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POImplementing C extension functions for PostgreSQL - how do I do this? (passing data between C/PostgreSQL)
 

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  1. POImplementing C extension functions for PostgreSQL - how do I do this? (passing data between C/PostgreSQL)
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    <p>I am writing a C extension for PostgreSQL (v 8.4). I am currently stuck on how to pass columnar data from PostgreSQL to my C functions. I also have a question of memory ownership, as PostgreSQL seems to do a lot of memory management beind the scenes.</p> <p>I would be grateful if someone could help me "join the dots", in order to get a basic skeleton code base upon which I could build the library from.</p> <p>This is what I have so far:</p> <pre><code>/*******************************************************/ /* C header file */ /*******************************************************/ typedef struct _myarray { double *data; size_t len; } MyArray; MyArray * NEW_MyArray(const size_t len); void Destroy_MyArray(MyArray * arr); size_t NumElements_MyArray(MyArray * arr); /* trivial function returns number of elements */ MyArray * NotTrivial_MyArray(MyArray * arr); /* non trivial function returns MyArray (a float8[] in PG) */ double HeapFunc_MyArray(MyArray * arr); /* allocs from heap */ /*******************************************************/ /* C Source file */ /*******************************************************/ /* utility conversion funcs */ /* How do I convert from the structure returned by array_agg to float8[] (or int4[] ?) */ MyArray * NEW_MyArray(const size_t len){ /* Do I use palloc0() or calloc() here ? */ } void Destroy_MyArray(MyArray * arr){ /* Do I use pfree() or free() here ? */ } size_t NumElements_MyArray(MyArray * arr){ assert(arr != 0); return arr-&gt;len; } MyArray * NotTrivial_MyArray(MyArray * arr){ assert(arr != 0); MyArray * ptr = NEW_MyArray(arr-&gt;len); return ptr; } double HeapFunc_MyArray(MyArray * arr){ /* Create temporary variables on heap (use palloc0() or calloc()?) */ /* Cleanup temp variables (use pfree() or free() ? */ return 42/1.0; } /*******************************************************/ /* PostgreSQL wrapper funcs implementation source file */ /*******************************************************/ /* Prototypes */ PG_FUNCTION_INFO_V1(test_num_elements); PG_FUNCTION_INFO_V1(test_not_trivial); PG_FUNCTION_INFO_V1(test_heapfunc); Datum test_num_elements(PG_FUNCTION_ARGS); Datum test_not_trivial(PG_FUNCTION_ARGS); Datum test_heapfunc(PG_FUNCTION_ARGS); Datum test_num_elements(PG_FUNCTION_ARGS) { /* Convert data returned by array_agg() into MyArray * (how?) */ /* invoke NumElements_MyArray() */ /* Do I free temporary MyArray * ptr or will PG clean up - if I have to clean up (like I suspect), do I use pfree() or free() ?*/ PG_RETURN_INT32(result); } Datum test_not_trivial(PG_FUNCTION_ARGS) { /* Ditto, as above */ PG_RETURN_POINTER(/* utility function to convert MyArray* to float8[] equiv for PG (how) */); } Datum test_heapfunc(PG_FUNCTION_ARGS) { /* Ditto, as above */ PG_RETURN_FLOAT8(result); } -- SQL FUNCTIONS CREATE OR REPLACE FUNCTION test_num_elements(float8[]) RETURNS int4 AS '$libdir/pg_testlib.so' LANGUAGE 'c'; CREATE OR REPLACE FUNCTION test_not_trivial(float8[]) RETURNS float8[] AS '$libdir/pg_testlib.so' LANGUAGE 'c'; CREATE OR REPLACE FUNCTION test_heapfunc(float8[]) RETURNS float8 AS '$libdir/pg_testlib.so' LANGUAGE 'c'; -- SQL TEST SELECT test_num_elements(array_agg(salary)) FROM employees; SELECT test_not_trivial(array_agg(salary)) FROM employees; SELECT test_heapfunc(array_agg(salary)) FROM employees; </code></pre> <p>In summary, my questions are:</p> <ol> <li>How do I convert columnar data from array_agg() into a C array of doubles (or ints as the case may be)</li> <li>How do I convert a C array of ints (or doubles) back into int4[] or float8[] for consumption in PostgreSQL?</li> <li>Memory allocation principles - do I use the PostgreSQL memory management functions palloc()/ pfree() or can I use calloc/free?. Also, when using the PG mem funcs, am I responsible for freeing memory I allocated?</li> </ol>
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