CODEWITHSUNDEEP
cgdbdynamic-arraysvariable-length-array
Phi
Phi

Reputation: 814

GDB C how to print VLA entries (dynamic array)

I post this after a quick search on stackoverflow...

I got trouble to print a VLA entry from GDB, here is my streamlined C fragment.

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

typedef struct  
{ int i, j; 
} c_t;

int f(int l, int c, c_t a[l][c])
{ for(int i=0;i<l;i++)
  { for(int j=0;j<c;j++)
    { a[i][j].i=i; a[i][j].j=j; 
    }
  }
}

int main(int ac, char **av)
{ int l=(int)atoi(av[1]), c=(int)atoi(av[2]);
  c_t a[l][c];
  return(f(l,c,a));
}

I compile it

$ cc -o g g.c -g -O0

Then run GDB on it, and after a couple of iteration in f() I try to display a[i][j]

$ gdb  ./g
GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
...[snip]...
Reading symbols from ./g...done.
(gdb) r 2 3
Breakpoint 1 at 0x76b: file g.c, line 17.

Breakpoint 1, main (ac=3, av=0x7fffffffe608) at g.c:17
17      { int l=(int)atoi(av[1]), c=(int)atoi(av[2]);
(gdb) n
18        c_t a[l][c];
(gdb) 
19        return(f(l,c,a));
(gdb) s
f (l=2, c=3, a=0x7fffffffe440) at g.c:8
8       int f(int l, int c, c_t a[l][c])
(gdb) n
9       { for(int i=0;i<l;i++)
(gdb) 
10        { for(int j=0;j<c;j++)
(gdb) 
11          { a[i][j].i=i; a[i][j].j=j; 
(gdb) 
10        { for(int j=0;j<c;j++)
(gdb) 
11          { a[i][j].i=i; a[i][j].j=j; 
(gdb) 
10        { for(int j=0;j<c;j++)
(gdb) 
11          { a[i][j].i=i; a[i][j].j=j; 
(gdb) 
10        { for(int j=0;j<c;j++)
(gdb) 
9       { for(int i=0;i<l;i++)
(gdb) p a[0][0]
Cannot perform pointer math on incomplete types, try casting to a known type, or void *.
(gdb)

From there I am stuck, tried various cast with no success.

Thanx in advance for any pointers (may be pointers are better than arrays :) )

Phi

Upvotes: 2

Views: 360

Answers (3)

jxh
jxh

Reputation: 70472

When you print a from the debugger, you should get a big hint:

...
Breakpoint 1, f (l=2, c=3, a=0x7fffffffe1b0) at x.c:8
8       int f(int l, int c, c_t a[l][c])
(gdb) p a
$1 = (c_t (*)[variable]) 0x7fffffffe1b0

The debugger understands that a is a pointer to a VLA of c_t, but feigns ignorance on the actual length of that array. However, you know that it is 3 (the value of c). So you can simply cast it as such.

(gdb) p c
$2 = 3
(gdb) p (c_t (*)[3])a
$3 = (c_t (*)[3]) 0x7fffffffe1b0
(gdb) p $3[0][0]
$4 = {i = -7552, j = 32767}

Upvotes: 3

Antti Haapala -- Слава Україні
Antti Haapala -- Слава Україні

Reputation: 133978

You can check the dimensions from the variables and make a pointer to an array of defined size. It is not optimal, of course. But that way you can even see the entire array:

(gdb) p ((c_t (*)[3])a)[0][1]
$30 = {i = 0, j = 1}
(gdb) p ((c_t (*)[3])a)[1][1]
$31 = {i = 1, j = 1}
(gdb) p *((c_t (*)[2][3])a)
$29 = {{{i = 0, j = 0}, {i = 0, j = 1}, {i = 0, j = 2}}, {{i = 1, j = 0}
{i = 1, j = 1}, {i = 1, j = 2}}}

Upvotes: 2

Stephan Schlecht
Stephan Schlecht

Reputation: 27126

What's supposed to work always, it's actually pointer arithmetic.

You can use something like the following commands:

(gdb)  p *((c_t*)a+0*3+0)
$3 = {i = 0, j = 0}
(gdb)  p *((c_t*)a+0*3+1)
$4 = {i = 0, j = 1}
(gdb)  p *((c_t*)a+0*3+2)
$5 = {i = 0, j = 2}
(gdb)  p *((c_t*)a+1*3+0)
$6 = {i = 1, j = 0}
(gdb)  p *((c_t*)a+1*3+1)
$7 = {i = 1, j = 1}
(gdb)  p *((c_t*)a+1*3+2)
$8 = {i = 1, j = 2}

BTW: not related to your question: The function signature for f specifies a return value of type int, but does not return anything.

Upvotes: 2

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