Chapter 3. lldb

Consider a C source file named fibonacci.c with the following content:

#include <stdio.h>
#include <limits.h>

int main (int argc, char *argv[]) {
  unsigned long int a = 0;
  unsigned long int b = 1;
  unsigned long int sum;

  while (b < LONG_MAX) {
    printf("%ld ", b);
    sum = a + b;
    a = b;
    b = sum;
  }
  return 0;
}

Enable the debug information and compile the fibonacci.c with the following command:

Start debugging the program with lldb:

The output indicates that the program fibonacci is ready for debugging.

This example assumes that you have successfully compiled the fibonacci.c file as shown in Example 3.1, “Running the lldb Utility on the fibonacci Binary File”

Set two breakpoints at line 10 by running the following commands:

(lldb) b 10
Breakpoint 1: where = fibonacci`main + 33 at fibonacci.c:10, address = 0x000000000040054e

(lldb)  breakpoint set -f fibonacci.c --line 10
Breakpoint 2: where = fibonacci`main + 33 at fibonacci.c:10, address = 0x000000000040054e

This example assumes that you have successfully followed the instructions in Example 3.2, “Setting a New Breakpoint”.

Display the list of currently set breakpoints:

(lldb) breakpoint list
Current breakpoints:
1: file = 'fibonacci.c', line = 10, exact_match = 0, locations = 1
  1.1: where = fibonacci`main + 33 at fibonacci.c:10, address = fibonacci[0x000000000040054e], unresolved, hit count = 0

2: file = 'fibonacci.c', line = 10, exact_match = 0, locations = 1
  2.1: where = fibonacci`main + 33 at fibonacci.c:10, address = fibonacci[0x000000000040054e], unresolved, hit count = 0

This example assumes that you have successfully compiled the fibonacci.c file.

Set a new breakpoint at line 7:

(lldb) b 7
Breakpoint 3: where = fibonacci`main + 31 at fibonacci.c:9, address = 0x000000000040054c

Remove this breakpoint:

(lldb) breakpoint clear -l 7 -f fibonacci.c
1 breakpoints cleared:
3: file = 'fibonacci.c', line = 7, exact_match = 0, locations = 1

This example assumes that you have successfully followed the instructions in Example 3.2, “Setting a New Breakpoint”.

Execute the fibonacci binary file in lldb:

(lldb) run
Process 21054 launched: 'fibonacci' (x86_64)
Process 21054 stopped
* thread #1, name = 'fibonacci', stop reason = breakpoint 1.1
    frame #0: fibonacci`main(argc=1, argv=0x00007fffffffdeb8) at fibonacci.c:10
   7      unsigned long int sum;
   8
   9      while (b < LONG_MAX) {
-> 10       printf("%ld ", b);
   11       sum = a + b;
   12       a = b;
   13       b = sum;

Execution of the program stops at the breakpoint set in Example 3.2, “Setting a New Breakpoint”.

This example assumes that you have successfully followed the instructions in Example 3.5, “Executing the fibonacci Binary File in lldb”. Execution of the fibonacci binary stopped after reaching the breakpoint at line 10.

Display the current values of variables a and b:

(lldb) print a
$0 = 0
(lldb) print b
$1 = 1

This example assumes that you have successfully followed the instructions in Example 3.5, “Executing the fibonacci Binary File in lldb”. The execution of the fibonacci binary stopped after reaching the breakpoint at line 10.

Resume the execution:

(lldb) continue
Process 21580 resuming
Process 21580 stopped
* thread #1, name = 'fibonacci', stop reason = breakpoint 1.1
    frame #0: fibonacci`main(argc=1, argv=0x00007fffffffdeb8) at fibonacci.c:10
   7      unsigned long int sum;
   8
   9      while (b < LONG_MAX) {
-> 10       printf("%ld ", b);
   11       sum = a + b;
   12       a = b;
   13       b = sum;

The execution stops the next time it reaches a breakpoint. (In this case it is the same breakpoint.) Execute the next three lines of code:

(lldb) step -c 3
Process 21580 stopped
* thread #1, name = 'fibonacci', stop reason = step in
    frame #0: fibonacci`main(argc=1, argv=0x00007fffffffdeb8) at fibonacci.c:11
   8
   9      while (b < LONG_MAX) {
   10       printf("%ld ", b);
-> 11       sum = a + b;
   12       a = b;
   13       b = sum;
   14     }

Verify the current value of the sum variable:

(lldb) print sum
$2 = 2