我正在尝试为GnuTLS编译我的代码，但在编译时会出现以下错误。
我已检查gnutls.h是否存在于我的系统中/usr/include/gnutls/。
还有什么可能是原因？
gcc -o tls.o tls.c
/tmp/ccfyZ1Bd.o: In function `main':
tls.c:(.text+0x1c): undefined reference to `gnutls_global_init'
tls.c:(.text+0x28): undefined reference to `gnutls_anon_allocate_client_credentials'
tls.c:(.text+0x3c): undefined reference to `gnutls_init'
tls.c:(.text+0x58): undefined reference to `gnutls_priority_set_direct'
tls.c:(.text+0x74): undefined reference to `gnutls_credentials_set'
tls.c:(.text+0x79): undefined reference to `tcp_connect'
tls.c:(.text+0x91): undefined reference to `gnutls_transport_set_ptr'
tls.c:(.text+0x9d): undefined reference to `gnutls_handshake'
tls.c:(.text+0xdc): undefined reference to `gnutls_perror'
tls.c:(.text+0x109): undefined reference to `gnutls_record_send'
tls.c:(.text+0x125): undefined reference to `gnutls_record_recv'
tls.c:(.text+0x154): undefined reference to `gnutls_strerror'
tls.c:(.text+0x1e6): undefined reference to `gnutls_bye'
tls.c:(.text+0x1f2): undefined reference to `tcp_close'
tls.c:(.text+0x1fe): undefined reference to `gnutls_deinit'
tls.c:(.text+0x20a): undefined reference to `gnutls_anon_free_client_credentials'
tls.c:(.text+0x20f): undefined reference to `gnutls_global_deinit'
collect2: ld returned 1 exit status