GNU-Make does not recompile when a header file changed

Question

GNU-Make does not recompile when hdr.h file changed. As below printed lines, it did not try to recompile even main.d file is generated. Can you guide me why it happend?

hdr.h

#ifndef __HDR_H__  

#define LOOP_CNT 1000  

#endif /* __HDR_H__ */  

main.c

#include <stdio.h>  
#include "hdr.h"  

int main(void)  
{  
    int i, sum = 0;  
    for (i = 0; i < LOOP_CNT; i++) sum += i;  
    (void)printf("sum = %d\n", sum);  
    return 0;  
}  

Makefile

SUFFIXES += .d

.PHONY: clean  

OBJECTS = $(patsubst %.c,%.o,$(wildcard *.c))  
CC = armcc  
LD = armcc  
CFLAGS += 

# Default target  
all: sum  

sum : $(OBJECTS)  
    $(CC) $(CFLAGS) -o $@ $^  

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

# Generating dependency files  
%.d : %.c  
    @$(CC) -M $< > $@  

# Include dependency file to have gcc recompile necessary sources  
include $(patsubst %.c,%.d,$(wildcard *.c))  
#$(info $(patsubst %.c,%.d,$(wildcard *.c)))

clean:  
    rm -f *.o *.d core $(EXEC_NAME)  

Here is printed line in second.

C:\project\dep>make all
Makefile:24: main.d: No such file or directory
armcc    -o main.o -c main.c
armcc    -o sum main.o

C:\project\dep>make all
make: Nothing to be done for `all'. 

main.d file is generated as below.

__image.axf: main.c
__image.axf: C:\Program Files\ARM\RVCT\Data\4.1\713\include\windows\stdio.h
__image.axf: hdr.h

Answer 1

As a quick and dirty Makefile fix for rebuilding if headers change I just list all my header files and then add $(HEADERS) as a dependency in the part that builds the object files from the C src files. Its not as efficient as it could be but I find it to be good enough, i.e.

HEADERS = \
    my_header.h \
    my_other_header.h

$(BUILD_DIR)/%.o: %.c $(HEADERS)
    $(LINK.c) $< -c -o $@

Answer 2

Generally speaking, if a source file needs to be compiled to create an object file, then its dependency file also needs to be rebuilt.

So, instead of having a separate target for the .d files, simple regenerate it when compiling. To achieve this, a simple approach is to replace

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

# Generating dependency files  
%.d : %.c  
    @$(CC) -M $< > $@  

with

$(OBJECTS) : %.o : %.c   
    $(CC) $(CFLAGS) -o $@ -c $<  
    @$(CC) -M $< > $@  

Note that the dependency file will only be regnerated if compilation succeeds. If compilation fails, the object file will be deleted, so recompilation would be forced regardless of whether the dependency file is up to date.