How to fix Paho MQTT undefined reference to `SHA1_Init’

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. theĀ ssl andĀ 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