1. CS201 – Lecture 2 GDB, The C Library
Raoul Rivas
Portland state university

2. Announcements CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

3. Multidimensional Dynamically Allocated Arrays
Direct access support.
Same as Multidimensional Static Arrays
No direct allocation support
Declare a single dimension array of pointers (Rows)
For each row allocate an array (Columns)
Char**
Char*
array
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0x510 X Y Z A B C D E F G H I
What’s the value of array[1][2]?
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

4. Multidimensional Dynamically Allocated Arrays
char** array;
int i;
array = (char**)malloc(sizeof(char*)*4);
for(i=0; i < 4; i++){
array[i] = (char*)malloc(sizeof(char)*3); }
array[2][0]=‘D’;
for(i=0; i < 4; i++) {
free (array[i]);
free(array);
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array X Y Z A B C D E F G H I
Char**
Char*
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

5. Alternate Method Store the data in a single dimension array array
Flatten array into a single dimension
Compute offsets during access
Can’t use multiple subscripts
array X Y Z A B C X Y Z A B C D E F G H I D E F G H I 11
array = (char*)malloc(sizeof(char*)*4*3);
idx=2*3+1;
array[idx]=‘E’;
free(array);
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

6. The C Library Standard library available across all C compilers
Part of ANSI C
Many implementations
Microsoft C Runtime Library
GNU C library
Provides many services
File Access
Read and write to console
String Manipulation
Mathematical functions
Time and Date functions
Divided into modules
Include header file with definitions before you can use them
#include <header.h>
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

7. Back to Hello, world
Include stdio.h header file (so we can use printf)
#include <stdio.h> main() { printf(“hello, world”); }
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

8. String Manipulation Assumes all strings are C-Strings strlen(char *s1)
char arrays with NULL character at the end (‘\0’)
strlen(char *s1)
Returns the number of character in the string not including the NULL character
strncpy(char *dest, char *src, int n)
Copies at most n characters of src on top of dest
strncmp(char *s1, char *s2, int n)
Compares up to n characters of s1 with s2
Returns 0 if s1== s2. Returns less than 0 if s1 < s2. Returns more than 0 if s1 > s2
strncat(char* s1, char *s2, int n)
Appends up to n characters of s2 at the end of s1
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

9. Is the manually added NULL necessary? Why?
String Manipulation
#include <stdio.h>
#include <string.h>
int main () {
char str1[]= "To be or not to be";
char str2[40];
char str3[6];
/* copy to sized buffer */
strncpy ( str2, str1, sizeof(str2) );
/* partial copy (only 5 chars): */
strncpy ( str3, str2, 5 );
/* null character manually added */
str3[5] = '\0';
return 0; }
Is the manually added NULL necessary? Why?
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

10. File I/O FILE *fopen(const char *filename, const char *mode);
Opens/Creates a file for read or write. Returns a pointer to a FILE structure
Mode: “r” - Read, “w”- Write, “a” - Append, “b” - Binary mode
int fclose(FILE *a_file);
Close the file
ASCII Read/Write
fprintf(), fscanf(), fgets()
Usually read one line at a time
Good for Text, CSV files and files with line breaks
Binary Read/Write
Write of data in binary format
Read/Write M-blocks of N-bytes at a time
Use fwrite() and fread()
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

11. Console I/O stdin stdout
Console I/O is like reading from a file. You can use the same file functions just specify “stdin” and ‘stdout” as the file pointers
There are some specific functions for stdin and stdout are provided in the C library
Most used function for console writing is printf
Sometimes the File and Console versions have different behaviors or parameters
fgets() does not behave the same a gets()
Data read from the file/console must be validated
fscanf() and scanf() are subject to many attacks
fgets() is the safest way to read from the file/console
Parse the string afterwards and convert to the specific types
stdin
stdout
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

12. String to Numbers Various functions based on the type
strtol, strtoll, stroul, etc
long int strtol (const char* str, char** endptr, int base);
Parses a number inside a string and outputs it as a long
str contains the string to parse
endptr is an optional return value to the remaining of the string after the number
base is the Base of the number (hex, octal, decimal, etc). Use zero for autodetect based on the format.
int main () {
char str1[]=“ ”;
long number;
number=strtol(str1, NULL, 10);
return 0; }
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

13. Numbers to Strings
Use sprint to create a string with the format you need
sprintf(char* str, char *format, …)
str contains the destination string. Make sure is pre-allocated to the maximum possible length.
Limits.h contains macros with the largest and smallest number supported for each type.
log 10 (𝑀𝑎𝑥𝑁𝑢𝑚) +1, not counting the sign
Format is the format string used by the printf family of functions
printf, fprintf, sprint
int main () {
char str1[12];
int number= ;
sprintf(str1, “%d”, number);
return 0; }
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

