Print the values of variables of a structure array in GDB

Question

Here is the definition of structure in C:

typedef struct projection {
    angle_t angle;
    int size;
    element_t *element;
} projection_t;

projection_t *projections;
projections = (projection_t *)malloc(sizeof(projection_t)*m);

An array of this structure is defined and values are assigned to each projection's variables. Using GDB, I am trying to display the value of the size of each projection. Printing the values one by one is a heavy process.

I know that the content of arrays can be displayed with print *projections@len but I do not know if it is possible to access easily the values of projection's size with the @ operand.

-1 You rejected two very good answers that suggested using python or a few instructions in gdb as not being convenient, and then modified your C code instead? You should have clarified your definition of "easily" so readers wouldn't waste their time trying to help you. — user1619508
– user1619508, Commented Jan 21, 2014 at 12:02
These answers bring great information to solve the problem, as I highlight in my comments. It is definitely neither a waste of time nor should it be considered as rejected as I am sure it can help other users that know Python. Yes, I consider adding few instructions in the C code and using the 'call' function a good alternative as it might be helpful for everyone. — denaitre
– denaitre, Commented Jan 21, 2014 at 12:12

nikpel7 · Accepted Answer · 2014-01-20 14:19:20Z

16

While not using the @ operand you can try the following to achieve your goal:

(gdb)set $i=0
(gdb) set $end=m
(gdb) while ($i < $end)
 >p projections[$i++].size
 >end

or use

p projections[index].size

to print the size for the given index.

edited Jan 20, 2014 at 14:19

answered Jan 20, 2014 at 14:13

nikpel7

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3 Comments

Hasturkun Over a year ago

The documentation suggests something similar, without the loop.

nikpel7 Over a year ago

Yes, but without the loop no boundary check is provided. Meaning that if someone goes on pressing <RET> then he might end up inspecting memory out of the specified size of the projections array.

denaitre Over a year ago

Thanks for the reply. Yes, both work but are not convenient in my opinion. The first one requires to write a loop and associated variables, and the second one is what I meant by "one by one" in my post.

score 15 · Accepted Answer · 2014-01-21 04:30:00Z

You can register python pretty prointer: https://sourceware.org/gdb/onlinedocs/gdb/Writing-a-Pretty_002dPrinter.html and use it to get something like this:

(gdb) p *projections@10
$1 = {10, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
(gdb)

This is example of a python pretty printer:

>cat my_printer.py
class projection_printer:
    def __init__(self, val):
        self.val = val

    def to_string(self):
        return str(self.val['size'])


import gdb.printing

def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter("")
    pp.add_printer('projection', '^projection$', projection_printer)
    return pp
gdb.printing.register_pretty_printer( gdb.current_objfile(),  build_pretty_printer())

This is a test program:

>cat main.cpp
#include <stdlib.h>

typedef struct angle
{
  int a;
} angle_t;


typedef struct projection {
    angle_t angle;
    int size;
} projection_t;


int main()
{
  projection_t *projections;
  projections = (projection_t *)malloc(sizeof(projection_t)*10);
  projections[0].size = 10;
  projections[0].angle.a = 20;

  return 0;
}

And this is a gdb session:

>gdb -q -x my_printer.py a.out
Reading symbols from /home/a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x4005ac: file main.cpp, line 18.
Starting program: /home/a.out

Temporary breakpoint 1, main () at main.cpp:18
18        projections = (projection_t *)malloc(sizeof(projection_t)*10);
(gdb) n
19        projections[0].size = 10;
(gdb)
20        projections[0].angle.a = 20;
(gdb)
22        return 0;
(gdb) p *projections@10
$1 = {10, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
(gdb)

Thanks for the reply :) I voted up as it works and is it supported by GDB's documentation. However, this solution relies on Python which is not a simple solution in my case.

denaitre · Accepted Answer · 2014-01-21 12:02:04Z

1

I did not want to write a loop each time I launch GDB, and pretty-printers rely on Python which does not fit me. Printing the value of variables from non contiguous memory spaces seems not to be easily feasible in GDB.

I end up finding an alternative way which seems to be the easiest one. I wrote a C function in the code to print the values I need to get:

void print(projection_t *projections, int size)
{
    int i;
    for(i=0; i<size; i++)
        printf("proj_%d: size=%d\n", i, projections[i].size);
}

It is possible to call the function from GDB when I want to print the size of each projections:

(gdb) call print(projections, len)
proj_0: size=1027
proj_1: size=1024
proj_2: size=1027
proj_3: size=1030

edited Jan 21, 2014 at 12:02

answered Jan 21, 2014 at 11:54

denaitre

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Comments

Jonathan Koren · Accepted Answer · 2020-06-15 20:37:17Z

1

Define a function that prints out each element in a loop. You only need to define the function once. Set $m equal to number of projections you allocated. Then, you can print out everything with one command line input.

(gdb) set $m=4 

(gdb) define f
>set $i=0
>while ($i<$m)
> p projections[$i++].size
> end
>end

(gdb) f

answered Jun 15, 2020 at 20:37

Jonathan Koren

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1 Comment

YuryChu Over a year ago

f - is short for "frame <#N>" command. Pick up another name.

bbecheriu · Accepted Answer · 2021-09-01 20:23:29Z

0

(gdb) p *(projection_t (*)[10])projections

edited Sep 1, 2021 at 20:23

answered Sep 1, 2021 at 20:18

bbecheriu

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