C-Interop and FFI

C++ uses name mangling to encode type information into function symbols, enabling overloading. C Does not mangle names — each function has a single symbol matching its source name. Interoperating Between C and C++ requires careful management of linkage, data layouts, and exception boundaries.

5.1 `extern "C"` Linkage [N4950 §9.9]

C++ uses name mangling to encode type information into function symbols, enabling overloading. C Does not mangle names — each function has a single symbol matching its source name. The extern "C" Linkage specification disables name mangling, making a C++ function callable from C (and vice Versa).

extern "C" {

// These functions are exported with unmangled C linkage:
// Symbol names: "add" and "multiply" (no type encoding)
int add(int a, int b) {
    return a + b;
}

double multiply(double a, double b) {
    return a * b;
}

}  // extern "C"

// Without extern "C", these would have mangled names:
//   _Z3addii      (add(int, int))
//   _Z8multiplydd (multiply(double, double))
int subtract(int a, int b) {
    return a - b;
}

Formal Semantics of `extern "C"`

By [N4950 §9.9], the extern "C" linkage specification has three effects:

Name mangling is disabled. The symbol name in the object file is the literal function name, not an encoded representation of the signature.
Language linkage is set to C. This affects how the function is called (C calling convention) and how entities are looked up.
Overloading is prohibited. Within an extern "C" block, you cannot have two functions with the same name — the linker would see duplicate symbols.

`extern "C"` and Function Overloading

Since name mangling is disabled, you cannot overload functions with extern "C" linkage:

extern "C" {
    int process(int x);     // OK: symbol is "process"
    // int process(double); // ERROR: duplicate symbol "process" — no mangling to disambiguate
}

This is not a limitation of the linkage specification per se, but a consequence of the linker”s Requirement for unique symbol names within a translation unit.

`extern "C"` and Member Functions

extern "C" cannot be applied to member functions. Only free functions and variables can have C Language linkage [N4950 §9.9.1]:

class Foo {
    // extern "C" void bar();  // ERROR: member functions cannot have C linkage
    static void bar();         // OK: but still has C++ linkage (name mangled)
};

`extern "C"` and Static Members

Static member functions have C++ linkage even if declared in a class. If you need a C-callable Static member function, you must provide a non-member wrapper:

extern "C" void foo_c_wrapper(void* self) {
    // reinterpret self to the actual class type
    // call the static or non-static member function
}

`constexpr` and `extern "C"`

A function declared with both constexpr and extern "C" linkage is valid since C++17. The Function can be used in constant expressions and also has C linkage for linking purposes [N4950 §9.9]:

extern "C" constexpr int square(int x) {
    return x * x;
}

static_assert(square(5) == 25);  // OK: constexpr evaluation
// Symbol "square" has C linkage for linking purposes

`extern "C"` and `noexcept`

Functions with extern "C" linkage are implicitly noexcept unless declared otherwise [N4950 §14.5]. This is because C has no exception mechanism, so a C-linkage function that throws violates The C ABI contract:

extern "C" void c_function();  // implicitly noexcept

extern "C" void throwing_c_function() noexcept(false);  // explicitly non-noexcept — allowed
                                                        // but dangerous: exceptions may cross
                                                        // the C ABI boundary

5.2 Calling C from C++

The standard C library headers are wrapped with extern "C" by the C++ standard library headers. When you #include <cstring>The declarations are automatically given C linkage. For your own C Libraries, use extern "C":

// my_c_api.h — the C header
#ifndef MY_C_API_H
#define MY_C_API_H

#ifdef __cplusplus
extern "C" {
#endif

typedef struct {
    double x;
    double y;
} Point;

Point point_create(double x, double y);
double point_distance(const Point* a, const Point* b);
void point_translate(Point* p, double dx, double dy);

