GNU Make Mastery Part 7: Automatic Dependency Generation

Sample Source Files

main.h

/* main.h — top-level config */
#ifndef MAIN_H
#define MAIN_H
#include "utils.h"
#include "network.h"
#define APP_NAME "demo"
#endif

utils.h

/* utils.h */
#ifndef UTILS_H
#define UTILS_H
void utils_greet(const char *name);
int  utils_add(int a, int b);
#endif

network.h

/* network.h */
#ifndef NETWORK_H
#define NETWORK_H
void net_connect(const char *host, int port);
void net_disconnect(void);
#endif

crypto.h

/* crypto.h */
#ifndef CRYPTO_H
#define CRYPTO_H
void crypto_hash(const char *input, char *output, int outlen);
#endif

main.c

/* main.c — includes several headers to create a realistic dep chain */
#include <stdio.h>
#include "main.h"
#include "crypto.h"

int main(void) {
    char hash[64];
    crypto_hash("hello", hash, 64);
    utils_greet(hash);
    return 0;
}

GCC Dependency Flags

-M and -MM

-M instructs GCC to output a Make dependency rule listing all headers (including system headers). -MM is usually preferred — it omits system headers (/usr/include/) since those rarely change.

flowchart LR
    SRC["main.c
#include utils.h"] -->|"gcc -MMD -c"| OBJ["main.o"]
    SRC -->|"gcc -MMD -c"| DEP["main.d
main.o: main.c utils.h"]
    DEP -->|"-include *.d"| MAKE["Makefile
reads .d files"]
    MAKE -->|"utils.h modified?"| REBUILD{"Rebuild
needed?"}
    REBUILD -->|"Yes"| RECOMPILE["Recompile main.c"]
    REBUILD -->|"No"| SKIP["Skip (up to date)"]

    style SRC fill:#e8f4f4,stroke:#3B9797
    style DEP fill:#f0f4f8,stroke:#16476A
    style MAKE fill:#132440,stroke:#132440,color:#fff
                        

# Run manually to see what's generated:
gcc -MM main.c
# Output:
# main.o: main.c main.h config.h utils.h

-MD and -MMD — During Compilation

These flags make GCC write the dependency output to a .d file alongside normal compilation — no separate preprocessing pass needed. -MMD = -MM -MD (omit system headers, write to .d).

# GCC writes main.d during the regular compile:
gcc -Wall -c -MMD -MP main.c -o main.o
# Creates: main.o  AND  main.d

# Example contents of main.d:
main.o: main.c main.h config.h utils.h
main.h:       # phony target added by -MP
config.h:     # (prevents error if header is deleted)
utils.h:

-MP — Phony Targets for Headers

When a header is deleted and it still appears in a .d file, Make errors with "No rule to make target deleted_header.h". The -MP flag adds a phony (empty) target for each header, so Make silently ignores missing ones and just rebuilds the affected .o.

-MF — Custom Dependency File Path

# Write dependency file to a specific path (e.g., in a separate dep dir):
gcc -MMD -MP -MF build/deps/main.d -c main.c -o build/main.o

Including .d Files in Make

Include the generated dependency files at the end of your Makefile using -include (the - prefix suppresses errors on first build when .d files don't exist yet).

CC       := gcc
CFLAGS   := -Wall -Wextra -std=c11 -O2
SRCDIR   := src
BUILDDIR := build
DEPDIR   := $(BUILDDIR)/deps

SRCS  := $(wildcard $(SRCDIR)/*.c)
OBJS  := $(patsubst $(SRCDIR)/%.c,$(BUILDDIR)/%.o,$(SRCS))
DEPS  := $(patsubst $(SRCDIR)/%.c,$(DEPDIR)/%.d,$(SRCS))

TARGET := $(BUILDDIR)/myapp

.PHONY: all clean

all: $(TARGET)

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

# The key: -MMD -MP to generate .d file, -MF to specify output path
$(BUILDDIR)/%.o: $(SRCDIR)/%.c
	@mkdir -p $(BUILDDIR) $(DEPDIR)
	$(CC) $(CFLAGS) -MMD -MP -MF $(DEPDIR)/$*.d -c $< -o $@

# Include all .d files — silent on first build (no .d files yet)
-include $(DEPS)

clean:
	rm -rf $(BUILDDIR)

Generated Headers & Out-of-Source Builds

Some projects generate headers (e.g., from .proto files or configure scripts). These headers must be listed as prerequisites of the rules that generate them and as order-only prerequisites of the compile rules that use them.

