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PRQL C library

Description

This module compiles PRQL as a library (both .a and .so are generated). This allows embedding in languages that support FFI — for example, Golang.

Linking

See examples/minimal-c/Makefile.

Copy the .a and .so files in a convenient place and add the following compile flags to Go (cgo):

CGO_LDFLAGS="-L/path/to/libprqlc_c.a -lprqlc -pthread -ldl" go build

Examples

For a minimal example, see examples/minimal-c/main.c.

Below is an example from an actual application that is using PRQL in Go.

package prql

/*


#include <stdlib.h>

int to_sql(char *prql_query, char *sql_query);
int to_json(char *prql_query, char *json_query);

*/
import "C"

import (
    "errors"
    "strings"
    "unsafe"
)

// ToSQL converts a PRQL query to SQL
func ToSQL(prql string) (string, error) {
    // buffer length should not be less than 1K because we may get an error
    // from the PRQL compiler with a very short query
    cStringBufferLength := 1024

    // allocate a buffer 3 times the length of the PRQL query to store the
    // generated SQL query
    if len(prql)*3 > cStringBufferLength {
        cStringBufferLength = len(prql) * 3
    }

    // preallocate the buffer
    cstr := C.CString(strings.Repeat(" ", cStringBufferLength))
    defer C.free(unsafe.Pointer(cstr))

    // convert the PRQL query to SQL
    res := C.to_sql(C.CString(prql), cstr)
    if res == 0 {
        return C.GoString(cstr), nil
    }

    return "", errors.New(C.GoString(cstr))
}

// ToJSON converts a PRQL query to JSON
func ToJSON(prql string) (string, error) {
    // buffer length should not be less than 1K because we may get an error
    cStringBufferLength := 1024
    if len(prql)*3 > cStringBufferLength {
        cStringBufferLength = len(prql) * 10
    }

    // preallocate the buffer
    cstr := C.CString(strings.Repeat(" ", cStringBufferLength))
    defer C.free(unsafe.Pointer(cstr))

    // convert the PRQL query to SQL
    res := C.to_json(C.CString(prql), cstr)
    if res == 0 {
        return C.GoString(cstr), nil
    }

    return "", errors.New(C.GoString(cstr))
}

Development

Headers

The C & C++ header files prqlc.h & prqlc.hpp were generated using cbindgen. To generate a new one run:

task build-prqlc-c-header

...or copy & paste the commands from the Taskfile.

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