#ifdef __cplusplus
}  // extern "C"
#endif

#endif  // MY_C_API_H

// my_c_api.c — the C implementation
#include "my_c_api.h"
#include <math.h>

Point point_create(double x, double y) {
    Point p = {x, y};
    return p;
}

double point_distance(const Point* a, const Point* b) {
    double dx = a->x - b->x;
    double dy = a->y - b->y;
    return sqrt(dx * dx + dy * dy);
}

void point_translate(Point* p, double dx, double dy) {
    p->x += dx;
    p->y += dy;
}

// main.cpp — calling C from C++
#include "my_c_api.h"
#include <cstdio>
#include <memory>

int main() {
    auto deleter = [](Point* p) {
        std::printf("Destroying point\n");
        delete p;
    };
    std::unique_ptr<Point, decltype(deleter)> p(
        new Point(point_create(3.0, 4.0)),
        deleter
    );

    Point origin = point_create(0.0, 0.0);
    double dist = point_distance(p.get(), &origin);
    std::printf("Distance from origin: %f\n", dist);  // 5.000000

    point_translate(p.get(), 1.0, 1.0);
    dist = point_distance(p.get(), &origin);
    std::printf("After translate: %f\n", dist);  // ~4.242641
}

Memory Ownership Across the Boundary

When a C function returns a heap-allocated pointer, the C++ caller must know how to free it. If the C library uses mallocThe C++ code must use free (not delete):

extern "C" {
    // C function that allocates with malloc
    char* c_create_buffer(size_t size);
    void c_destroy_buffer(char* buf);
}

// C++ code using the C allocator
std::unique_ptr<char, decltype(&c_destroy_buffer)> buf(
    c_create_buffer(1024),
    c_destroy_buffer  // uses the correct C deallocator
);

5.3 Calling C++ from C

Calling C++ functions from C requires a C-compatible entry point — a function with extern "C" Linkage that wraps the C++ implementation:

// widget.cpp — C++ implementation
#include <string>
#include <vector>

class Widget {
    std::string name_;
    std::vector<int> data_;
public:
    Widget(const char* name) : name_(name) {}
    void add_value(int v) { data_.push_back(v); }
    const char* get_name() const { return name_.c_str(); }
    int get_value(int index) const {
        return (index < static_cast<int>(data_.size())) ? data_[index] : -1;
    }
};

// C-compatible opaque handle
extern "C" {
    // Opaque pointer type — C code never sees the full definition
    typedef struct WidgetOpaque* WidgetHandle;

    WidgetHandle widget_create(const char* name) {
        return reinterpret_cast<WidgetHandle>(new Widget(name));
    }

    void widget_destroy(WidgetHandle h) {
        delete reinterpret_cast<Widget*>(h);
    }

    void widget_add_value(WidgetHandle h, int v) {
        reinterpret_cast<Widget*>(h)->add_value(v);
    }

    const char* widget_get_name(WidgetHandle h) {
        return reinterpret_cast<Widget*>(h)->get_name();
    }

    int widget_get_value(WidgetHandle h, int index) {
        return reinterpret_cast<Widget*>(h)->get_value(index);
    }
}

/* widget_user.c — calling C++ from C */
#include <stdio.h>

/* Opaque type — only declared, never defined in C */
typedef struct WidgetOpaque* WidgetHandle;

WidgetHandle widget_create(const char* name);
void widget_destroy(WidgetHandle h);
void widget_add_value(WidgetHandle h, int v);
const char* widget_get_name(WidgetHandle h);
int widget_get_value(WidgetHandle h, int index);

int main(void) {
    WidgetHandle w = widget_create("sensor-1");
    widget_add_value(w, 10);
    widget_add_value(w, 20);
    widget_add_value(w, 30);

    printf("Widget: %s\n", widget_get_name(w));
    printf("Value[0] = %d\n", widget_get_value(w, 0));
    printf("Value[1] = %d\n", widget_get_value(w, 1));
    printf("Value[2] = %d\n", widget_get_value(w, 2));

    widget_destroy(w);
    return 0;
}

`void*` Instead of `reinterpret_cast`

For maximum portability across platforms where C and C++ may have different pointer representations, Use void* handles and pass data through C-compatible types:

// More portable C API using void*
extern "C" {
    typedef void* WidgetHandle;

