The tcgetpgrp() should fail with -1 and should set errno when the file descriptor is a pipe descriptor.  It returns 0.  Note that file descriptors created with socket pair do return -1.  The program below outputs s=0 and errno=0, but it should output s=-1 and errno non-zero.

How-To-Repeat: #include        <unistd.h>
#include        <stdio.h>
#include        <errno.h>
int main(int argc, char *argv[])
{
        int s,pv[2];
        pipe(pv);
        errno = 0;
        s = tcgetpgrp(pv[1]);
        fprintf(stderr,"s=%d errno=%d\n",s,errno);
        return(0);
}