Preprocessor

SafeC includes a preprocessor that supports a safe subset of C preprocessing directives. By default, it operates in safe mode, which restricts features that lead to hard-to-debug macro issues. A compatibility mode is available for full C macro support.

Directives

#include

Include other SafeC files or C headers:

#include "mymodule.h"          // local include (quoted)
#include <stdio.h>             // system include (angle brackets)

System C headers are imported natively via the CHeaderImporter, which extracts function and typedef declarations from clang -ast-dump=json. See C Interop for details.

Use -I <dir> to add include search paths.

#define (Object-Like)

Object-like macros define simple text substitutions:

#define MAX_SIZE 1024
#define VERSION "2.0"
#define DEBUG

In safe mode, function-like macros are rejected:

// ERROR in safe mode:
#define MAX(a, b) ((a) > (b) ? (a) : (b))

Use const variables and generic functions instead:

const int MAX_SIZE = 1024;

generic<T: Numeric>
T max(T a, T b) {
    if (a > b) return a;
    return b;
}

#undef

Remove a previously defined macro:

#define FEATURE_X
// ... use FEATURE_X ...
#undef FEATURE_X

#pragma once

Prevent a header from being included multiple times:

#pragma once

// header contents...

Conditional Compilation

#ifdef / #ifndef

Test whether a macro is defined:

#ifdef DEBUG
    printf("debug: x = %d\n", x);
#endif

#ifndef NDEBUG
    assert(x > 0);
#endif

#if / #elif / #else / #endif

General conditional compilation with constant expressions:

#if PLATFORM == 1
    // Linux-specific code
#elif PLATFORM == 2
    // macOS-specific code
#else
    // Generic fallback
#endif

Nesting

Conditional blocks can be nested:

#ifdef FEATURE_A
    #ifdef FEATURE_B
        // both A and B enabled
    #else
        // only A enabled
    #endif
#endif

Predefined Macros

Macro	Value	Description
__FILE__	String literal	Current source file name
__LINE__	Integer literal	Current line number

Unlike C, SafeC deliberately omits __TIME__ and __DATE__ to support reproducible builds.

Command-Line Flags

-D <name>[=value]

Define a macro from the command line:

./build/safec program.sc -D DEBUG -D MAX_SIZE=2048 --emit-llvm -o program.ll

-I <dir>

Add an include search directory:

./build/safec program.sc -I ./include -I ../lib/include --emit-llvm -o program.ll

--no-import-c-headers

Disable the native C header import mechanism. With this flag, #include <stdio.h> will not invoke clang for AST extraction. You must provide extern declarations manually.

--compat-preprocessor

Enable compatibility mode with full C preprocessor features (see below).

Safe Mode vs Compatibility Mode

Safe Mode (Default)

The default preprocessor mode restricts features that are common sources of bugs in C:

Feature	Safe Mode	Rationale
Object-like #define	Allowed	Simple constant substitution
Function-like macros	Rejected	Use const + generics instead
## (token pasting)	Rejected	Obfuscates code
# (stringification)	Rejected	Obfuscates code
#include	Allowed	File inclusion is necessary
#ifdef / #if	Allowed	Conditional compilation is necessary
#pragma once	Allowed	Header guard

Compatibility Mode (--compat-preprocessor)

When working with legacy C code or headers that require full macro support, enable compatibility mode:

./build/safec legacy.sc --compat-preprocessor --emit-llvm -o legacy.ll

In compatibility mode, function-like macros, token pasting, and stringification are all permitted.

Compile-Time Alternatives

SafeC provides first-class language features that replace most preprocessor use cases:

C Preprocessor Pattern	SafeC Alternative
#define PI 3.14159	const double PI = 3.14159;
#define MAX(a,b) ...	generic<T> T max(T a, T b) { ... }
#ifdef DEBUG	if const (DEBUG) { ... }
#define ARRAY_SIZE 100	const int ARRAY_SIZE = 100;
Conditional function body	if const with consteval conditions

Using language-level constructs instead of macros provides:

• Type safety: const and generics are type-checked
• Scoping: const variables respect lexical scope
• Debuggability: no invisible text substitution
• IDE support: go-to-definition, rename, hover all work correctly

Example

#pragma once
#define VERSION 1

#ifdef DEBUG
    #define LOG_LEVEL 3
#else
    #define LOG_LEVEL 0
#endif

const int MAX_CONNECTIONS = 128;

// Prefer const over #define for typed constants
const double TIMEOUT_SEC = 30.0;

// Prefer generic functions over function-like macros
generic<T: Numeric>
T clamp(T val, T lo, T hi) {
    if (val < lo) return lo;
    if (val > hi) return hi;
    return val;
}