Thread: How can I pass an array from PostgreSQL to C (libpq)?
I'm using the C library (lipq) to grab data from a field that contains a float array (float4[]). I'd like to fill an array in my C program with these values. Unfortunately, Postgres returns the array as a large string instead of individual values. Now I could perform "select array_var[1], array_var[2], array_var[3], ..., array_var[4]" and have each array value come back as a different field, but this seems fairly messy. I suppose I could also go through and parse the characters in the string returned. Has anyone else used any other strategies to fill their C array with an array returned from the libpq? Perhaps there are some SQL functions for arrays that might exist? Thanks. -Tony
Tony Reina wrote: > I'm using the C library (lipq) to grab data from a field that contains > a float array (float4[]). I'd like to fill an array in my C program > with these values. Unfortunately, Postgres returns the array as a > large string instead of individual values. > > Now I could perform "select array_var[1], array_var[2], array_var[3], > ..., array_var[4]" and have each array value come back as a different > field, but this seems fairly messy. I suppose I could also go through > and parse the characters in the string returned. > > Has anyone else used any other strategies to fill their C array with > an array returned from the libpq? Perhaps there are some SQL functions > for arrays that might exist? > PQgetvalue is always going to give you the string representation of the field value *unless* you specify a binary cursor. In that case you'll get all requested fields in binary form, and it's your responsibility to deal with making sense of what you get. There are some backend macros and functions that can help you get at array values. Take a look at src/utils/adt/arrayfuncs.c and src/include/utils/array.h. You might want to read through contrib/array and/or contrib/intarray for examples. You could also consider writing your array manipulation code in a user defined C function and just return the end result to the libpq program. It's hard to know if this is an option without more detail on what you're trying to do. Joe
Yes, in the past I had used binary cursor and things seemed to work fine. However, recently when I ran the binary cursor stuff, the array ends up getting shifted to the right one index. So what should be placed in array_var[n] was getting placed in array var[n+1]. Plus, I was always a little concerned with the binary cursor when going across platforms (e.g. endianess, alignment). I ended up just writing a little script that parses the string for the comma separators. That works fine, but wanted to see if others had more elegant solutions. I'll take a look at the /utils directory you mentioned. Perhaps there are functions that I'm not aware of (e.g. I didn't know there was a built in function called array_dims that gives the array dimensions). -Tony --On Wednesday, March 26, 2003 11:07 AM -0800 Joe Conway <mail@joeconway.com> wrote:r > > PQgetvalue is always going to give you the string representation of the > field value *unless* you specify a binary cursor. In that case you'll get > all requested fields in binary form, and it's your responsibility to deal > with making sense of what you get. There are some backend macros and > functions that can help you get at array values. Take a look at > src/utils/adt/arrayfuncs.c and src/include/utils/array.h. You might want > to read through contrib/array and/or contrib/intarray for examples. > > You could also consider writing your array manipulation code in a user > defined C function and just return the end result to the libpq program. > It's hard to know if this is an option without more detail on what you're > trying to do. > > Joe >
On Wed, 26 Mar 2003, G. Anthony Reina wrote: See contrib/intarray > Yes, in the past I had used binary cursor and things seemed to work fine. > However, recently when I ran the binary cursor stuff, the array ends up > getting shifted to the right one index. So what should be placed in > array_var[n] was getting placed in array var[n+1]. Plus, I was always a > little concerned with the binary cursor when going across platforms (e.g. > endianess, alignment). > > I ended up just writing a little script that parses the string for the > comma separators. That works fine, but wanted to see if others had more > elegant solutions. I'll take a look at the /utils directory you mentioned. > Perhaps there are functions that I'm not aware of (e.g. I didn't know there > was a built in function called array_dims that gives the array dimensions). > > -Tony > > > --On Wednesday, March 26, 2003 11:07 AM -0800 Joe Conway > <mail@joeconway.com> wrote:r > > > > PQgetvalue is always going to give you the string representation of the > > field value *unless* you specify a binary cursor. In that case you'll get > > all requested fields in binary form, and it's your responsibility to deal > > with making sense of what you get. There are some backend macros and > > functions that can help you get at array values. Take a look at > > src/utils/adt/arrayfuncs.c and src/include/utils/array.h. You might want > > to read through contrib/array and/or contrib/intarray for examples. > > > > You could also consider writing your array manipulation code in a user > > defined C function and just return the end result to the libpq program. > > It's hard to know if this is an option without more detail on what you're > > trying to do. > > > > Joe > > > > > ---------------------------(end of broadcast)--------------------------- > TIP 2: you can get off all lists at once with the unregister command > (send "unregister YourEmailAddressHere" to majordomo@postgresql.org) > -- ================================================================== Achilleus Mantzios S/W Engineer IT dept Dynacom Tankers Mngmt Nikis 4, Glyfada Athens 16610 Greece tel: +30-210-8981112 fax: +30-210-8981877 email: achill@matrix.gatewaynet.com mantzios@softlab.ece.ntua.gr