On Mon, Nov 21, 2016 at 11:22 AM, Poul Kristensen <bcc5...@gmail.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
