Predefined Macros 预定义的宏

Here is a Quick Reference Table (Cheat Sheet) for Predefined Macros in C. These are automatically defined by the compiler and are essential for debugging, cross-platform development, and version control.

1. Standard ISO C Macros

These macros are supported by almost all compliant C compilers (ANSI C / C99 / C11).

Macro	Type	Description	Example Output
`__FILE__`	`string`	Name of the current source file.	`"main.c"`
`__LINE__`	`int`	Current line number in the source file.	`42`
`__DATE__`	`string`	Date of compilation (Mmm dd yyyy).	`"Dec 23 2025"`
`__TIME__`	`string`	Time of compilation (hh:mm:ss).	`"14:30:00"`
`__func__`	`string`	Name of the current function (C99+).	`"myFunction"`
`__STDC__`	`int`	Defined as `1` if the compiler is standard-compliant.	`1`
`__STDC_HOSTED__`	`int`	`1` if running on an OS, `0` if bare-metal (embedded).	`1`

2. C Standard Version Detection (`__STDC_VERSION__`)

Used to check which C standard the compiler is using.

Value (`long`)	C Standard
Undefined	C89 / ANSI C
`199409L`	C94
`199901L`	C99
`201112L`	C11
`201710L`	C17
`202311L`	C23 (Expected)

Usage:

#if __STDC_VERSION__ >= 199901L
    // Use C99 features like 'long long' or variable declarations in for-loops
#endif

3. Operating System (OS) Detection

Used for cross-platform conditional compilation.

Macro	Target OS	Notes
`_WIN32`	Windows (32/64-bit)	Defined for both x86 and x64 on Windows.
`_WIN64`	Windows (64-bit)	Only defined for 64-bit Windows.
`__linux__`	Linux	Ubuntu, Debian, CentOS, etc.
`__APPLE__`	macOS / iOS	Used with `__MACH__`.
`__ANDROID__`	Android	Specific to Android NDK.
`__unix__`	Unix	Generic Unix systems.

Usage:

#ifdef _WIN32
    #include <windows.h>
#elif defined(__linux__)
    #include <unistd.h>
#endif

4. Compiler Detection

Used to handle compiler-specific keywords or optimizations.

Macro	Compiler	Format / Note
`__GNUC__`	GCC (GNU)	Also defined by Clang/ICC as they emulate GCC.
`__clang__`	Clang / LLVM	Specific to Clang.
`_MSC_VER`	Microsoft Visual C++	Integer value (e.g., `1900` for VS2015).
`__MINGW32__`	MinGW	Windows port of GCC.

5. Architecture Detection

Used to optimize for specific hardware (CPU).

Macro	Architecture
`__x86_64__` / `_M_X64`	x86_64 (64-bit Intel/AMD)
`__i386__` / `_M_IX86`	x86 (32-bit Intel/AMD)
`__arm__` / `_M_ARM`	ARM (32-bit)
`__aarch64__`	ARM64 (64-bit)

6. Useful Non-Standard Extensions

Supported by most major compilers (GCC, Clang, MSVC) but technically not ISO C.

Macro	Description
`__COUNTER__`	Expands to an integer starting at 0 and increments by 1 every time it is used. Great for generating unique variable names.
`__cplusplus`	Defined if the file is being compiled as C++. Used for `extern "C"` blocks.
`__VA_OPT__`	(C23/C++20) Handles the trailing comma in variadic macros cleanly.

Quick Example: "Universal" Header

A common pattern combining these macros to set up a project environment.

#ifndef MY_CONFIG_H
#define MY_CONFIG_H

// 1. Check C++ compatibility
#ifdef __cplusplus
extern "C" {
#endif

// 2. OS Specific Definitions
#if defined(_WIN32)
    #define PLATFORM_NAME "Windows"
    #define PATH_SEP '\\'
#elif defined(__linux__)
    #define PLATFORM_NAME "Linux"
    #define PATH_SEP '/'
#elif defined(__APPLE__)
    #define PLATFORM_NAME "macOS"
    #define PATH_SEP '/'
#else
    #define PLATFORM_NAME "Unknown"
#endif

// 3. Debug Helper
#define LOG_LOC() printf("File: %s, Line: %d, Func: %s\n", \
                          __FILE__, __LINE__, __func__)

#ifdef __cplusplus
}
#endif

#endif