    WidgetHandle widget_create(const char* name) {
        return static_cast<void*>(new Widget(name));
    }

    void widget_destroy(WidgetHandle h) {
        delete static_cast<Widget*>(h);
    }
}

:::caution The reinterpret_cast approach works on platforms where C and C++ share the same ABI (pointer size, struct layout, calling convention). This is true for x86-64 Linux/macOS (both use the System V ABI). On platforms with divergent C/C++ ABIs, use void* handles and pass data through C-compatible types only. :::

5.4 ABI Boundaries: Name Mangling and Layout

At a C/C++ boundary, several ABI properties must align:

Property	C ABI	C++ ABI (Itanium, used on Linux/macOS)
Name mangling	None — symbol = function name	Encodes types, namespaces, templates
Calling convention	System V AMD64 (x86-64)	Same as C (on System V platforms)
Struct layout	Same as C++ POD	Same as C for POD; non-POD differs
Exception handling	N/A (no exceptions)	Zero-cost with unwind tables
`bool` size	1 byte (implementation-def)	Same as C (implementation-defined)

Itanium C++ ABI vs MSVC C++ ABI

On Linux and macOS, the Itanium C++ ABI is used for name mangling, virtual table layout, and Exception handling. On Windows, MSVC uses a different C++ ABI. This means that C++ libraries Compiled with GCC/Clang cannot be linked with MSVC-compiled C++ code (even with extern "C" on the C-compatible parts). The C-compatible parts work fine across ABIs; only C++-specific features (classes, templates, exceptions) are incompatible.

Name Mangling Examples

The Itanium C++ ABI encodes the full function signature into the symbol name:

// Symbol: _Z3addii
void add(int, int);

// Symbol: _Z3adddd
void add(double, double);

// Symbol: _ZN3lib3addEii
namespace lib { void add(int, int); }

// Symbol: _ZNK4Base3fooEv
struct Base { virtual void foo() const; };

// Symbol: _Z3maxIiERKT_S2_
template<typename T> const T& max(const T&, const T&);

With extern "C"All of these become addLosing the type information. This is why Overloading is not possible within extern "C" blocks.

#include <cstddef>
#include <cstdio>

// Verifying struct layout compatibility across the C boundary
extern "C" {
struct CPoint {
    double x;
    double y;
    // No virtual functions, no non-POD members → layout is identical in C and C++
};

// This struct has a C-compatible layout
struct CPoint make_cpoint(double x, double y) {
    CPoint p{x, y};
    return p;
}

// C++-specific struct — NOT safe to pass across the boundary
struct ComplexPoint {
    double x, y;
    virtual double magnitude() const { /* ... */ return 0.0; }
    // vtable pointer changes the layout — first member is NOT at offset 0
    // sizeof(ComplexPoint) >= 24 (8-byte vptr + 2*8 bytes)
};
}

int main() {
    static_assert(sizeof(CPoint) == 16);
    static_assert(offsetof(CPoint, x) == 0);
    static_assert(offsetof(CPoint, y) == 8);

    std::printf("sizeof(CPoint) = %zu\n", sizeof(CPoint));       // 16
    std::printf("sizeof(ComplexPoint) = %zu\n", sizeof(ComplexPoint)); // 24 (on x86-64)
}

5.5 Data Marshalling: Ensuring Compatible Layouts

When passing data across a C/C++ boundary, ensure that:

Structs are POD (Plain Old Data) or standard-layout: no virtual functions, no base classes with virtual functions, no non-static data members of reference type, all non-static data members have the same access control.
Fixed-width types are used (int32_tNot int).
No padding surprises: use static_assert and offsetof to verify layout, or #pragma pack / alignas to control it.
No C++ exceptions cross the boundary: exceptions thrown in C++ code called from C unwind through C frames, which have no unwind information — undefined behavior. Catch all exceptions before returning to C code.

Proof of Struct Layout Compatibility

Claim: A standard-layout struct with only fundamental type members has identical layout in C and C++ on the same platform.

