is a chunk of text that has a name. Whenever this name appears in code, the preprocessor replaces the name with the text. Macros are indicated with #define For example:

#define ARR_SIZE 1024
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))

int main() {
    int arr1[ARR_SIZE];
    int arr2[ARR_SIZE];
    int arr3[ARR_SIZE];

    for (int i = 0; i < ARR_SIZE; ++i) {
        arr3[i] = min(arr1[i], arr2[i]);
    }
}

In this code, the preprocessor will replace ARR_SIZE with 1024, and it will replace

arr3[i] = min(arr1[i], arr2[i]);

with

arr3[i] = ((arr1[i]) < (arr2[i]) ? (arr1[i]) : (arr2[i]));

Macros can be much more complex than the example above. You can find more information in the GCC docs

What is a segfault?

A segfault occurs when you try to access a piece of memory that "does not belong to you." There are several things that can cause a segfault including

  1. Accessing an array out of bounds. Note that accessing an array out of bounds will not always lead to a segfault. The index at which a segfault will occur is somewhat unpredictable.
  2. Derefrencing a null pointer.
  3. Accessing a pointer that has been free'd (free is not in the scope of this lab).
  4. Attempting to write to read-only memory. For example, strings created with the following syntax are read only. This means that you cannot alter the value of the string after you have created it. In other words, it is immutable.
char *my_str = "Hello";

However, a string created using the following syntax is mutable.

char my_str[] = "hello";

Why is the first string immutable while the second string is mutable? The first string is stored in the data portion of memory which is read-only while the second string is stored on the stack.

What is a header file?

Header files allow you to share functions and macros across different source files. For more info, see the GCC header docs.

Common GDB Errors

GDB is skipping over lines of code

This could mean that your source file is more recent than your executable. Exit GDB, recompile your code with the -g flag, and restart gdb.

GDB isn't loading my file

You might see an error like this "not in executable format: file format not recognized" or "No symbol table loaded. Use the "file" command."

This means that you called gdb on the source file (the one ending in .c) instead of the executable. Exit GDB and make sure that you call it with the executable.

How do I switch between the code window and the console?

CGDB presents a vim-like navigation interface: Press i on your keyboard to switch from the code window to the console. Press Esc to switch from the console to the code window.

GDB presents a readline/emacs-like navigation interface: Press Ctrl + X then O to switch between windows.

I'm stuck in the code window

Press i on your keyboard. This should get you back to the console.

The text UI is garbled

Refresh the GDB text UI by pressing Ctrl + l.