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Debugging tools like gdb (GNU Debugger) are essential for debugging C programs. gdb allows you to examine what is happening inside a program while it runs or what it was doing at the moment it crashed. Here is a detailed guide on how to use gdb for debugging C programs:
Setting Up
1. Compile with Debugging Information:
To use gdb, you need to compile your C program with the -g flag which includes debugging information in the executable.
gcc -g -o myprogram myprogram.c
Starting gdb
2. Starting gdb:
Run ‘gdb‘ with your executable:
gdb myprogram
Basic Commands
3. Basic gdb Commands:
- run (r): Starts your program.
- break (b): Sets a breakpoint at a function or line number.
- next (n): Executes the next line of code, stepping over function calls.
- step (s): Executes the next line of code, stepping into function calls.
- continue (c): Continues running your program until the next breakpoint.
- print (p): Prints the value of a variable.
- backtrace (bt): Displays the call stack.
Example Debugging Session
Let's walk through an example debugging session.
Code Example
#include <stdio.h>
void function1(int);
void function2(int);
int main() {
int x = 5;
function1(x);
return 0;
}
void function1(int a) {
printf("Function1: %d\n", a);
function2(a + 1);
}
void function2(int b) {
printf("Function2: %d\n", b);
}
Debugging Steps
1. Compile the Code with Debugging Information:
gcc -g -o myprogram myprogram.c
2. Start gdb:
gdb myprogram
3. Set a Breakpoint at main:
(gdb) break main
Breakpoint 1 at 0x40052a: file myprogram.c, line 6.
4. Run the Program:
(gdb) run
Starting program: /path/to/myprogram
Breakpoint 1, main () at myprogram.c:6
6 int x = 5;
5. Step Through the Code:
(gdb) next
7 function1(x);
(gdb) next
Function1: 5
8 return 0;
6. Set a Breakpoint in function2:
(gdb) break function2
Breakpoint 2 at 0x400557: file myprogram.c, line 15.
7. Continue to the Next Breakpoint:
(gdb) continue
Continuing.
Breakpoint 2, function2 (b=6) at myprogram.c:15
15 printf("Function2: %d\n", b);
8. Print Variable b:
(gdb) print b
$1 = 6
9. Continue Running:
(gdb) continue
Continuing.
Function2: 6
[Inferior 1 (process 10345) exited normally]
Analyzing a Crash
When a program crashes, gdb can help determine what caused the crash.
1. Run the Program:
(gdb) run
2. If the Program Crashes:
Program received signal SIGSEGV, Segmentation fault.
0x00000000004004b6 in function2 (b=6) at myprogram.c:15
15 printf("Function2: %d\n", b);
3. Inspect the Call Stack:
(gdb) backtrace
#0 0x00000000004004b6 in function2 (b=6) at myprogram.c:15
#1 0x000000000040049a in function1 (a=5) at myprogram.c:10
#2 0x0000000000400478 in main () at myprogram.c:7
4. Inspect Variables:
(gdb) print b
$1 = 6
Useful gdb Tips
List Source Code:
(gdb) list
Inspect Memory:
(gdb) x/10x &variable
Conditional Breakpoints:
(gdb) break function2 if b > 5
Watchpoints: (stop when a variable changes)
(gdb) watch variable
Quit gdb:
(gdb) quit
Using these commands and techniques, you can effectively debug your C programs using gdb.