Proof:

By [N4950 §7.7.2], a standard-layout class has the same layout as a corresponding C struct with the same members in the same order.
By [N4950 §7.7.2.1], each non-static data member is allocated at an offset that satisfies its alignment requirement, and the alignment of the struct is the maximum alignment of its members.
C struct layout follows the same rules (ISO C 6.2.5p20, 6.7.2.1p15): each member is placed at an offset satisfying its alignment, with padding inserted as needed.
Since both C and C++ use the same alignment rules for fundamental types (int``doubleEtc.) on the same platform, the resulting layout is byte-for-byte identical. QED.

What Breaks Layout Compatibility

The following C++ features break layout compatibility with C:

Feature	Effect on Layout
Virtual functions	Adds vtable pointer ( 8 bytes at offset 0)
Virtual base classes	Adds vtable pointer and virtual base offset
Multiple inheritance	May add pointer adjustments for base-to-derived
Non-standard-layout members	Reference members, `std::string`Etc.
Different compiler flags	-fpack-struct``#pragma pack changes padding
Different alignment (`alignas`)	Adds padding not present in the C struct

#include <cstdint>
#include <cstddef>
#include <cstdio>

// Correct: POD struct with explicit layout guarantees
struct PacketHeader {
    static_assert(offsetof(PacketHeader, magic) == 0);
    int32_t magic;
    int32_t version;
    int64_t timestamp;
    int32_t payload_size;
    int32_t checksum;
};

static_assert(sizeof(PacketHeader) == 24);
static_assert(alignof(PacketHeader) == 8);

// Incorrect: this struct would break C interop
struct BadPacket {
    std::string payload;  // Non-POD: heap allocation, non-trivial destructor
    virtual void validate() {}  // vptr changes layout
};

Controlling Layout with `#pragma pack`

When interfacing with a C library that uses non-default packing (common in network protocols and File formats), use #pragma pack to match the layout:

#pragma pack(push, 1)  // 1-byte alignment — no padding
struct NetworkHeader {
    uint8_t  type;
    uint32_t length;
    uint16_t flags;
};
#pragma pack(pop)

static_assert(sizeof(NetworkHeader) == 7);  // 1 + 4 + 2 = 7, no padding

:::caution #pragma pack changes the alignment of all members in the struct, which can cause Misaligned access on strict-alignment architectures (ARM, SPARC). Use with caution and document the Rationale. :::

5.6 Dynamic Library Loading with `dlfcn.h`

POSIX systems provide dlopen``dlsym``dlcloseAnd dlerror for loading shared libraries at Runtime. This enables plugin architectures and runtime code loading.

// plugin.cpp — compiled into libplugin.so
// $ g++ -shared -fPIC -o libplugin.so plugin.cpp

#include <cstdint>

extern "C" {

int32_t plugin_version() {
    return 1;
}

int32_t plugin_compute(int32_t x, int32_t y) {
    return x * x + y * y;
}

const char* plugin_name() {
    return "quadratic_plugin";
}

}  // extern "C"

// loader.cpp — dynamically loads and uses the plugin
// $ g++ -std=c++17 -o loader loader.cpp -ldl
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <dlfcn.h>
#include <string>

struct PluginAPI {
    int32_t (*version)();
    int32_t (*compute)(int32_t, int32_t);
    const char* (*name)();
};

PluginAPI load_plugin(const char* path) {
    void* handle = dlopen(path, RTLD_NOW);
    if (!handle) {
        std::fprintf(stderr, "dlopen failed: %s\n", dlerror());
        std::exit(1);
    }

    // Clear any existing error
    dlerror();

    auto load_sym = [&](const char* name) -> void* {
        void* sym = dlsym(handle, name);
        char* err = dlerror();
        if (err) {
            std::fprintf(stderr, "dlsym(%s) failed: %s\n", name, err);
            std::exit(1);
        }
        return sym;
    };

