Cannot run debug after printf statement in gdb

Question

trying to debug a small program but cannot continue debugging after printf(), please help! this is the code:

#include <stdio.h>
#include <stdlib.h>

struct dll {
    struct dll* prev;
    int data;
    struct dll* next;
};
struct dll* head;

int main() {
    struct dll *p1, *p2, *p3, *p4, *p5, *temp;
    struct dll * add=NULL;
    p1 = (struct dll *)malloc(sizeof(struct dll));
    p2 = (struct dll *)malloc(sizeof(struct dll));
    p3 = (struct dll *)malloc(sizeof(struct dll));
    p4 = (struct dll *)malloc(sizeof(struct dll));
    p5 = (struct dll *)malloc(sizeof(struct dll));
    p1->prev = NULL;
    p1->data = 1;
    p1->next = p2;
    p2->prev = p1;
    p2->data = 2;
    p2->next = p3;
    p3->prev = p2;
    p3->data = 3;
    p3->next = p4;
    p4->prev = p3;
    p4->data = 4;
    p4->next = p5;
    p5->prev = p4;
    p5->data = 5;
    p5->next = NULL;
    head=p1;

    int count=0, pos=0,i=0;
    printf("add of p1::%p add of p2::%p add of p3::%p add of p4::%p add of p5::%p\n", p1, p2, p3, p4, p5);
    for ( temp = p1; temp != NULL; temp = temp->next ){
        count++;
    }
    if(count%2==0)
        puts("even nodes so no midlle node");
    else if(count%2!=0)
    {
        pos=count/2;
        printf("pos::%d\n",pos+1);
        for ( i=0; i<=pos; i++)

        {
            for ( temp = p1;temp != NULL;temp = temp->next )
            {

                if(i==pos)
                    printf("middle node is %p\n",temp);
            }
        }
    }
    return 0;
}

Debugging output:

(gdb) b 47
Breakpoint 1 at 0x8048643: file sony.c, line 47.
(gdb) b 57
Breakpoint 2 at 0x8048696: file sony.c, line 57.
(gdb) run
Starting program: /home/jeevan/Documents/jvt/ds/sony 
add of p1::0x804b008 add of p2::0x804b018 add of p3::0x804b028 add of p4::0x804b038 add of p5::0x804b048
pos::3

Breakpoint 1, main () at sony.c:47
47  for ( i=0; i<=pos; i++)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
47  for ( i=0; i<=pos; i++)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
47  for ( i=0; i<=pos; i++)
(gdb) s
50  for ( temp = p1;temp != NULL;temp = temp->next )
(gdb) s
53  if(i==pos)
(gdb) s
54  printf("middle node is %p\n",temp);
(gdb) s
__printf (format=0x804879d "middle node is %p\n") at printf.c:28
28  printf.c: No such file or directory.
(gdb) s
__x86.get_pc_thunk.bx () at ../sysdeps/i386/i686/multiarch/strcat.S:55
55  ../sysdeps/i386/i686/multiarch/strcat.S: No such file or directory.
(gdb) s
__printf (format=0x804879d "middle node is %p\n") at printf.c:32
32  printf.c: No such file or directory.
(gdb) s
33  in printf.c
(gdb) s
_IO_vfprintf_internal (s=0xb7fb3ac0 <_IO_2_1_stdout_>, format=format@entry=0x804879d "middle node is %p\n", 
    ap=ap@entry=0xbfffef44 "\b\260\004\b\030\260\004\b(\260\004\b8\260\004\bH\260\004\b\001") at vfprintf.c:235
235 vfprintf.c: No such file or directory.
(gdb) s
270 in vfprintf.c

After printf() I am stuck in a loop, I don’t understand what it is telling and how much time should I wait to go to next statement?

