GDB Breakpoints and Watchpoints using awatch rwatch Examples

In this tutorial we’ll cover couple of advanced GDB functions including conditional breakpoints, and watchpoints.

We’ve also used a simple C program as an example to explain these GDB debugger concepts.

GDB Conditional Breakpoints

A breakpoint, brakes everytime your program reaches a specific place. You can also specify a condition for breakpoints. A condition is a boolean expression. A conditional breakpoint will break, only if the boolean expression evaluates to TRUE.

Assume that you are calling a function in for loop, and every 500th time, the program is crashing. Conditional breakpoint comes to rescue in those situations, where you can set a condition and break the execution only if the condition matches.

#include <stdio.h>
int main() {
        int i=0;
        for (i=0;i<1000;i++) {
                printf("%d\n",i);
        }
}

(gdb) b 8 if i=500
Breakpoint 2 at 0x400524: file a.c, line 8.
(gdb) run
Starting program: /home/lakshmanan/./a.out 

Breakpoint 1, main () at a.c:6
6		int i=0;
(gdb) c
Continuing.

Breakpoint 2, main () at a.c:8
8			printf("%d\n",i);
(gdb) p i
$1 = 500

In the above gdb session, the program break when the value of i is 500.

What is a Watchpoint?

Watchpoint are similar to breakpoints which we discussed in our previous GDB – Breakpoints article. But unlike breakpoints which are set for functions or lines of code, watchpoints are set on variables. When those variables are read/written, watchpoint will be triggered and the program execution will stop.

Please note that the variable you want to watch must be in the current scope. So make sure you set a breakpoint within the scope of the variable to watch it.

#include <stdio.h>

int main() {
        int a=10;
        int b=15;
        b = a+b;
        printf("%d\n",b);
}

$ cc -g watch.c -o watch

Setting Read/Write watchpoint using ‘awatch’

Use the awatch command to set a read/write watchpoint on a variable. The syntax for awatch is, ‘awatch’ followed by the variable name.

$ gdb ./watch

(gdb) break 1
Breakpoint 1 at 0x400514: file a.c, line 1.
(gdb) run
Starting program: /home/lakshmanan/Personal/./a.out 

Breakpoint 1, main () at a.c:4
4		int a=10;
(gdb) awatch b
Hardware access (read/write) watchpoint 2: b
(gdb) n
5		int b=15;
(gdb) n
Hardware access (read/write) watchpoint 2: b

Old value = 0
New value = 15
main () at a.c:7
7		b = a+b;
(gdb) n
Hardware access (read/write) watchpoint 2: b

Old value = 15
New value = 25
main () at a.c:9
9		printf("%d\n",b);

In the above example, whenever the given variable is read/written, gdb prints the old and new value and stops the execution of the program.

Setting a read watchpoint for a variable using ‘rwatch’

rwatch is the command which is used to set a read watchpoint in gdb. Once set, whenever the given variable is read, the program will stop execution. The syntax is similar to ‘awatch’.

(gdb) rwatch b
Hardware read watchpoint 2: b
(gdb) c
Continuing.
Hardware read watchpoint 2: b

Value = 25
0x000000000040052b in main () at a.c:9
9		printf("%d\n",b);

Similarly you can use ‘watch’ to set a write watchpoint in gdb.

Deleting a watchpoint

Watchpoints will be displayed in the breakpoints list.

So, you can use info breakpoints to list the watchpoints and you can delete/disable similar to breakpoints as shown below.

(gdb) info breakpoints 
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000000000400514 in main at a.c:1
	breakpoint already hit 1 time
2       hw watchpoint  keep y                      b
	breakpoint already hit 1 time
(gdb) delete 2