I had this problem compiling x11/sessre port:

"
===>  Building for sessreg-1.0.7
make  all-recursive
Making all in man
  GEN      filenames.sed
  GEN      sessreg.1
  CC       sessreg.o
sessreg.c: In function 'main':
sessreg.c:276: warning: assignment discards qualifiers from pointer target type
sessreg.c:281: warning: implicit declaration of function 'ttyslot'
sessreg.c:281: warning: nested extern declaration of 'ttyslot'
sessreg.c: In function 'set_utmp':
sessreg.c:387: warning: declaration of 'line' shadows a global declaration
sessreg.c:93: warning: shadowed declaration is here
sessreg.c: In function 'set_utmpx':
sessreg.c:463: warning: declaration of 'line' shadows a global declaration
sessreg.c:93: warning: shadowed declaration is here
sessreg.c: In function 'Xslot':
sessreg.c:556: warning: declaration of 'ttys_file' shadows a global declaration
sessreg.c:113: warning: shadowed declaration is here
sessreg.c:556: warning: declaration of 'host_name' shadows a global declaration
sessreg.c:108: warning: shadowed declaration is here
sessreg.c: In function 'findslot':
sessreg.c:616: warning: declaration of 'host_name' shadows a global declaration
sessreg.c:108: warning: shadowed declaration is here
sessreg.c:632: warning: assignment discards qualifiers from pointer target type
  CCLD     sessreg
sessreg.o: In function `main':
sessreg.c:(.text+0xc7a): undefined reference to `ttyslot'
*** [sessreg] Error code 1

Stop in /usr/ports/x11/sessreg/work/sessreg-1.0.7.
*** [all-recursive] Error code 1

Stop in /usr/ports/x11/sessreg/work/sessreg-1.0.7.
*** [all] Error code 1

Stop in /usr/ports/x11/sessreg/work/sessreg-1.0.7.
*** [do-build] Error code 1

Stop in /usr/ports/x11/sessreg.
root:[172] 
"

I found ttyslot in man page. Then I made a little thisprogram to test:

"
#include        <stdio.h>
#include        <unistd.h>


main()
{
        printf( "%d\n",ttyslot() );
}
"

Result:

"
goffredo:[1004] cc -s -o ttyslot_test ttyslot_test.c
/tmp//ccyJQC88.o: In function `main':
ttyslot_test.c:(.text+0xa): undefined reference to `ttyslot'

"

I worked around downloading and installing the old package below:

ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.0-RELEASE/packages/x11/sessreg-1.0.7.tbz

How-To-Repeat: Compile this program:

"
#include        <stdio.h>
#include        <unistd.h>


main()
{
        printf( "%d\n",ttyslot() );
}
"