Linking multiple C libraries to C create executable [duplicate] - c

Question

This question already has answers here:
Undefined Reference to `pthread_init' When Using -lpthread Flag:
(1 answer)
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Why does order in which input libraries are specified matter?
(3 answers)
Closed 5 years ago.
I am trying create a C executable which depends on multiple C static libraries.
I have two libraries :
../libs/libsulibs.a and ../ppm_client/libppm_client.a. libppm_client.a calls some functions of libsulibs.a
Here are my Makefiles
../libs/Makefile
LIBS=-lpthread
CC=gcc
CFLAGS=-Wall -g
INCLUDES=-I .
OBJ=WheelTimer.o threadApi.o LinkedListApi.o
TARGET=libsulibs.a
${TARGET}:$(OBJ)
ar rs ${TARGET} $(OBJ)
%.o:%.c
${CC} ${CFLAGS} -c ${INCLUDES} $<
clean:
rm *.o
rm ${TARGET}
I successfully create libsulibs.a through this makefile
../ppm_client/Makefile
CC=gcc
CFLAGS=-Wall -g
INCLUDES=-I . -I ../libs -I ../ppm
OBJ=ppm_pkt_enums.o ppm_client.o ppm_client_sock.o
TARGET=libppm_client.a
${TARGET}:$(OBJ)
ar rs ${TARGET} ${OBJ}
ppm_pkt_enums.o:ppm_pkt_enums.c
gcc -g -c ${INCLUDES} ppm_pkt_enums.c -o ppm_pkt_enums.o
ppm_client.o:ppm_client.c
gcc -g -c ${INCLUDES} ppm_client.c -o ppm_client.o
ppm_client_sock.o:ppm_client_sock.c
gcc -g -c ${INCLUDES} ppm_client_sock.c -o ppm_client_sock.o
clean:
rm *.o
rm ${TARGET}
This makefile too successfully create the libppm_client.a.
Now, in current dir, I have main Makefile to create executable
CC=gcc
CFLAGS=-g -Wall
STANDARD_LIBS=-lpthread
PPM_OBJ=ppm.o ppm_main.o
LIBS_OBJ=libs/LinkedListApi.o libs/threadApi.o libs/WheelTimer.o
PPM_LIBS=../libs/libsulibs.a
PPM_CLIENT_LIBS=../ppm_client/libppm_client.a
INCLUDES=-I . -I ../libs -I ../ppm_client
TARGET:exe ${PPM_LIBS} ${PPM_CLIENT_LIBS}
ppm.o:ppm.c
gcc -g -c ${INCLUDES} ppm.c -o ppm.o
ppm_main.o:ppm_main.c
gcc -g -c ${INCLUDES} ppm_main.c -o ppm_main.o
${PPM_LIBS}:
(cd ../libs; make)
${PPM_CLIENT_LIBS}:
(cd ../ppm_client; make)
exe:${PPM_LIBS} ${PPM_OBJ} ${STANDARD_LIBS} ${PPM_CLIENT_LIBS}
${CC} ${CFLAGS} ${PPM_OBJ} -o exe -L ../libs -lsulibs -L ../ppm_client -lppm_client ${STANDARD_LIBS}
clean:
rm *.o
(cd ../libs; make clean)
(cd ../ppm_client; make clean)
rm exe
But when I run make, it ends up with the error : undefined reference to pthread_init . pthread_init is a function defined in libsulibs.a and is dependent on standard pthread library.
vm#vm:~/Documents/PeriodicPacketManager/ppm$ make
(cd ../libs; make)
make[1]: Entering directory `/home/vm/Documents/PeriodicPacketManager/libs'
gcc -Wall -g -c -I . WheelTimer.c
gcc -Wall -g -c -I . threadApi.c
gcc -Wall -g -c -I . LinkedListApi.c
ar rs libsulibs.a WheelTimer.o threadApi.o LinkedListApi.o
ar: creating libsulibs.a
make[1]: Leaving directory `/home/vm/Documents/PeriodicPacketManager/libs'
gcc -g -c -I . -I ../libs -I ../ppm_client ppm.c -o ppm.o
gcc -g -c -I . -I ../libs -I ../ppm_client ppm_main.c -o ppm_main.o
(cd ../ppm_client; make)
make[1]: Entering directory `/home/vm/Documents/PeriodicPacketManager/ppm_client'
gcc -g -c -I . -I ../libs -I ../ppm ppm_pkt_enums.c -o ppm_pkt_enums.o
gcc -g -c -I . -I ../libs -I ../ppm ppm_client.c -o ppm_client.o
gcc -g -c -I . -I ../libs -I ../ppm ppm_client_sock.c -o ppm_client_sock.o
ar rs libppm_client.a ppm_pkt_enums.o ppm_client.o ppm_client_sock.o
ar: creating libppm_client.a
make[1]: Leaving directory `/home/vm/Documents/PeriodicPacketManager/ppm_client'
gcc -g -Wall ppm.o ppm_main.o -o exe -L ../libs -lsulibs -L ../ppm_client -lppm_client -lpthread
../ppm_client/libppm_client.a(ppm_client_sock.o): In function `ppm_client_init_socket':
/home/vm/Documents/PeriodicPacketManager/ppm_client/ppm_client_sock.c:138: undefined reference to `pthread_init'
collect2: error: ld returned 1 exit status
make: *** [exe] Error 1
Kindly help what am I missing here.

Answer 1

Woa !!
#Antti Haapalam
Changing the order of libraries worked.
In Main Makefile, i changed the order from :
${CC} ${CFLAGS} ${PPM_OBJ} -o exe -L ../libs -lsulibs -L ../ppm_client -lppm_client ${STANDARD_LIBS}
To:
${CC} ${CFLAGS} ${PPM_OBJ} -o exe -L ../ppm_client -lppm_client -L ../libs -lsulibs ${STANDARD_LIBS}
Can anyone comment the reasoning ?