Compilation error in GCC on Linux

I am trying to compile my code for GnuTLS, but on compiling it gives the following errors.

I have checked gnutls.h is present on my system in /usr/include/gnutls/.

What else can be the cause?

gcc -o tls.o tls.c  
/tmp/ccfyZ1Bd.o: In function `main':  
tls.c:(.text开发者_运维问答+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

This is a linking error. You need to include some library with the -l flag. Taking a wild guess without knowing your setup, I would try -lgnutls