Re: Postgresql 9.5 and Shell scripts/variables vs. C programming/defining a value to be used

I think I understand.
When I use this in my code I get
"undefined reference to `PQexecParms'
when compiling.

references in main is

const char *conninfo; /* connection string to the database */
PGconn *conn; /* connection to the database */
PGresult *res; /* result of sql query */
int nFields; /* print out the attribute names */
int i; / * print the columns */
j;

Is the a reserved reference to use with

Reserved res = PQexecParms(conn.... )

Then I assume that I have to use another reference than res.

Thanks.

/Poul

2016-11-22 0:48 GMT+01:00 John McKown <john(dot)archie(dot)mckown(at)gmail(dot)com>:

> On Mon, Nov 21, 2016 at 11:22 AM, Poul Kristensen <bcc5226(at)gmail(dot)com>
> wrote:
>
>> Thank you for fast repons!
>>
>> The $1 substitution below. I assume that it refers to "joe's place". But
>> it is not very clear to me, how "joe's place" will appear instead of $1
>> when running. Where is it possiible to read more about this? There just
>> is'nt much about substitution in C online. Any recommended books to buy?
>>
>>
>> /* Here is our out-of-line parameter value */
>> paramValues[0] = "joe's place";
>>
>> res = PQexecParams(conn,
>> "SELECT * FROM test1 WHERE t = $1",
>> 1, /* one param */
>> NULL, /* let the backend deduce param type */
>> paramValues,
>> NULL, /* don't need param lengths since text */
>> NULL, /* default to all text params */
>> 1); /* ask for binary results */
>> }
>>
>> /Poul
>>
>>
>>
> ​It is described better here: https://www.postgresql.org/
> docs/9.6/static/libpq-exec.html
> than I can do. But I just noticed a mistake in your code, or maybe just
> something left out. I would say:
>
> char *value1 = "joe's place";
> ​​
> char **paramV
> ​a​
> lues = &value1;
> ​ /* closer match to the documentation's syntax */​
>
> //char *paramValues[] = {"joe's place"}; /* same as above, different
> syntax */
> //
> //char *paramValues[1]; /* this looks to be missing */
> //paramValues[0]="joe's place"; /* what you had */
> res = PQexecParms(conn,
> "SELECT * FROM test1 WHERE t = $1",
> 1, /* there is only 1 entry in paramValues array */
> paramValues, /* address of parameter value array */
> NULL, /* don't need param lengths since text */
> NULL, /* defaul to all text params */
> 1); /* return all values as binary */
>
> Well, you have an array of pointers to characters called paramValues. The
> $1 refers to whatever is pointed to by paramValues[0]​, which is a pointer
> to value1 which is a C "string". Basically in the second parameter, the
> command, the $n is used as a 1-based index into the paramValues[] array.
> This means that the actual C language array value is one less (since C
> arrays are 0-based). Which means that "$n" (n>=1) in the "command" string
> refers to value pointed to by paramValues[n-1]. The 3rd value, 1 in this
> case, tells PQexecParms how many entries there are in the paramValues[]
> array. I guess this is a type of validity check that the $n in the command
> string is not too large for the array.
>
> Note: please keep the discussion on the list, not to me personally. It may
> be of help to others (or maybe not, I don't know.)
>
> --
> Heisenberg may have been here.
>
> Unicode: http://xkcd.com/1726/
>
> Maranatha! <><
> John McKown
>

--
Med venlig hilsen / Best regards
Poul Kristensen
Linux-OS/Virtualizationexpert and Oracle DBA