C++ C++ Debugging and Debug-prevention Tools & Techniques Segfault analysis with GDB
Example
Lets use the same code as above for this example.
#include <iostream>
void fail() {
int *p1;
int *p2(NULL);
int *p3 = p1;
if (p3) {
std::cout << *p2 << std::endl;
}
}
int main() {
fail();
}
First lets compile it
g++ -g -o main main.cpp
Lets run it with gdb
gdb ./main
Now we will be in gdb shell. Type run.
(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/opencog/code-snippets/stackoverflow/a.out
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400850 in fail () at debugging_with_gdb.cc:11
11 std::cout << *p2 << std::endl;
We see the segmentation fault is happening at line 11. So the only variable being used at this line is pointer p2. Lets examine its content typing print.
(gdb) print p2
$1 = (int *) 0x0
Now we see that p2 was initialized to 0x0 which stands for NULL. At this line, we know that we are trying to dereference a NULL pointer. So we go and fix it.
