C Interoperability

For the ABI responsibilities and exact declaration rules, see Attributes, Directives, and Interoperability.

DQ is designed to call C libraries directly. Interop code normally combines external declarations, C-compatible types, pointers, and #linklib.

External Functions

Declare C functions with [[external]].

function printf(fmt : ^char, ...) -> int  [[external]]

When the DQ name should differ from the C symbol, pass the symbol name.

function c_fprintf(stream : pointer, fmt : ^char, ...) -> int  [[external('fprintf')]]

Variable Arguments

C-style variadic functions use ....

[[external]] function printf(fmt : ^char, ...) -> int

printf("value = %d\n", 42)

Use C-compatible argument types and format strings that match the external ABI.

External Global Variables

External global variables can be imported with [[external('name')]].

var libc_stdout : pointer [[external('stdout')]]

C Strings

Use ^char, str.pchar, cstring, and cstring(n) for C-style strings.

function WriteStr(s : cstring):
    var p : ^char = s.pchar
    while p^ <> 0:
        putchar(p^)
        p += 1
    endwhile
endfunc

cstring(n) declares fixed storage.

var buf : cstring(127) = "hello"

str has an enforced trailing zero and can be passed with .pchar without copying. Internal zero bytes are still possible; a C API using zero-terminated semantics sees only the prefix before the first zero byte.

Generic Pointers

Use pointer for opaque C pointers.

[[external]] function fopen(path : ^char, mode : ^char) -> pointer
[[external]] function fclose(file : pointer) -> int

Cast to typed pointers before dereferencing.

Struct Layout

Use structs for C-compatible records when layout is known.

struct SPoint:
    x : int32
    y : int32
endstruct

Attributes such as [[packed]], [[align(n)]], and [[volatile]] are available for low-level layout/access cases where supported by the compiler. Memory-mapped register fields can use [[regrw]], [[regro]], and [[regwo]] for volatile read/write, read-only, and write-only access respectively.

Linking Libraries

Use #linklib to request an external library.

#linklib('z')
[[external]] function zlibVersion() -> ^char

Standard Package Declarations

The stdpkg/libc modules provide some existing C declarations.

use libc/stdio

ABI Responsibility

DQ type checking verifies the DQ side of the declaration, but the programmer is responsible for matching the real C ABI: integer widths, pointer types, struct layout, calling convention, variadic arguments, ownership, and lifetime.