# Makefile Variables

**Variables** in Make store strings (compiler names, flags, file lists). Use `VAR = value` to define, `$(VAR)` to expand.

```makefile
CC = gcc
CFLAGS = -O3 -Wall -g
SRCS = main.c util.c helper.c
OBJS = main.o util.o helper.o

app: $(OBJS)
	$(CC) $(CFLAGS) -o app $(OBJS)
```

`$(CC)` expands to `gcc`, `$(CFLAGS)` to `-O3 -Wall -g`. Changing `CC` once updates every recipe that uses it.

**Automatic variables** are special—Make sets them for each recipe:

```makefile
$@   target filename
$<   first prerequisite
$^   all prerequisites (space-separated)
$+   all prerequisites (with duplicates)
```

Example:

```makefile
main.o: main.c
	$(CC) $(CFLAGS) -c $< -o $@
```

`$<` is `main.c`, `$@` is `main.o`. The recipe becomes: `gcc -O3 -Wall -g -c main.c -o main.o`.

**Generic rule with automatic variables:**

```makefile
%.o: %.c
	$(CC) $(CFLAGS) -c $< -o $@
```

This rule matches any `.o` depending on `.c`: `main.o` from `main.c`, `util.o` from `util.c`, etc. `$<` is the matching `.c` file, `$@` is the `.o` file.

**Variable assignment modes:**

```makefile
VAR = value              # lazy (expands when used)
VAR := value             # immediate (expands when defined)
VAR ?= value             # conditional (only if not set)
VAR += value             # append
```

`=` is most common. `:=` is useful when you need the value immediately (e.g., in conditionals).

**Built-in variables:**

```makefile
CC       C compiler (default: cc)
CFLAGS   C compiler flags
LDFLAGS  linker flags
LDLIBS   libraries to link
```

Make has defaults for these; override in your Makefile or on the command line: `make CFLAGS=-O2`.

**Command-line variables:**

```bash
make CFLAGS=-O2         # override in this run
make CC=clang           # use a different compiler
```

These take precedence over Makefile definitions.

**String functions:** Variables can use functions like `patsubst` (pattern substitution):

```makefile
SRCS = main.c util.c
OBJS = $(patsubst %.c, %.o, $(SRCS))   # main.o util.o
```

`patsubst` replaces patterns. More on functions in [[makefile-functions]].