CS102A - Lesson 11

Basic Typedef

typedef int Length;

Length len, maxlen;
Length *lengths[];

typedef char *String;
String p, ineptr[MAXLINES], alloc(int);
int strcmp(String, String);
p = (String) malloc(100);

Struct Typedef

typedef struct tnode *Treeptr;

typedef struct tnode{   // the tree node:
    char *word;             // points to the text
    int count;              // number of occurrences
    Treeptr left;           // left child
    Treeptr right;          // right child
} Treenode;

Treeptr talloc(void)
{
    return (Treeptr) malloc(sizeof(Treenode));
}

What is GDB?

• A debugger for C and C++ (along with Pascal, Objective-C, and Fortran).
• Does 4 main things:
  • Start your program, specifying anything that might affect its behavior
  • Make your program stop on specified conditions
  • Examine what has happened when your program stopped
  • Change things in your program so you can experiment with correcting the effects of one bug

Sample Segmentation Fault Program

#include <stdio.h>

int main()
{
    int a, b, c;
    char d, e, f;
    a = 1;
    b = 2;
    d = 'D';
    e = 'E';

    int *a_ptr, *b_ptr, *c_ptr;
    char *d_ptr, *e_ptr, *f_ptr;

    a_ptr = &a;
    b_ptr = a_ptr;
    c_ptr = b_ptr;

    d_ptr = &d;
    e_ptr = d_ptr;
    f_ptr = d_ptr;

    char *ptr = NULL; // Null pointer
    printf("%c", *ptr);

    return 0;
}

Compiling a Program for Debugging

• g++ -g segfault.cpp
  • The -g flag enables built-in debugging support; note that the file size is slightly bigger and it takes more time
• gdb
  • Start the gdb interface
• (gdb) help
  • get the help menu

Running a Program in GDB

• (gdb) file a.exe
  • loads the executable in gdb
• (gdb) run
  • runs the executable
• (gdb) continue
  • continues the program

Breakpoints Instructions

• Places in code to tell gdb to stop so you can analyze the code
• (gdb) break segfault.cpp:8
  • Note that it stops before an actionable line (initialization/assignment/function call)
  • Can also break at a specific function w/ the function name
• (gdb) run

Checking Variable Values

• (gdb) step
  • steps through the program line by line; enter to repeat
• (gdb) next
  • goes inside a function; more on that later

Checking Variable Values

• (gdb) print a
• (gdb) print a_ptr
• (gdb) print *a_ptr
• (gdb) print ptr - NULL Pointer
• (gdb) print *ptr - can't access!

Stepping into Functions Sample

#include <stdio.h>

void sample_function()
{
    int a, b, c;
    a = 101;
    b = 102;
    char d, e, f;
    d = 'd';
    e = 'e';

    return;
}

int main()
{
    sample_function();

    int a, b, c;
    char d, e, f;
    a = 1;
    b = 2;
    d = 'D';
    e = 'E';

    int *a_ptr, *b_ptr, *c_ptr;
    char *d_ptr, *e_ptr, *f_ptr;

    a_ptr = &a;
    b_ptr = a_ptr;
    c_ptr = b_ptr;

    d_ptr = &d;
    e_ptr = d_ptr;
    f_ptr = d_ptr;

    char *ptr = NULL; // Null pointer
    printf("%c", *ptr);

    return 0;
}

Stepping into a Function

• (gdb) break segfault_func.cpp:17
• (gdb) step
  • Note that this time, we actually entered the function
  • Also note that the variables are in the scope of the function, not main

Modified Program

#include <stdio.h>

void sample_function()
{
    int a, b, c;
    a = 101;
    b = 102;
    char d, e, f;
    d = 'd';
    e = 'e';

    return;
}

int main()
{
    sample_function();

    int a, b, c;
    char d, e, f;
    a = 1;
    b = 2;
    d = 'D';
    e = 'E';
    a = 11;
    a = 111;

    int *a_ptr, *b_ptr, *c_ptr;
    char *d_ptr, *e_ptr, *f_ptr;

    a_ptr = &a;
    b_ptr = a_ptr;
    c_ptr = b_ptr;

    d_ptr = &d;
    e_ptr = d_ptr;
    f_ptr = d_ptr;

    char *ptr = NULL; // Null pointer
    printf("%c", *ptr);

    return 0;
}

GDB Watch

• gdb
• (gdb) file a.exe
• (gdb) break segfault_watch.cpp:17
• (gdb) run
• (gdb) watch a - whenever a changes, it will print the old and new values and stop before the next line
• (gdb) next

Other GDB Commands

• breaktrace - stack trace leading to a seg fault
• where - same as backtrace, but in middle of program (e.g. in a function or in main)
• finish - runs until current function finishes
• info breakpoints - shows all declared breakpoints
• clear file:linenum - deletes breakpoint
• delete - deletes all breakpoints

Conditional Breakpoints

• (gdb) break segfault_watch.cpp:27 if a >=100
  • stops at line 27 if the variable a is greater than 100

Fork Example

https://github.com/ECE160B-Fall2018/forkme