BUILDDIR := build

# Rule to generate a header from a .proto file
$(BUILDDIR)/protocol.pb.h: protocol.proto
	protoc --c_out=$(BUILDDIR) $<              # $< = protocol.proto

# Compile rule: order-only prereq (|) ensures header is generated first
# but does NOT trigger recompile when header's timestamp changes
$(BUILDDIR)/handler.o: src/handler.c | $(BUILDDIR)/protocol.pb.h
	$(CC) $(CFLAGS) -I$(BUILDDIR) -MMD -MP -c $< -o $@

For out-of-source builds where .d files contain absolute or different-prefix paths, use -MF explicitly to control where dependency files are written.

Hands-On Milestone

# Production Makefile with automatic dependency generation

# ── Variables ────────────────────────────────────────
CC       := gcc
CFLAGS   := -Wall -Wextra -std=c11 -O2
SRCDIR   := src
BUILDDIR := build
TARGET   := $(BUILDDIR)/myapp

SRCS  := $(wildcard $(SRCDIR)/*.c)                               # find all .c files
OBJS  := $(patsubst $(SRCDIR)/%.c,$(BUILDDIR)/%.o,$(SRCS))       # src/foo.c → build/foo.o
DEPS  := $(OBJS:.o=.d)                # .o → .d (dep files generated by -MMD)

.PHONY: all clean info

all: $(TARGET)

# ── Link ──
$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^             # $^ = all .o files

# ── Compile (with auto-dep generation) ──
$(BUILDDIR)/%.o: $(SRCDIR)/%.c
	@mkdir -p $(BUILDDIR)
	$(CC) $(CFLAGS) -MMD -MP -c $< -o $@  # -MMD writes .d file, -MP adds phony header targets

# Pull in generated .d files (silent on first build when none exist)
-include $(DEPS)

clean:
	rm -rf $(BUILDDIR)

info:
	@echo "CC       = $(CC)"
	@echo "CFLAGS   = $(CFLAGS)"
	@echo "TARGET   = $(TARGET)"
	@echo "SRCDIR   = $(SRCDIR)"
	@echo "SRCS     = $(SRCS)"
	@echo "OBJS     = $(OBJS)"
	@echo "DEPS     = $(DEPS)"

# --- Setup: create src/ directory with all source files ---
mkdir -p src

cat > src/main.h << 'EOF'
#ifndef MAIN_H
#define MAIN_H
#include "utils.h"
#include "network.h"
#define APP_NAME "demo"
#endif
EOF

cat > src/utils.h << 'EOF'
#ifndef UTILS_H
#define UTILS_H
void utils_greet(const char *name);
int  utils_add(int a, int b);
#endif
EOF

cat > src/network.h << 'EOF'
#ifndef NETWORK_H
#define NETWORK_H
void net_connect(const char *host, int port);
void net_disconnect(void);
#endif
EOF

cat > src/crypto.h << 'EOF'
#ifndef CRYPTO_H
#define CRYPTO_H
void crypto_hash(const char *input, char *output, int outlen);
#endif
EOF

cat > src/main.c << 'EOF'
#include <stdio.h>
#include <string.h>
#include "main.h"
#include "crypto.h"
int main(void) {
    char hash[64];
    crypto_hash("hello", hash, 64);
    utils_greet(hash);
    return 0;
}
EOF

cat > src/utils.c << 'EOF'
#include <stdio.h>
#include "utils.h"
void utils_greet(const char *name) { printf("Hello, %s!\n", name); }
int  utils_add(int a, int b)       { return a + b; }
EOF

cat > src/network.c << 'EOF'
#include <stdio.h>
#include "network.h"
void net_connect(const char *host, int port) { printf("Connecting to %s:%d\n", host, port); }
void net_disconnect(void) { printf("Disconnected\n"); }
EOF

cat > src/crypto.c << 'EOF'
#include <stdio.h>
#include <string.h>
#include "crypto.h"
void crypto_hash(const char *input, char *output, int outlen) {
    snprintf(output, outlen, "hash(%s)", input);
}
EOF

# --- Try it: verify dependency tracking ---
make info                   # display all variable values
make                        # full build
ls build/*.d                # see generated .d dependency files
cat build/main.d            # inspect dependency chain for main.c

touch src/utils.h           # simulate header change
make                        # only files including utils.h rebuild

touch src/crypto.h          # simulate another header change
make                        # only crypto-dependent files rebuild
make clean                  # remove build artifacts