CS201 Lecture 2 GDB, The C Library - Raoul Rivas · CS201 –Lecture 2 GDB, The C Library. Announcements CS201: Computer Science Programming Raoul Rivas (Based on slides from Bryant

RAOUL RIVAS

PORTLAND STATE UNIVERSITY

CS201 – Lecture 2GDB, The C Library

Announcements

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

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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)


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0x100

0x123

0x230

0x510

array

Char** Char*

What’s the value of array[1][2]?

X Y Z

A B C

D E F

G H I

Multidimensional Dynamically Allocated Arrays


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0x100

0x123

0x230

0x510

array X Y Z

A B C

D E F

G H I

Char** Char*

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);

Alternate Method▪ Store the data in a single dimension array▪ Flatten array into a single dimension

▪ Compute offsets during access

▪ Can’t use multiple subscripts


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array = (char*)malloc(sizeof(char*)*4*3);idx=2*3+1;array[idx]=‘E’; free(array);

X Y ZA B CD E FG H I

X Y Z A B C D E F G H I

array

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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


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#include <header.h>

Back to Hello, world


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#include <stdio.h>

main()

{

printf(“hello, world”);

}

Include stdio.h header file (so we can use printf)

String Manipulation▪ Assumes all strings are C-Strings▪ 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


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String Manipulation


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#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?

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()


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Console I/O▪ 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


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stdin

stdout

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.


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int main (){char str1[]=“16734893”;long number;

number=strtol(str1, NULL, 10);return 0;

}

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.

▪ log10(𝑀𝑎𝑥𝑁𝑢𝑚) + 1, not counting the sign

▪ Format is the format string used by the printf family of functions

▪ printf, fprintf, sprint


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int main (){char str1[12];int number= 16734893;

sprintf(str1, “%d”, number);return 0;

}

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


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#define NDEBUG

assert(condition);

Assertions

▪ Not a user or runtime error

▪ Assert Fails and memcpy never executes


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int main(){

int str1[] = {1,2,3,4,5}int str2[3];

assert(sizeof(str1) == sizeof(str2));memcpy(str2, str1, sizeof(str1));

}

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


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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


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(gdb) help mycommand

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


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gdb myprogram

(gdb) file myprogram

(gdb) run

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


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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


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(gdb) break file5.c:67

(gdb) break my_func

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!


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(gdb) step

(gdb) next

Next vs. Step


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int main(){

int val=5, result;

res=factorial(val);printf(“%d\n”, result);return 0;

}

int factorial(int n){

int i, retv=1;

for(i=1; i<= n; i++) {

retv*= 1; }

return retv;}

Step

Next

Printing a variable▪ To print a variable use the command “print”

▪ GDB can print variables members of structures

▪ Dereference pointer

▪ Support self-referential structures


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(gdb) print myvar

(gdb) print mystruct.myvar

(gdb) print *myptr

(gdb) print mylist->next->next

Watchpoint▪ 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


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(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

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


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(gdb) break breakpoint if condition

(gdb) break file1:6 if i > ARRAYSIZE

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!


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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


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CS201 Lecture 2 GDB, The C Library - Raoul Rivas · CS201 –Lecture 2 GDB, The C Library. Announcements CS201: Computer Science Programming Raoul Rivas (Based on slides from Bryant

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