Makefile cant find libraries

February 14, 2023

I’m trying to write my own makefile for a paho.mqtt project on a Raspberry Pi 4.

I’ve downloaded & tested the paho.mqtt install and its all working as expected.

So I’m now testing some C code but I just cant figure out the makefile (I’m new to this), my file so far,

NAME = mqtt_test
OBJ = $(NAME).o
LIBS = -libpaho-mqtt3c -libpaho-mqtt3cs
CFLAGS = -Wall -I/usr/local/include -L/usr/local/lib
CC = gcc
EXTENSION = .c

all: $(NAME)

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

$(NAME): $(OBJ)
    $(CC) -o $@ $^ $(CFLAGS) $(LIBS)

.PHONY: clean

clean:
    @rm -f *.o *~ core $(NAME)

This returns,

gcc -o mqtt_test mqtt_test.o -Wall -I/usr/local/include  -L/usr/local/lib -libpaho-mqtt3c -libpaho-mqtt3cs
/usr/bin/ld: cannot find -libpaho-mqtt3c
/usr/bin/ld: cannot find -libpaho-mqtt3cs
collect2: error: ld returned 1 exit status
make: *** [makefile:14: mqtt_test] Error 1

I’ve checked & the includes and libraries are in the directories I put after the-I and -L flags.

When I look in /usr/bin there is no ld but there are paho files prefixed with paho_ but no library files.

What am I missing?

>Solution :

You don’t use -libpaho-mqtt3c (etc.)

The option is -l so when you write -libpaho-mqtt3c the linker is looking for libraries named ibpaho-mqtt3c which of course do not exist: that would be either libibpaho-mqtt3c.a or libibpaho-mqtt3c.so.

You want to use -lpaho-mqtt3c: remove the lib at the front and the extension .a or .so, and add in the option -l.