C API

PetraDB provides a native shared library (libpetradb-engine.so / .dylib) with a C API modeled after SQLite. The library is self-contained — no JVM or runtime needed.

Download from GitHub Releases or build from source with sbt engineNative/nativeLink. See Getting Started with C for setup instructions.

Include petradb.h and link with -lpetradb-engine.

Quick Start

#include "petradb.h"

int db = petradb_open();
int conn = petradb_connect(db);

petradb_exec(conn, "CREATE TABLE users (id SERIAL PRIMARY KEY, name TEXT)");
petradb_exec(conn, "INSERT INTO users (name) VALUES ('Alice')");

int cur = petradb_prepare(conn, "SELECT id, name FROM users");
while (petradb_step(cur) == PETRADB_ROW) {
    int id = petradb_column_int(cur, 0);
    const char *name = petradb_column_text(cur, 1);
    printf("%d: %s\n", id, name);
}
petradb_finalize(cur);
petradb_close(db);

Database Lifecycle

int petradb_open(void);                       // in-memory database
int petradb_open_persistent(const char *path); // persistent database
int petradb_close(int db);                     // close and free
int petradb_connect(int db);                   // create a session

Execute Statements

int petradb_exec(int conn, const char *sql);   // returns affected row count, -1 on error

Cursors (Query Results)

int petradb_prepare(int conn, const char *sql); // returns cursor handle
int petradb_step(int cursor);                   // PETRADB_ROW (1), PETRADB_DONE (0), PETRADB_ERROR (-1)
int petradb_finalize(int cursor);               // close cursor

Column Metadata

int petradb_column_count(int cursor);
const char *petradb_column_name(int cursor, int index);

Column Values

int         petradb_column_type(int cursor, int index);     // PETRADB_INTEGER/FLOAT/TEXT/BLOB/NULL
int         petradb_column_int(int cursor, int index);
long long   petradb_column_int64(int cursor, int index);
double      petradb_column_double(int cursor, int index);
const char *petradb_column_text(int cursor, int index);     // do NOT free
const void *petradb_column_blob(int cursor, int index);     // do NOT free
int         petradb_column_bytes(int cursor, int index);    // byte length of text/blob
int         petradb_column_is_null(int cursor, int index);

User-Defined Functions

Register native C functions callable from SQL, triggers, and stored procedures:

void my_double(int ctx, int argc, const int* argv) {
    if (petradb_value_is_null(argv[0])) {
        petradb_result_null(ctx);
        return;
    }
    int x = petradb_value_int(argv[0]);
    petradb_result_int(ctx, x * 2);
}

petradb_create_function(db, "my_double", 1, NULL, my_double);
// Now usable: SELECT my_double(age) FROM users;

Registration

typedef void (*petradb_func_callback)(int ctx, int argc, const int* argv);
int petradb_create_function(int db, const char* name, int nargs, void* user_data, petradb_func_callback fn);

Reading Arguments

int         petradb_value_int(int value);
long long   petradb_value_int64(int value);
double      petradb_value_double(int value);
const char *petradb_value_text(int value);
int         petradb_value_type(int value);
int         petradb_value_is_null(int value);

Setting Results

void petradb_result_int(int ctx, int value);
void petradb_result_int64(int ctx, long long value);
void petradb_result_double(int ctx, double value);
void petradb_result_text(int ctx, const char* value);
void petradb_result_null(int ctx);
void petradb_result_error(int ctx, const char* msg);  // aborts the SQL statement

User Data

Pass application-specific context through user_data:

static int call_count = 0;

void my_counter(int ctx, int argc, const int* argv) {
    int *p = (int*)petradb_user_data(ctx);
    (*p)++;
    petradb_result_int(ctx, *p);
}

petradb_create_function(db, "call_count", 0, &call_count, my_counter);

Error Handling

const char *petradb_errmsg(void);   // last error message, empty string if none

All functions that return handles return 0 on error. Functions that return status codes return -1 on error. After any error, call petradb_errmsg() for details. Successful operations clear the error.

Type Constants

Constant	Value	Description
PETRADB_INTEGER	1	Integer value
PETRADB_FLOAT	2	Floating-point value
PETRADB_TEXT	3	Text string
PETRADB_BLOB	4	Binary data
PETRADB_NULL	5	SQL NULL

Memory Management

• String pointers from petradb_column_text, petradb_value_text, and petradb_errmsg are owned by PetraDB. Do NOT free them. They remain valid until the next call that returns a string.
• The shared library includes Scala Native’s garbage collector. No manual memory management is needed for database objects — just call petradb_finalize and petradb_close when done.

Language Bindings

The C API works with any language that supports C FFI:

• Rust: unsafe extern "C" declarations + -lpetradb-engine
• Python: ctypes.cdll.LoadLibrary("libpetradb-engine.so")
• Go: // #cgo LDFLAGS: -lpetradb-engine + import "C"
• Ruby: FFI::Library from the ffi gem