14. Assertions
Assertions are runtime checks placed on a program that are only checked during development
They are ignored for “production” code.
We define the macro NDEBUG to indicate that our code is “production” code
An Assertion must be true at the time of its execution or the program will stop.
To check an assertion we call the assert() function
Do not use assertions to catch user or runtime errors
#define NDEBUG
assert(condition);
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

15. Assertions Not a user or runtime error
int main() {
int str1[] = {1,2,3,4,5}
int str2[3];
assert(sizeof(str1) == sizeof(str2));
memcpy(str2, str1, sizeof(str1)); }
Not a user or runtime error
Assert Fails and memcpy never executes
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

16. GDB Overview Inspect program execution and state at any point in time.
Set breakpoints
Read variable, dump stack
Useful to find a wide range of bugs
Must compile the program to include “Symbols”
Use the –g or –ggdb flag
If possible use –O0 to disable optimizations
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

17. GDB Interactive Shell GDB is based on an interactive shell
The shell prompt symbol is “(gdb)”
History of past command
TAB for autocompletion
Integrated help
Call the help command followed by the name of the command you need info about
(gdb) help mycommand
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

18. Using GDB You can specify the program to load in the command line
Or you can load the program using the “file” command in the gdb prompt “(gdb)”
To run your program use the “run” command
If your code crashes you will see additional crash information
gdb myprogram
(gdb) file myprogram
(gdb) run
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

19. How to debug a program Debugging is a lot like fishing
Stop execution at possible suspecting points or functions
Look for the values of each variable until you can determine the problem
Step through specific functions or loops to find any problems in your code
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

20. Breakpoints
Breakpoints stop execution of a program at a specific point
Use the “break” command
Stop at a specific line in a source file
Stop at line 67 of file5.c
Stop at a specific function
Stop at function my_func()
After setting a breakpoint start your program using the “run” command
(gdb) break file5.c:67
(gdb) break my_func
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

21. Navigating the Code
GDB stops the execution of your program at a breakpoint
GDB command prompt will appear
To step through the code use the “step” command
Executes one instruction and stops. If the line is a function it will step into your function
Visual Studio calls this “Step Into”
To execute an entire function as if it was a single instruction use the command “next”
Visual Studio calls this “Step Over”
For assembly use stepi and nexti!
(gdb) step
(gdb) next
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

22. Next vs. Step int factorial(int n) { int i, retv=1; Step
for(i=1; i<= n; i++)
retv*= 1; }
return retv;
Step
int main() {
int val=5, result;
res=factorial(val);
printf(“%d\n”, result);
return 0; }
Next
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

23. Printing a variable To print a variable use the command “print”
GDB can print variables members of structures
Dereference pointer
Support self-referential structures
(gdb) print myvar
(gdb) print mystruct.myvar
(gdb) print *myptr
(gdb) print mylist->next->next
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

24. When GDB stops if we set a Watchpoint on variable val?
A watchpoint is similar to a breakpoint, except that it stops the execution when the value of a variable change
No associated position in the code
Variable is the stopping trigger
Use the “watch” command to set a watchpoint on a variable
Pause execution when the variable myvar changes
(gdb) watch myvar
int main() {
int val=5, result;
result= val + 1;
val= 25; }
When GDB stops if we set a Watchpoint on variable val?
Val=25
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

25. Conditional Breakpoints
Stop in a breakpoint only if certain condition is met
Good to debug array or loop boundaries
Cases where code fails with certain values
Use the following syntax:
Example
Stop at line 6 of file1 if i > ARRAYSIZE
(gdb) break breakpoint if condition
(gdb) break file1:6 if i > ARRAYSIZE
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

26. Debugging in Windows GDB is the most popular debugger in Linux
In Windows you can use WinDBG or the more user friendly Visual Studio Debugger
Breakpoints, Watchpoints, Conditional Breakpoints
Windows keeps Symbols in separate Program Database Files (PDB)
PDB files must be loaded into WinDbg to produce any meaningful information
A PDB file is specific to a particular build!
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)

27. Summary
Dynamically allocated multidimensional arrays are usually declared as an array of pointers to one dimension arrays
GDB is the GNU Debugger.
Inspect variables, breakpoints, watchpoints, step by step execution
The C library is a standard ANSI library
String Manipulations, File Access, Console I/O, Memory Allocation
CS201: Computer Science Programming
Raoul Rivas (Based on slides from Bryant and O’Hallaron)