# Makefile Conditionals

**Conditional directives** allow different behavior based on variables or environment. Common: `ifeq`, `ifneq`, `ifdef`, `ifndef`.

**`ifeq` (equal):**

```makefile
ifeq ($(OS), Linux)
    LDFLAGS = -lrt
else
    LDFLAGS = -framework CoreFoundation
endif
```

If `$(OS)` equals `Linux`, set Linux-specific flags. Otherwise, use macOS flags.

**`ifneq` (not equal):**

```makefile
ifneq ($(PLATFORM), windows)
    CFLAGS = -fPIC
endif
```

If `$(PLATFORM)` is not `windows`, add position-independent code flag.

**`ifdef` (defined):**

```makefile
ifdef DEBUG
    CFLAGS += -g -O0
else
    CFLAGS += -O3
endif
```

If `DEBUG` is defined (even if empty), use debug flags. Otherwise, optimize.

**`ifndef` (not defined):**

```makefile
ifndef INSTALL_PREFIX
    INSTALL_PREFIX = /usr/local
endif
```

If `INSTALL_PREFIX` is not set, use a default.

**Detecting the platform:**

```makefile
UNAME_S := $(shell uname -s)
ifeq ($(UNAME_S), Linux)
    LDLIBS = -lrt
endif
ifeq ($(UNAME_S), Darwin)
    LDLIBS = -framework CoreFoundation
endif
```

Detect the OS at make time and set appropriate flags.

**Conditional compilation:**

```makefile
ifdef ENABLE_TESTS
    SRCS += test.c
    TEST_TARGET = run_tests
else
    TEST_TARGET =
endif

app: $(SRCS)
	gcc -o app $(SRCS)

test: $(TEST_TARGET)
	@echo "Tests skipped (use: make ENABLE_TESTS=1)"

$(TEST_TARGET): test.c
	./run_tests
```

Enable tests with `make ENABLE_TESTS=1 test`.

**Nested conditionals:**

```makefile
ifdef RELEASE
    CFLAGS = -O3
    ifdef STRIP
        LDFLAGS += -s
    endif
else
    CFLAGS = -g -O0
endif
```

Conditionals can be nested. Use indentation for clarity.

**Conditional includes:**

```makefile
ifdef CONFIG_FILE
    include $(CONFIG_FILE)
else
    include default_config.mk
endif
```

Include different configuration files based on conditions.

**Example: Debug vs Release builds:**

```makefile
ifdef DEBUG
    CFLAGS = -g -O0 -DDEBUG
    TARGET = app_debug
else
    CFLAGS = -O3
    TARGET = app
endif

$(TARGET): main.o util.o
	gcc -o $@ main.o util.o

clean:
	rm -f *.o app app_debug
```

Build with `make DEBUG=1` for debug, `make` for release.