Re: Printing a 2D array in a C program

Neven Sajko
2016-03-04T16:15:32Z

On 4 March 2016 at 15:42, Jan Kratochvil <jan.kratochvil@redhat.com> wrote:
> On Fri, 04 Mar 2016 15:27:14 +0100, Neven Sajko wrote:
>> In my C program there is a square matrix implemented as an array
>> of arrays, which prints nicely in GDB with `info locals`.
>
> I do not understand how that can happen.
> You should always provide sample code / example.
>
>
>> But how can that nice output be accomplished if you are not in
>> the function in which the array was declared, but just have it
>> passed as a function parameter.
>
> If it is a pointer (as you say C, not C++ reference) you should be able to:
> (gdb) print *thatpointername
> Then GDB should IMO print it the same as a local variable is printed.
>
>
> Printing of C++ vectors of vectors (which is BTW wrong data structure for a 2D
> matrix anyway) as a matrix was implemented by Chris Moller as a Python Pretty
> printer in some versions of Fedora GDB but it was later discontinued. The
> initial implementation was:
> http://pkgs.fedoraproject.org/cgit/rpms/gdb.git/commit/?id=6068e6305ed7d05b4a919c28aa5bcb737e1f163b
> (gdb) p test2
> $2 =
> {
> {0 1 }
> {2 3 }
> {4 5 }
> }
>
>
>
> Jan
Thanks for your answer, Jan.

In my question I asked about C (not C++), I even mentioned the
gcc -std=gnu90 option.

My code is thus:

enum {
sz = 17
};

void p(int m[sz][sz], int n) {
int i;
for (i=1; i<n; i++) {
int j;
for (j=i-1; 0<=j; j--) {
m[i][j] = abs(3*m[i-1][j] +2*m[i][j+1]) % 9340506;
}
}
}

void f(int n) {
int m[sz][sz];

g(m, n);

p(m, n);

r(m, n);
}

So, when I am in f, `info locals` prints it like {{a11, ...,
a1n}, ..., {an1, ..., ann}}.
But in p `print *m` just gets me {a11, ..., a1n}.