How to fix makefile to properly include -lcrypto to avoid linker error: undefined reference to `EVP_CIPHER_CTX_new'

Question

First of all I am trying to use

#include <openssl/evp.h>

I am getting the common linker error:

undefined reference to `EVP_CIPHER_CTX_new'

which I know can be solved by using -lcrypto when compiling. I am able to do this when compiling manually ie

g++ encrv2.cpp -o encrv2 -lcrypto 

though I am trying to create a Makefile and putting my encryption/decryption functions in a seperate header file (AesGcm) and am unable to get around the linker issue. Below is my Makefile so far and the output when I try and make all.

CXX = g++
CXXFLAGS = -std=c++14
LDFLAGS= -lcrypto

all: encrv2

AesGcm.o: AesGcm.cpp AesGcm.h
    $(CXX) $(CXXFLAGS) -c AesGcm.cpp $(LDFLAGS)

encrv2.o: encrv2.cpp AesGcm.h
    $(CXX) $(CXXFLAGS) -c encrv2.cpp $(LDFLAGS)

encrv2: encrv2.o AesGcm.o

clean:
    rm -f *~ *.o encrv2 

Giving the following error:

cc -lcrypto  encrv2.o AesGcm.o   -o encrv2
AesGcm.o: In function `AesGcm::encrypt(unsigned char*, int, unsigned char*, int, unsigned char*, unsigned char*, unsigned char*, unsigned char*)':
AesGcm.cpp:(.text+0x3e): undefined reference to `EVP_CIPHER_CTX_new'
AesGcm.cpp:(.text+0x58): undefined reference to `EVP_aes_256_gcm'
AesGcm.cpp:(.text+0x77): undefined reference to `EVP_EncryptInit_ex'
AesGcm.cpp:(.text+0xae): undefined reference to `EVP_EncryptInit_ex'
AesGcm.cpp:(.text+0xe0): undefined reference to `EVP_EncryptUpdate'
AesGcm.cpp:(.text+0x111): undefined reference to `EVP_EncryptUpdate'
AesGcm.cpp:(.text+0x148): undefined reference to `EVP_EncryptFinal_ex'
AesGcm.cpp:(.text+0x17b): undefined reference to `EVP_CIPHER_CTX_ctrl'
AesGcm.cpp:(.text+0x197): undefined reference to `EVP_CIPHER_CTX_free'
AesGcm.o: In function `AesGcm::decrypt(unsigned char*, int, unsigned char*, int, unsigned char*, unsigned char*, unsigned char*, unsigned char*)':
AesGcm.cpp:(.text+0x30a): undefined reference to `EVP_CIPHER_CTX_new'
AesGcm.cpp:(.text+0x324): undefined reference to `EVP_aes_256_gcm'
AesGcm.cpp:(.text+0x343): undefined reference to `EVP_DecryptInit_ex'
AesGcm.cpp:(.text+0x37a): undefined reference to `EVP_DecryptInit_ex'
AesGcm.cpp:(.text+0x3ac): undefined reference to `EVP_DecryptUpdate'
AesGcm.cpp:(.text+0x3dd): undefined reference to `EVP_DecryptUpdate'
AesGcm.cpp:(.text+0x410): undefined reference to `EVP_CIPHER_CTX_ctrl'
AesGcm.cpp:(.text+0x441): undefined reference to `EVP_DecryptFinal_ex'
AesGcm.cpp:(.text+0x450): undefined reference to `EVP_CIPHER_CTX_free'
collect2: error: ld returned 1 exit status
<builtin>: recipe for target 'encrv2' failed
make: *** [encrv2] Error 1

Answer 1

You're using the wrong variable.

LDFLAGS are intended to be used for linker flags such as -L , etc. that need to come "earlier" in the link line. That variable is not intended to have libraries that are added to the link line.

You should be using the LDLIBS variable for that:

LDLIBS = -lcrypto

The default rule for linking, which is what you're using since you don't write one yourself, looks something like this:

%: %.o
#  recipe to execute (built-in):
        $(CC) $(LDFLAGS) $(TARGET_ARCH) $^ $(LDLIBS) -o $@

Note that LDLIBS comes after your object files, which is what you need to avoid this linker error. Order of object files versus libraries, and between libraries themselves, is very important to the linker.