# Makefile Functions

**Make functions** manipulate strings and lists. Common ones: `wildcard`, `patsubst`, `subst`, `filter`, `foreach`.

**`wildcard`** expands a glob pattern to a list of files:

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

`$(wildcard *.c)` lists all `.c` files in the current directory. Essential for avoiding hardcoded file lists.


**`patsubst`** replaces a pattern in a list:

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

Syntax: `$(patsubst pattern, replacement, list)`. Replace `%.c` with `%.o` in the list.

**`subst`** replaces a substring:

```makefile
SRCS = main.c util.c
OBJS = $(subst .c, .o, $(SRCS))        # simple replacement
```

Unlike `patsubst`, `subst` doesn't use patterns—it replaces the exact string.

**`filter`** selects items matching a pattern:

```makefile
SOURCES = main.c util.c test.h helper.c
C_FILES = $(filter %.c, $(SOURCES))    # main.c util.c helper.c
```

`$(filter %.c, ...)` keeps only `.c` files.

**`filter-out`** removes items matching a pattern:

```makefile
ALL_OBJS = main.o util.o test.o
REAL_OBJS = $(filter-out test.o, $(ALL_OBJS))  # excludes test.o
```

**`foreach`** loops over a list:

```makefile
DIRS = src lib test
$(foreach DIR, $(DIRS), $(DIR)/Makefile)    # expands to src/Makefile lib/Makefile test/Makefile
```

**`shell`** runs a shell command and captures output:

```makefile
VERSION = $(shell git describe --tags)
HOSTNAME = $(shell hostname)

all:
	@echo "Building version $(VERSION) on $(HOSTNAME)"
```

`$(shell ...)` runs the command and uses its output. Useful for dynamic values.

**`if`** for conditional expansion:

```makefile
PLATFORM = $(if $(findstring Linux, $(shell uname)), linux, other)
```

`$(if condition, then, else)` expands to `then` if `condition` is non-empty, else `else`.

**Chaining functions:**

```makefile
SRCS = $(wildcard src/*.c)
OBJS = $(patsubst src/%.c, build/%.o, $(SRCS))
```

Read from inner to outer: `wildcard` finds `.c` files, `patsubst` transforms paths. Powerful when combined.

**Performance note:** Functions like `shell` are expensive (spawn a process). Avoid calling them in hot loops or on every recipe. Use them sparingly in variable definitions (set once).