The man page of getlogin(3) says that L_cuserid is declared in <stdio.h>. But if I try to compile (gcc-4.7.0-5.fc17.x86_64) a little source using L_cuserid #include <unistd.h> #include <stdio.h> main () { fprintf (stderr,"L_cuserid = %d",L_cuserid); } I get an error: x.c: In function ‘main’: x.c:4:34: error: ‘L_cuserid’ undeclared (first use in this function) x.c:4:34: note: each undeclared identifier is reported only once for each function it appears in The source compiles correctly if I use -D_XOPEN_SOURCE Kind regards -- Joachim Backes <joachim.backes(a)rhrk.uni-kl.de&gt; https://www-user.rhrk.uni-kl.de/~backes