    PluginAPI api{};
    api.version = reinterpret_cast<int32_t(*)()>(load_sym("plugin_version"));
    api.compute = reinterpret_cast<int32_t(*)(int32_t, int32_t)>(load_sym("plugin_compute"));
    api.name    = reinterpret_cast<const char*(*)()>(load_sym("plugin_name"));
    return api;
}

int main() {
    auto plugin = load_plugin("./libplugin.so");

    std::printf("Plugin: %s v%d\n", plugin.name(), plugin.version());
    std::printf("compute(3, 4) = %d\n", plugin.compute(3, 4));  // 25

    // In production code, store handle and call dlclose(handle) when done
}

:::caution The RTLD_NOW flag resolves all symbols at load time, surfacing missing symbol errors Immediately. RTLD_LAZY defers resolution to first use, which can mask errors and cause crashes at Unpredictable points. For plugin loading, prefer RTLD_NOW.

Windows Equivalent: `LoadLibrary` and `GetProcAddress`

On Windows, the equivalent functions are LoadLibraryA/LoadLibraryW``GetProcAddressAnd FreeLibrary. The pattern is identical but the API is different:

#include <windows.h>

extern "C" {
    typedef int32_t (*ComputeFn)(int32_t, int32_t);
}

int main() {
    HMODULE hmod = LoadLibraryA("plugin.dll");
    if (!hmod) {
        return 1;
    }

    ComputeFn compute = reinterpret_cast<ComputeFn>(GetProcAddress(hmod, "plugin_compute"));
    if (!compute) {
        FreeLibrary(hmod);
        return 1;
    }

    int32_t result = compute(3, 4);  // 25
    FreeLibrary(hmod);
    return 0;
}

5.7 `nothrow new` and C Interop

When allocating memory in C++ that will be freed by C code (or vice versa), you must ensure Compatible allocation. C++ new throws std::bad_alloc on failure; C malloc returns NULL. Use nothrow new to match C’s error-reporting convention:

#include <new>
#include <cstdlib>

extern "C" void* allocate_buffer(size_t size) {
    // nothrow new returns nullptr on failure, matching malloc semantics
    return ::operator new(size, std::nothrow);
}

extern "C" void deallocate_buffer(void* ptr) {
    ::operator delete(ptr, std::nothrow);
}

By [N4950 §7.6.2.7], ::operator new(size, std::nothrow) returns a null pointer on allocation Failure instead of throwing. This matches the malloc contract that C code expects.

5.8 Callback Functions Across the Boundary

When C code passes a callback function pointer to C++ code, the callback must have C linkage. If the Callback is a C++ function, it must be wrapped:

extern "C" {

// C library function that takes a callback
void c_library_set_callback(void (*callback)(int event_code));

}

// C++ callback wrapper
extern "C" void my_callback_wrapper(int event_code) {
    // Inside this wrapper, we are back in C++ context
    // We can use C++ features (exceptions, std::string, etc.)
    // but must not let exceptions escape
    try {
        // C++ implementation
    } catch (...) {
        // Swallow — exceptions must not cross the C boundary
    }
}

void register_callback() {
    c_library_set_callback(my_callback_wrapper);
}

Function Pointer Type Compatibility

A function pointer with C linkage and a function pointer with C++ linkage are different types [N4950 §7.3.8]. You cannot assign one to the other without a cast:

extern "C" typedef void (*CFuncPtr)(int);
typedef void (*CppFuncPtr)(int);

CFuncPtr   c_ptr = nullptr;
CppFuncPtr cpp_ptr = nullptr;

// c_ptr = cpp_ptr;   // ERROR: different types
c_ptr = reinterpret_cast<CFuncPtr>(cpp_ptr);  // OK with cast, but dangerous

On platforms where C and C++ share the same calling convention (virtually all modern platforms), This cast is safe. But it is technically undefined behavior by the Standard.

