Stack trace

From Free net encyclopedia

A stack trace (also called backtrace) is a report of the active stack frames instantiated by the execution of a program.

Stack traces are often generated when a program terminates abnormally. The last few stack frames often indicate where the bug that caused the abnormal termination occurs.

All debuggers can produce stack traces. gdb prints a stack trace with the bt (or where) command.

For example, this intentionally ill-written C program will segfault (i.e., crash) in the function function_2:

#include <stdio.h>

int
main(void)
{
int x;
printf("This program will demonstrate gdb\n");
x=function_1();
printf("%d", x); 
return 0;
}

int
function_1(void)
{
int x = function_2(24);
return x;
}

int
function_2(int x)
{
int *y = (int *)x;
return *y;
}

To get an informative stack trace from a debugger, one has to compile the program with debugging information. With gcc, that is done by compiling the program with the -g option. If one then attempts to run the program in gdb, and obtain a backtrace, one would get

#0 0x080483cb in function_2 ()
#1 0x080483b4 in function_1 ()
#2 0x08048385 in main ()
#3 0x4003ddc6 in __libc_start_main () from /lib/libc.so.6

This shows that the function __libc_start_main called main, which in turn called function_1 and then function_2, whose stack frame is at the top of the stack, and it is indeed this function which is in error, the statement:

int *y = (int *)x;

attempts to create a pointer pointing to a nonsensical memory location at the decimal address 24, which is usually inaccessible by programs running normally.