gdb cannot index into an array and print value

Question

I have an object a of type vector<char>. I know it is 10 elements long. But, when I do p a[5] in gdb, I get an error saying gdb could not resolve symbol operator[].

void foo()
{
    vector<char> a (10, 10); // put a breakpoint here.
}

Could you please suggest me a way to print the value at index in gdb? Right now, I did something really lame -

char c1=a[1]; char c2 = a[2]; char c3 = a[3]; // ... so on and forth

Can I trust the values in c1, c2, etc

P.S. I have built my code with DEBUG symbols.

grigor · Accepted Answer · 2016-08-09 02:42:14Z

2

A vector is not the same as an array or a pointer. A vector typically has a pointer to the start, which you can use to access elements. Try something like this

p *(a._M_impl._M_start+5)

Here I'm accessing the pointer to the start of the array, adding 5 to get the element at index 5.

answered Aug 9, 2016 at 2:42

grigor

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1 Comment

user202729 Over a year ago

Note that a._M_impl._M_start[5] works as well, and it's possible to use tab-completion.

Sam Varshavchik · Accepted Answer · 2016-08-09 02:37:20Z

1

This works without issues with gdb 7.11.1

(gdb) b main
Breakpoint 1 at 0x400bd3: file t.C, line 6.
(gdb) run
Starting program: /tmp/t 
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.23.1-8.fc24.x86_64

Breakpoint 1, main () at t.C:6
6       std::vector<char> a={'a','b','c','d','e','f'};
Missing separate debuginfos, use: dnf debuginfo-install libgcc-6.1.1-3.fc24.x86_64 libstdc++-6.1.1-3.fc24.x86_64
(gdb) n
8       std::cout << "Ok" << std::endl;
(gdb) p a
$1 = std::vector of length 6, capacity 6 = {97 'a', 98 'b', 99 'c', 100 'd', 
  101 'e', 102 'f'}
(gdb) p a[2]
$2 = 99 'c'

The answer here is to update to the current version of gcc and gdb.

answered Aug 9, 2016 at 2:37

Sam Varshavchik

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2 Comments

The Vivandiere Over a year ago

Did you write a custom pretty printer?

Sam Varshavchik Over a year ago

No. Stock gdb, on Fedora 24. No customizations.

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