Thread: newbie libpq question...

I'm trying to use the libpq library for the first time and I'm
experiencing what seems to be a very basic problem.  When I run the
following:
       res = PQexec(conn, "select blah blah blah");       test_int=PQntuples(res);       fprintf(stdout,"number of
rows:%f\n",test_int);       test_int=PQnfields(res);       fprintf(stdout,"number of cols: %f\n",test_int);
 

where "blah blah blah" is the selection criteria, I get zero rows and
zero columns.  When I do the exact same "select" query through the psql
interface I get 547 rows and 3 columns.  I've checked that
PQresultStatus = PGRES_TUPLES_OK, so it seems like it completed the
command correctly.  I'm new to postgres and I haven't used C in a long
while so I'm a little rusty there.  Am I missing something obvious?

thanks in advance for any insight,
dh

Doug Homoelle <homoelle@ll.mit.edu> writes:
>         res = PQexec(conn, "select blah blah blah");
>         test_int=PQntuples(res);
>         fprintf(stdout,"number of rows: %f\n",test_int);
>         test_int=PQnfields(res);
>         fprintf(stdout,"number of cols: %f\n",test_int);

If test_int is an integer, you want %d or %i in the format, not %f ...
        regards, tom lane

On Mon, 08 Nov 2004 17:36:09 -0500
Doug Homoelle <homoelle@ll.mit.edu> wrote:
> I'm trying to use the libpq library for the first time and I'm
> experiencing what seems to be a very basic problem.  When I run the
> following:
> 
>         res = PQexec(conn, "select blah blah blah");
>         test_int=PQntuples(res);
>         fprintf(stdout,"number of rows: %f\n",test_int);
>         test_int=PQnfields(res);
>         fprintf(stdout,"number of cols: %f\n",test_int);
> 
> where "blah blah blah" is the selection criteria, I get zero rows and

I think you need "number of cols: %d\n" there.  Those functions returns
int.

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On Mon, Nov 08, 2004 at 06:17:31PM -0500, Tom Lane wrote:
> Doug Homoelle <homoelle@ll.mit.edu> writes:
> If test_int is an integer, you want %d or %i in the format, not %f ...

Plus, the singular for "criteria" is "criterion."  :-P

That worked.  I guess I would have expected an error if there was a type
mismatch, but like I said, my C is pretty rusty.

> I think you need "number of cols: %d\n" there.  Those functions returns
> int.

Doug Homoelle wrote:
> That worked.  I guess I would have expected an error if there was a type
> mismatch, but like I said, my C is pretty rusty.
> 

If you are using gcc you can add -Wformat to your compile options to 
have it report if your printf() arguments don't match the format 
specifier - I don't know if other compilers do this, or how to turn it 
on if they do.

Note that for the compiler to know if your printf() arguments are 
correct, you have to supply the format string as a string literal to the 
call.  As an example, see the following program:


$ cat print.c

#include <stdio.h>

int main(void)
{    const char *format = "d = %f\n";    int d = 0;
    printf(format, d);
    d = 1;    printf("d = %f\n", d);
    return 0;
}

$ gcc -Wall -W print.c -o print
print.c: In function `main':
print.c:11: warning: double format, different type arg (arg 2)



-- 
Edmund Bacon <ebacon@onesystem.com>