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SKILL.md

---
name: make
description: GNU Make skill for C/C++ build systems. Use when writing or debugging Makefiles, understanding pattern rules and automatic dependency generation, managing CFLAGS/LDFLAGS, converting ad-hoc compile commands into maintainable Makefiles, or diagnosing incremental build issues. Activates on queries about Makefiles, make targets, pattern rules, phony targets, dependency tracking, recursive make, or make errors.
---

# GNU Make

## Purpose

Guide agents through idiomatic Makefile patterns for C/C++ projects: phony targets, pattern rules, automatic dependency generation, and common build idioms.

## Triggers

- "How do I write a Makefile for my C project?"
- "My Makefile rebuilds everything every time"
- "How do I add dependency tracking to Make?"
- "What does `$@`, `$<`, `$^` mean?"
- "I'm getting 'make: Nothing to be done for all'"
- "How do I convert my shell compile script to a Makefile?"

## Workflow

### 1. Minimal correct Makefile for C

```makefile
CC      := gcc
CFLAGS  := -std=c11 -Wall -Wextra -g -O2
LDFLAGS :=
LDLIBS  :=

SRCS    := $(wildcard src/*.c)
OBJS    := $(SRCS:src/%.c=build/%.o)
TARGET  := build/prog

.PHONY: all clean

all: $(TARGET)

$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

build/%.o: src/%.c | build
	$(CC) $(CFLAGS) -c -o $@ $<

build:
	mkdir -p build

clean:
	rm -rf build
```

Automatic variables:

- `$@` — target name
- `$<` — first prerequisite
- `$^` — all prerequisites (deduplicated)
- `$*` — stem (the `%` part in a pattern rule)
- `$(@D)` — directory part of `$@`

### 2. Automatic dependency generation

Without this, changing a header doesn't trigger a rebuild of `.c` files that include it.

```makefile
CC      := gcc
CFLAGS  := -std=c11 -Wall -Wextra -g -O2
DEPFLAGS = -MMD -MP            # -MMD: generate .d file; -MP: phony targets for headers

SRCS    := $(wildcard src/*.c)
OBJS    := $(SRCS:src/%.c=build/%.o)
DEPS    := $(OBJS:.o=.d)
TARGET  := build/prog

.PHONY: all clean

all: $(TARGET)

$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

build/%.o: src/%.c | build
	$(CC) $(CFLAGS) $(DEPFLAGS) -MF $(@:.o=.d) -c -o $@ $<

-include $(DEPS)    # '-' ignores errors on first build (no .d files yet)

build:
	mkdir -p build

clean:
	rm -rf build
```

### 3. Pattern rules cheatsheet

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

# Compile C++
%.o: %.cpp
	$(CXX) $(CXXFLAGS) -c -o $@ $<

# Generate assembly
%.s: %.c
	$(CC) $(CFLAGS) -S -o $@ $<

# Run a tool on each file
build/%.processed: src/%.raw | build
	mytool $< > $@
```

### 4. Common Make patterns

#### Debug and release builds

```makefile
BUILD ?= release

ifeq ($(BUILD),debug)
  CFLAGS += -g -Og -DDEBUG
else
  CFLAGS += -O2 -DNDEBUG
endif
```

Usage: `make BUILD=debug`

#### Parallel builds

```bash
make -j$(nproc)     # use all CPUs
make -j4            # exactly 4 jobs
```

Add `-Otarget` (or `-O`) for ordered output: `make -j$(nproc) -O`

#### Verbose output

```makefile
# In Makefile: suppress with @
build/%.o: src/%.c | build
	@echo "  CC  $<"
	@$(CC) $(CFLAGS) -c -o $@ $<
```

Override silence: `make V=1` if you guard with `$(V)`:

```makefile
Q := $(if $(V),,@)
build/%.o: src/%.c
	$(Q)$(CC) $(CFLAGS) -c -o $@ $<
```

#### Installing

```makefile
PREFIX ?= /usr/local

install: $(TARGET)
	install -d $(DESTDIR)$(PREFIX)/bin
	install -m 0755 $(TARGET) $(DESTDIR)$(PREFIX)/bin/
```

### 5. Multi-directory projects

For medium projects, avoid recursive make (fragile, slow). Use a flat Makefile that includes sub-makefiles:

```makefile
# project/Makefile
include lib/module.mk
include src/app.mk
```

```makefile
# lib/module.mk
LIB_SRCS := $(wildcard lib/*.c)
LIB_OBJS := $(LIB_SRCS:lib/%.c=build/lib_%.o)
OBJS      += $(LIB_OBJS)

build/lib_%.o: lib/%.c
	$(CC) $(CFLAGS) -c -o $@ $<
```

### 6. Common errors

| Error | Cause | Fix |
|-------|-------|-----|
| `No rule to make target 'foo.o'` | Missing source or rule | Check source path and pattern rule |
| `Nothing to be done for 'all'` | Targets up to date | Touch a source file or run `make clean` |
| `Circular dependency dropped` | Target depends on itself | Check dependency chain |
| `missing separator` | Tab vs spaces | Recipes must use a tab, not spaces |
| `*** multiple target patterns` | Pattern rule syntax error | Check `%` placement |
| Rebuilds everything every time | Timestamps wrong, or PHONY missing | Check `date`; ensure `all` is `.PHONY` |
| Header change not detected | No dep tracking | Add `-MMD -MP` and `-include $(DEPS)` |

For a full variable and function reference, see [references/cheatsheet.md](references/cheatsheet.md).

## Related skills

- Use `skills/build-systems/cmake` for CMake-based projects
- Use `skills/build-systems/ninja` for Ninja as a make backend
- Use `skills/compilers/gcc` for CFLAGS details