mirror of https://github.com/mkerrisk/man-pages
136 lines
5.1 KiB
Groff
136 lines
5.1 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright 1995 James R. Van Zandt <jrv@vanzandt.mv.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Changed Tue Sep 19 01:49:29 1995, aeb: moved from man2 to man3
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.\" added ref to /etc/utmp, added BUGS section, etc.
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.\" modified 2003 Walter Harms, aeb - added getlogin_r, note on stdin use
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.TH GETLOGIN 3 2003-08-24 "Linux 2.4" "Linux Programmer's Manual"
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.SH NAME
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getlogin, getlogin_r, cuserid \- get user name
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.SH SYNOPSIS
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.B #include <unistd.h>
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.sp
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.B "char *getlogin(void);"
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.br
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.BI "int getlogin_r(char *" buf ", size_t " bufsize );
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.sp
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.B #include <stdio.h>
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.sp
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.BI "char *cuserid(char *" string );
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.SH DESCRIPTION
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\fBgetlogin\fP returns a pointer to a string containing the name of
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the user logged in on the controlling terminal of the process, or a
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null pointer if this information cannot be determined. The string is
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statically allocated and might be overwritten on subsequent calls to
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this function or to \fBcuserid\fP.
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.PP
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\fBgetlogin_r\fP returns this same user name in the array
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.I buf
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of size
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.IR bufsize .
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.PP
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\fBcuserid\fP returns a pointer to a string containing a user name
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associated with the effective user ID of the process. If \fIstring\fP
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is not a null pointer, it should be an array that can hold at least
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\fBL_cuserid\fP characters; the string is returned in this array.
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Otherwise, a pointer to a string in a static area is returned. This
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string is statically allocated and might be overwritten on subsequent
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calls to this function or to \fBgetlogin\fP.
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.PP
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The macro \fBL_cuserid\fP is an integer constant that indicates how
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long an array you might need to store a user name. \fBL_cuserid\fP is
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declared in \fBstdio.h\fP.
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.PP
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These functions let your program identify positively the user who is
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running (\fBcuserid\fP) or the user who logged in this session
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(\fBgetlogin\fP). (These can differ when set-user-ID programs are
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involved.)
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.PP
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For most purposes, it is more useful to use the environment variable
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\fBLOGNAME\fP to find out who the user is. This is more flexible
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precisely because the user can set \fBLOGNAME\fP arbitrarily.
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.SH "RETURN VALUE"
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\fBgetlogin\fP returns a pointer to the user name when successful,
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and NULL on failure.
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\fBgetlogin_r\fP returns 0 when successful, and non-zero on failure.
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.SH ERRORS
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POSIX specifies
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.TP
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.B EMFILE
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The calling process already has the maximum allowed number of open files.
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.TP
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.B ENFILE
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The system already has the maximum allowed number of open files.
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.TP
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.B ENXIO
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The calling process has no controlling tty.
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.TP
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.B ERANGE
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(getlogin_r)
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The length of the user name, including final NUL, is larger than
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.IR bufsize .
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.LP
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Linux/glibc also has
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.TP
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.B ENOENT
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There was no corresponding entry in the utmp-file.
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.TP
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.B ENOMEM
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Insufficient memory to allocate passwd structure.
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.SH FILES
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.nf
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\fI/etc/passwd\fP password database file
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.br
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\fI/var/run/utmp\fP (traditionally \fI/etc/utmp\fP;
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some libc versions used \fI/var/adm/utmp\fP)
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.fi
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.SH "CONFORMING TO"
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POSIX.1. System V has a \fBcuserid\fP function which uses the real
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user ID rather than the effective user ID. The \fBcuserid\fP function
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was included in the 1988 version of POSIX, but removed from the 1990 version.
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.LP
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OpenBSD has \fBgetlogin\fP and \fBsetlogin\fP, and a username
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associated with a session, even if it has no controlling tty.
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.SH BUGS
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Unfortunately, it is often rather easy to fool getlogin().
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Sometimes it does not work at all, because some program messed up
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the utmp file. Often, it gives only the first 8 characters of
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the login name. The user currently logged in on the controlling tty
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of our program need not be the user who started it.
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Avoid getlogin() for security-related purposes.
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.LP
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Note that glibc does not follow the POSIX spec and uses stdin
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instead of
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.IR /dev/tty .
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A bug. (Other recent systems, like SunOS 5.8 and HPUX 11.11 and FreeBSD 4.8
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all return the login name also when stdin is redirected.)
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.LP
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Nobody knows precisely what cuserid() does; avoid it in portable programs.
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Or avoid it altogether: use getpwuid(geteuid()) instead, if that is
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what you meant.
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DO NOT USE cuserid().
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.SH "SEE ALSO"
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.BR geteuid (2),
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.BR getuid (2)
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