GNU make: What does '*** missing separator. Stop' mean for a source code file? - makefile

Question

I'm fairly new to Makefiles, and I am trying to create one for a C project I am building using gcc.
I am familiar with the error Makefile:<col>: *** missing separator. Stop. It has popped up before when I used spaces instead of tabs to precede rules in the Makefile. I just tried writing a Makefile for this particular project (being sure to use TAB character instead of spaces) and when I run the make command, I get a very nondescript error I do not understand how to fix: src/main.c:7: *** missing separator. Stop
My directory structure looks like this:
- projectfolder/
- Makefile
- bin/
- build/
- inc/
- src/
- main.c
- otherfolder/
- inc/
- common.h
- io.h
- src/
- io.c
main.c, which includes the main function, has the following imports:
#include <stdio.h>
#include <stdlib.h>
#include "common.h"
#include "io.h"
My goal is to make a bunch of .o and .d files from the files in both src/ and inc/ directories and put those into projectfolder/build/, so that they can then be used to make the final executable in projectfolder/bin/
Finally, here is the Makefile that is causing the issue:
# Compiler and linker
CC := gcc
# Target binary
TARGET := the-program
# File extensions
SRCEXT := c
# Directories
TARGETDIR := bin
BUILDDIR := build
SRCDIRS := src /home/myusername/otherfolder/src
INCDIRS := inc /home/myusername/otherfolder/inc
# gcc options
CCFLAGS := -Wall -Wextra -O3
#---------------------------------------------------------------------------------
#DO NOT EDIT BELOW THIS LINE
#---------------------------------------------------------------------------------
vpath %.$(SRCEXT) $(SRCDIRS)
vpath %.h $(INCDIRS)
CCFLAGS += $(addprefix -I,$(INCDIRS)) -MMD -MP
SRC := $(shell find $(SRCDIRS) -name *.$(SRCEXT))
OBJ := $(SRC:$(SRCDIRS)/%.$(SRCEXT)=$(BUILDDIR)/%.o)
DEP := $(OBJ:.o=.d)
TARGET := $(TARGETDIR)/$(TARGET)
# RULE: Default make
all: makedirs $(TARGET) clean
# RULE: Remake
remake: fullclean all
# RULE: Clean
clean:
rm -rf $(BUILDDIR)
# RULE: Full clean (objects and binaries)
fullclean: clean
rm -rf $(TARGETDIR)
# RULE: Make dirs
makedirs:
mkdir -p $(BUILDDIR)
mkdir -p $(TARGETDIR)
# RULE: Link
$(TARGET): $(OBJ)
$(CC) $(OBJ) -o $#
# RULE: Compile
$(OBJ): $(SRC)
$(CC) $(CCFLAGS) -c $< -o $#
# RULE: Non-file targets
.PHONY: all remake clean fullclean makedirs
# include dependencies
-include $(DEP)
What about this file would cause the described error? I am assuming that it is related to the Makefile itself rather than the C code actually contained in src/main.c since this does not appear to be a compilation error, but if I am wrong, let me know and I can change the post.

Answer 1

This error:
src/main.c:7: *** missing separator. Stop
is clearly being printed by make. Since we know the format of these messages is <makefile>:<linenumber>: <error>, we can see that make is trying to parse the file src/main.c as a makefile and obviously this cannot work.
How could make be doing such a thing? The obvious culprit is this line:
-include $(DEP)
If the variable DEP contains the file src/main.c, then make would try to include that as a makefile and parse it. So how is DEP set?
DEP := $(OBJ:.o=.d)
This changes all words in OBJ that end with .o to end with .d. Crucially, it should be noted that this makes no changes to words that do not end in .o. So, if OBJS contained src/main.c, this would pass it through unmodified to DEPS.
So how is OBJ set? Here are the relevant variables:
SRCDIRS := src /home/myusername/otherfolder/src
SRC := $(shell find $(SRCDIRS) -name *.$(SRCEXT))
OBJ := $(SRC:$(SRCDIRS)/%.$(SRCEXT)=$(BUILDDIR)/%.o)
Let's expand this: the SRC variable runs:
find src /home/myusername/otherfolder/src -name *.c
(you really should escape the *, either with backslash or quotes: it's very dangerous how you have this).
Now we'll say that SRC gets the value:
SRC := src/main.c /home/myusername/otherfolder/src/other.c
Now what does OBJ contain?
OBJ := $(SRC:src /home/myusername/otherfolder/src/%.$(SRCEXT)=build/%.o)
This clearly cannot work: you can't put multiple directories into SRCDIRS, but then treat it as if it contained only one word.

Answer 2

The problem is in the line
OBJ := $(SRC:$(SRCDIRS)/%.$(SRCEXT)=$(BUILDDIR)/%.o)
The pattern substitution works only where the pattern matches and leaves all other strings alone.
As a simple demonstration see this makefile (no rules, just variable assignments):
x := foo bar baz
y := $(x:ba%=gu%)
$(info $(x))
$(info $(y))
Output:
foo bar baz
foo gur guz
As you can see, foo still is there, unchanged although it didn't match the pattern ba%.
In your case you are passing an impossible to substitute pattern, the content of $(SRCDIRS), which is the two words: src /home/myusername/otherfolder/src to the substitution call, therefore main.c - which is either src/main.c or /home/myusername/otherfolder/src/main.c remains unchanged in your list, gets inadvertently flushed further into $(DEP) and is finally included as text with your last line -include $(DEP).
As a recommendation I want to point you to VPATH (see manual and here: https://www.cmcrossroads.com/article/basics-vpath-and-vpath), which is the better alternative to indicate the location of your sources to make.