Problem:
When trying to build your C++ application using Paho MQTT, you see a bunch of undefined reference error messages:
MQTTAsync.c:(.text+0x3482): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34a1): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34c0): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34df): undefined reference to `OpenSSL_version' /usr/bin/ld: MQTTAsync.c:(.text+0x34fe): undefined reference to `OpenSSL_version' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(MQTTAsyncUtils.c.o): in function `MQTTAsync_closeOnly': MQTTAsyncUtils.c:(.text+0x9b6): undefined reference to `SSL_SESSION_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(MQTTAsyncUtils.c.o): in function `MQTTAsync_receiveThread': MQTTAsyncUtils.c:(.text+0x463b): undefined reference to `SSL_set_session' /usr/bin/ld: MQTTAsyncUtils.c:(.text+0x5255): undefined reference to `SSL_get1_session' /usr/bin/ld: MQTTAsyncUtils.c:(.text+0x55bb): undefined reference to `SSL_get1_session' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `call_ssl_psk_cb': SSLSocket.c:(.text+0x11b): undefined reference to `SSL_get_SSL_CTX' /usr/bin/ld: SSLSocket.c:(.text+0x129): undefined reference to `SSL_CTX_get_ex_data' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_error': SSLSocket.c:(.text+0x1fa): undefined reference to `SSL_get_error' /usr/bin/ld: SSLSocket.c:(.text+0x22d): undefined reference to `ERR_print_errors_cb' /usr/bin/ld: SSLSocket.c:(.text+0x2f1): undefined reference to `ERR_get_error' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSL_CTX_info_callback': SSLSocket.c:(.text+0x376): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x381): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x3bc): undefined reference to `SSL_get_verify_result' /usr/bin/ld: SSLSocket.c:(.text+0x411): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x461): undefined reference to `SSL_get_current_cipher' /usr/bin/ld: SSLSocket.c:(.text+0x469): undefined reference to `SSL_CIPHER_get_name' /usr/bin/ld: SSLSocket.c:(.text+0x474): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4d1): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4dc): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x4e7): undefined reference to `SSL_state_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x503): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x50e): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x541): undefined reference to `SSL_alert_desc_string_long' /usr/bin/ld: SSLSocket.c:(.text+0x54c): undefined reference to `SSL_alert_type_string_long' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_initialize': SSLSocket.c:(.text+0x8ee): undefined reference to `OPENSSL_init_ssl' /usr/bin/ld: SSLSocket.c:(.text+0x90a): undefined reference to `OPENSSL_init_crypto' /usr/bin/ld: SSLSocket.c:(.text+0x916): undefined reference to `OPENSSL_init_ssl' /usr/bin/ld: SSLSocket.c:(.text+0x922): undefined reference to `OPENSSL_init_crypto' /usr/bin/ld: SSLSocket.c:(.text+0xa11): undefined reference to `CRYPTO_get_ex_new_index' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_createContext': SSLSocket.c:(.text+0xbee): undefined reference to `SSL_CTX_use_certificate_chain_file' /usr/bin/ld: SSLSocket.c:(.text+0xc48): undefined reference to `SSL_CTX_set_default_passwd_cb' /usr/bin/ld: SSLSocket.c:(.text+0xc55): undefined reference to `SSL_CTX_set_default_passwd_cb_userdata' /usr/bin/ld: SSLSocket.c:(.text+0xc67): undefined reference to `SSL_CTX_use_PrivateKey_file' /usr/bin/ld: SSLSocket.c:(.text+0xc9a): undefined reference to `SSL_CTX_load_verify_locations' /usr/bin/ld: SSLSocket.c:(.text+0xce5): undefined reference to `SSL_CTX_set_default_verify_paths' /usr/bin/ld: SSLSocket.c:(.text+0xd2e): undefined reference to `SSL_CTX_set_cipher_list' /usr/bin/ld: SSLSocket.c:(.text+0xd56): undefined reference to `SSL_CTX_set_ex_data' /usr/bin/ld: SSLSocket.c:(.text+0xd66): undefined reference to `SSL_CTX_set_psk_client_callback' /usr/bin/ld: SSLSocket.c:(.text+0xd8f): undefined reference to `SSL_CTX_ctrl' /usr/bin/ld: SSLSocket.c:(.text+0xdc5): undefined reference to `SSL_CTX_free' /usr/bin/ld: SSLSocket.c:(.text+0xe51): undefined reference to `SSL_CTX_set_alpn_protos' /usr/bin/ld: SSLSocket.c:(.text+0xeb9): undefined reference to `TLS_client_method' /usr/bin/ld: SSLSocket.c:(.text+0xec1): undefined reference to `SSL_CTX_new' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_setSocketForSSL': SSLSocket.c:(.text+0x1046): undefined reference to `SSL_CTX_set_info_callback' /usr/bin/ld: SSLSocket.c:(.text+0x1056): undefined reference to `SSL_CTX_set_msg_callback' /usr/bin/ld: SSLSocket.c:(.text+0x1075): undefined reference to `SSL_new' /usr/bin/ld: SSLSocket.c:(.text+0x10ab): undefined reference to `SSL_get_cipher_list' /usr/bin/ld: SSLSocket.c:(.text+0x10be): undefined reference to `SSL_set_fd' /usr/bin/ld: SSLSocket.c:(.text+0x113b): undefined reference to `SSL_ctrl' /usr/bin/ld: SSLSocket.c:(.text+0x121c): undefined reference to `SSL_CTX_set_verify' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_connect': SSLSocket.c:(.text+0x12b7): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x12bf): undefined reference to `SSL_connect' /usr/bin/ld: SSLSocket.c:(.text+0x135d): undefined reference to `SSL_get_peer_certificate' /usr/bin/ld: SSLSocket.c:(.text+0x138c): undefined reference to `X509_check_host' /usr/bin/ld: SSLSocket.c:(.text+0x13d4): undefined reference to `CRYPTO_free' /usr/bin/ld: SSLSocket.c:(.text+0x13f6): undefined reference to `X509_free' /usr/bin/ld: SSLSocket.c:(.text+0x144d): undefined reference to `X509_check_ip_asc' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_getch': SSLSocket.c:(.text+0x1569): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1579): undefined reference to `SSL_read' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_destroyContext': SSLSocket.c:(.text+0x1630): undefined reference to `SSL_CTX_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_close': SSLSocket.c:(.text+0x16b8): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x16c1): undefined reference to `SSL_shutdown' /usr/bin/ld: SSLSocket.c:(.text+0x16ce): undefined reference to `SSL_free' /usr/bin/ld: SSLSocket.c:(.text+0x16fd): undefined reference to `SSL_CTX_free' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_putdatas': SSLSocket.c:(.text+0x191f): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1930): undefined reference to `SSL_write' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_getdata': SSLSocket.c:(.text+0x1ca5): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1cb8): undefined reference to `SSL_read' /usr/bin/ld: SSLSocket.c:(.text+0x1d7f): undefined reference to `SSL_pending' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(SSLSocket.c.o): in function `SSLSocket_continueWrite': SSLSocket.c:(.text+0x1e2f): undefined reference to `ERR_clear_error' /usr/bin/ld: SSLSocket.c:(.text+0x1e3f): undefined reference to `SSL_write' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(Base64.c.o): in function `Base64_encodeDecode.part.0': Base64.c:(.text+0x1f): undefined reference to `BIO_f_base64' /usr/bin/ld: Base64.c:(.text+0x27): undefined reference to `BIO_new' /usr/bin/ld: Base64.c:(.text+0x2f): undefined reference to `BIO_s_mem' /usr/bin/ld: Base64.c:(.text+0x37): undefined reference to `BIO_new' /usr/bin/ld: Base64.c:(.text+0x45): undefined reference to `BIO_push' /usr/bin/ld: Base64.c:(.text+0x55): undefined reference to `BIO_set_flags' /usr/bin/ld: Base64.c:(.text+0x78): undefined reference to `BIO_write' /usr/bin/ld: Base64.c:(.text+0x8e): undefined reference to `BIO_ctrl' /usr/bin/ld: Base64.c:(.text+0x9e): undefined reference to `BIO_free_all' /usr/bin/ld: Base64.c:(.text+0xc1): undefined reference to `BIO_read' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_buildFrame.constprop.0': WebSocket.c:(.text+0x441): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `uuid_generate': WebSocket.c:(.text+0x721): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_connect': WebSocket.c:(.text+0x929): undefined reference to `RAND_bytes' /usr/bin/ld: /home/uli/.conan/data/paho-mqtt-c/1.3.9/_/_/package/6fbd1993993594bd3f9451023a96c5ec2eef0e19/lib/libpaho-mqtt3as.a(WebSocket.c.o): in function `WebSocket_upgrade': WebSocket.c:(.text+0x1dd9): undefined reference to `SHA1_Init' /usr/bin/ld: WebSocket.c:(.text+0x1def): undefined reference to `SHA1_Update' /usr/bin/ld: WebSocket.c:(.text+0x1dff): undefined reference to `SHA1_Final' collect2: error: ld returned 1 exit status
Solution
In addition to linking the Paho MQTT libraries, you also need to link OpenSSL, i.e. the ssl and crypto libraries. You can link them, for example using
target_link_libraries(myexecutable ssl crypto)
in CMake or by adding these linker flags for GCC:
-lssl -lcrypto