It's trying to locate the source code for printf, but can't find it. So it's just stepping through instruction by instruction. — Oliver Charlesworth
– Oliver Charlesworth, Commented Aug 30, 2014 at 17:01
You probably want to use next instead of step to iterate through your statements in gdb. — Bill Lynch
– Bill Lynch, Commented Aug 30, 2014 at 17:02
Yes thanks Oli Charlesworth, I don't know about next, I am learning on my own! I missed next somehow! I normally use s or si or stepi. Thanks for your time! — jeevan
– jeevan, Commented Aug 30, 2014 at 17:12
Yes thanks Sharth, I don't know about next, I am learning on my own! I missed next somehow! I normally use s or si or stepi. Thanks for your time! — jeevan
– jeevan, Commented Aug 30, 2014 at 17:13

wallyk · Accepted Answer · 2014-08-30 17:10:44Z

It runs okay without the debugger for me. And using "next" instead of "step" is important: (Note I have reformatted your code for readability.)

[wally@lenovoR61 ~]$ gdb t
GNU gdb (GDB) Fedora (7.3.50.20110722-16.fc16)
Copyright (C) 2011 Free Software Foundation, Inc.
...
Reading symbols from /home/wally/t...done.
(gdb) start
Temporary breakpoint 1 at 0x804842d: file t.c, line 14.
Starting program: /home/wally/t 

Temporary breakpoint 1, main () at t.c:14
14      p1 = malloc(sizeof(struct dll));
(gdb) s
__GI___libc_malloc (bytes=12) at malloc.c:2914
2914    {
(gdb) 
2919        = force_reg (__malloc_hook);
(gdb) 
2920      if (__builtin_expect (hook != NULL, 0))
(gdb) 
2921        return (*hook)(bytes, RETURN_ADDRESS (0));
(gdb) 
malloc_hook_ini (sz=12, caller=0x8048439) at hooks.c:30
30  {
(gdb) 
31    __malloc_hook = NULL;
(gdb) 
32    ptmalloc_init();
(and further into obscurity)

But, with next:

[wally@lenovoR61 ~]$ gdb t
GNU gdb (GDB) Fedora (7.3.50.20110722-16.fc16)
Copyright (C) 2011 Free Software Foundation, Inc.
 ...
Reading symbols from /home/wally/t...done.
(gdb) start
Temporary breakpoint 1 at 0x804842d: file t.c, line 14.
Starting program: /home/wally/t 

Temporary breakpoint 1, main () at t.c:14
14      p1 = malloc(sizeof(struct dll));
(gdb) n
15      p2 = malloc(sizeof(struct dll));
(gdb) 
16      p3 = malloc(sizeof(struct dll));
(gdb) 
17      p4 = malloc(sizeof(struct dll));
(gdb) 
18      p5 = malloc(sizeof(struct dll));
(gdb) 
19      p1->prev = NULL;    p1->data = 1;   p1->next = p2;
(gdb) 
20      p2->prev = p1;      p2->data = 2;   p2->next = p3;
(gdb) 
21      p3->prev = p2;      p3->data = 3;   p3->next = p4;
(gdb) 
22      p4->prev = p3;      p4->data = 4;   p4->next = p5;
(gdb) 
23      p5->prev = p4;      p5->data = 5;   p5->next = NULL;
(gdb) 
24      head=p1;
(gdb) 
26      int count=0, pos=0, i=0;
(gdb) 
27      printf("add of p1::%p add of p2::%p add of p3::%p add of p4::%p add of p5::%p\n", p1, p2, p3, p4, p5);
(gdb) 
add of p1::0x804a008 add of p2::0x804a018 add of p3::0x804a028 add of p4::0x804a038 add of p5::0x804a048
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
29          count++;
(gdb) 
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
29          count++;
(gdb) 
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
29          count++;
(gdb) 
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
29          count++;
(gdb) 
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
29          count++;
(gdb) 
28      for (temp = p1; temp != NULL; temp = temp->next)
(gdb) 
31      if (count%2==0)
(gdb) 
35          pos=count/2;
(gdb) 
36          printf("pos::%d\n",pos+1);
(gdb) 
pos::3
37          for (i=0; i<=pos; i++)
(gdb) 
39              for (temp = p1;temp != NULL;temp = temp->next)
(gdb) 
41                  if (i==pos)
(gdb) 
39              for (temp = p1;temp != NULL;temp = temp->next)
(gdb)
  (and continues to work well)

Ha yes by using next it is fine! Thanks I don't know about next, I am learning on my own! I normally use s or si or stepi. Thanks a lot!

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