5.9 Common Pitfalls at Language Boundaries

Exceptions crossing the boundary: A C++ exception that propagates through a C call stack is undefined behavior. Always wrap C++ entry points with try/catch(...).
Differing size_t between 32-bit and 64-bit code: If a 32-bit C library passes a pointer-sized value through an intIt will truncate on 64-bit systems.
Differing struct packing: MSVC defaults to 8-byte alignment; GCC defaults to natural alignment. Use explicit packing or fixed-width types.
Static initialization order fiasco: Global C++ objects with non-trivial constructors may not be initialized before a C main() calls them. Prefer the Construct On First Use idiom.
new/delete mismatch: Memory allocated with new in C++ must be freed with delete (not free()), and vice versa. If passing ownership of heap memory across the boundary, provide explicit create/destroy functions in the C API.
bool vs _Bool: C’s _Bool and C++‘s bool are distinct types. While they are compatible on most platforms, the Standard does not guarantee layout compatibility. Use int or a fixed-width type for flags passed across the boundary.
String ownership: If a C++ function returns a const char* pointing to a std::string’s internal buffer, the pointer is valid only as long as the std::string is alive. C code that stores this pointer will have a dangling reference once the std::string is destroyed. Return a copy or require the C code to copy immediately.

// Safe C++ entry point wrapping
#include <exception>

extern "C" {
    void safe_entry_point() {
        try {
            // ... C++ code that might throw ...
        } catch (const std::exception& e) {
            // Log and return error code — do NOT let exceptions escape
        } catch (...) {
            // Catch everything else
        }
    }
}

Construct On First Use Idiom

To avoid the static initialization order fiasco when C code calls into C++ during startup:

// Safe global accessor — avoids static init order issues
class Config {
    std::string name_;
public:
    Config() : name_("default") {}
    const char* name() const { return name_.c_str(); }
};

extern "C" const char* get_config_name() {
    static Config instance;  // Constructed on first use, not at static init time
    return instance.name();
}

By [N4950 §6.7.7], a local static variable with block scope is initialized on first control flow Passage through its declaration, which is guaranteed to be thread-safe since C++11. This avoids the Static initialization order problem entirely.

Summary

This topic covers the fundamental principles of c-interop and ffi, including the key equations, experimental methods, and applications relevant to the specification.

Key concepts include:

fundamental principles and equations
SI units and dimensional analysis
mathematical modelling of physical phenomena
experimental techniques and measurement
applications to real-world problems

A strong understanding of these principles, combined with regular practice of quantitative problems and past paper questions, is essential for success in examinations.

Worked Examples

Worked examples demonstrating the application of key concepts are covered in the detailed sub-pages linked above.

:::

Mark this page as reviewed

C-Interop and FFI

C-Interop and FFI

5.1 extern "C" Linkage [N4950 §9.9]

Formal Semantics of extern "C"

extern "C" and Function Overloading

extern "C" and Member Functions

extern "C" and Static Members

constexpr and extern "C"

extern "C" and noexcept

5.2 Calling C from C++

Memory Ownership Across the Boundary

5.3 Calling C++ from C

void* Instead of reinterpret_cast

5.4 ABI Boundaries: Name Mangling and Layout

Itanium C++ ABI vs MSVC C++ ABI

Name Mangling Examples

5.5 Data Marshalling: Ensuring Compatible Layouts

Proof of Struct Layout Compatibility

What Breaks Layout Compatibility

Controlling Layout with #pragma pack

5.6 Dynamic Library Loading with dlfcn.h

Windows Equivalent: LoadLibrary and GetProcAddress

5.7 nothrow new and C Interop

5.8 Callback Functions Across the Boundary

Function Pointer Type Compatibility

5.9 Common Pitfalls at Language Boundaries

Construct On First Use Idiom

See Also

Summary

Worked Examples

5.1 `extern "C"` Linkage [N4950 §9.9]

Formal Semantics of `extern "C"`

`extern "C"` and Function Overloading

`extern "C"` and Member Functions

`extern "C"` and Static Members

`constexpr` and `extern "C"`

`extern "C"` and `noexcept`

`void*` Instead of `reinterpret_cast`

Controlling Layout with `#pragma pack`

5.6 Dynamic Library Loading with `dlfcn.h`

Windows Equivalent: `LoadLibrary` and `GetProcAddress`

5.7 `nothrow new` and C Interop