Debugging CS 2022 Introduction to C Instructor Hussam Abu Libdeh Cornell University based on slides by Saikat Guha Fall 2011 Lecture 7 Debugging CS 2022 Fall 2011 Lecture 7 Before we begin I A quick note on arrays I I I We said that there are similarities between arrays and pointers You can use pointers as if they they are arrays i e ptr 1 But they are not exactly the same Debugging CS 2022 Fall 2011 Lecture 7 Before we begin I ptr1 ptr2 makes sense I I I Here we are assigning the value of variable ptr2 to the variable ptr1 The values just happen to be memory addresses array1 array2 does not make sense I I I array1 and array2 are the base addresses of the array but they are not full fledged pointers we can not have them point to different memory locations C does not automatically copy the values of one array to another what if they are different in size So expressions like array1 array2 and char str 100 argv 1 will give you compilation errors Debugging CS 2022 Fall 2011 Lecture 7 Print Debugging I I Manually insert debugging statements Debugging statements print to screen I I Caution stdout is buffered printf output may not appear before program crashes Solution stderr is unbuffered printf debugging fprintf stderr d p i p I I I I d int s char p any pointer see man page for others man 3 printf Debugging CS 2022 Fall 2011 Lecture 7 debug c Trace Information include stdio h int main int argc char argv fprintf stderr s d s t s n FILE LINE FUNCTION argv 0 fprintf stderr s d s t s n FILE LINE FUNCTION argv 1 fprintf stderr s d s t s n FILE LINE FUNCTION argv 2 trace c 5 main trace trace c 8 main hello trace c 11 main world Debugging CS 2022 Fall 2011 Lecture 7 GDB GNU Debugger I I I I Using printf is fine to get a quick idea about what might be wrong Using trace printing can give more info But no substitute for debugging Debugging allows us to I I I I I I step into the code see the execution path of our program examine the values of all variables set up breakpoints for careful examination get a better idea of what is going wrong GDB is a command line debugger for many languages including C I Not only debugger for C however Debugging CS 2022 Fall 2011 Lecture 7 GDB Commands I I I I I I I I I I I b function Breakpoint on entering function r args Run program list print C code n execute one statement s execute one step step into function calls c Continue running program p variable print the value of a variable bt Backtrace the stack fr num Make stackframe num current frame for printing variables q Quit help More GDB help Debugging CS 2022 Fall 2011 Lecture 7 GDB GNU Debugger saikat submit cs113 gcc g o cmd cmd c saikat submit cs113 cmd foo Segmentation fault saikat submit cs113 gdb cmd gdb b main Breakpoint 1 at 0x80483a4 file cmd c line 3 gdb r foo Breakpoint 1 main argc 1209306428 argv 0x4802f4c6 at cmd c 3 3 int main int argc char argv gdb n main argc 2 argv 0xbfb646e4 at cmd c 6 6 n atoi argv 1 gdb p argc 1 2 Debugging CS 2022 Fall 2011 Lecture 7 GDB GNU Debugger gdb p argv 0 2 0xbfb65c84 home netid cs113 cmd gdb c Continuing Program received signal SIGSEGV Segmentation fault strtol l internal from lib libc so 6 0x48045eae in gdb bt 0 0x48045eae in strtol l internal from lib libc so 6 1 0x48045c57 in strtol internal from lib libc so 6 2 0x48043511 in atoi from lib libc so 6 3 0x080483eb in main argc 2 argv 0xbfb646e4 at cmd c 7 gdb fr 3 3 0x080483eb in main argc 2 argv 0xbfb646e4 at cmd c 7 7 m atoi argv 2 gdb p argv 2 3 0x0 Debugging CS 2022 Fall 2011 Lecture 7 Things to try I I I I Crash a program by dereferencing a NULL pointer Crash a program by running out of stack space Crash a program by clobbering the stack e g the return address Crash a program by calling abort debug each of these cases using GDB Debugging CS 2022 Fall 2011 Lecture 7