Interacting with C

Calling C functions from Impala

Functions defined in C (or C++ functions marked with the extern “C” qualifier) can be made visible from Impala by using the extern keyword:

extern "C" {
    fn rand() -> int; // Make the C function rand visible from Impala
}

It is the programmer’s responsibility to ensure that the C signature matches the Impala signature.

Calling Impala functions from C

Impala functions can also be made available to C, using the extern keyword:

extern fn get_42() -> int {
    42
}

The C prototype corresponding to the function in the previous example would be:

int get_42(void);

Automatically generating a C interface

The Impala compiler contains an automatic C interface generator. It works by inspecting all functions marked extern (visible from C), and creating C signatures for them. If a structure type is used in the Impala function, the compiler will emit the equivalent structure in C.

As an example, if the file blob.impala contains the following lines:

struct Blob {
    i: int,
    f: float
}

extern fn sum_blob(blob: &Blob) -> float {
    blob.f + (blob.i as float)
}

The result of running the command:

$ impala blob.impala --emit-c-interface -o blob

Is the file blob.h containing:

/* blob.h : Impala interface file generated by impala */
#ifndef BLOB_H
#define BLOB_H

#ifdef __cplusplus
extern "C" {
#endif

struct Blob {
    int i;
    float f;
};

float sum_blob(struct Blob const* blob);

#ifdef __cplusplus
}
#endif

#endif /* BLOB_H */

Note that not all Impala types are supported in the interface generation tool.

Passing pointers to and from C

In general, it is safe to pass C pointers to Impala or the other way around. However, please note that some functions (in particular the runtime functions interacting with device memory) may require that a pointer is aligned to page boundary. In any case, the runtime functions can be called from both C or Impala to obtain aligned memory.