| .\" Copyright 1995 James R. Van Zandt <jrv@vanzandt.mv.com> |
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| .\" Changed Tue Sep 19 01:49:29 1995, aeb: moved from man2 to man3 |
| .\" added ref to /etc/utmp, added BUGS section, etc. |
| .\" modified 2003 Walter Harms, aeb - added getlogin_r, note on stdin use |
| .TH GETLOGIN 3 2015-12-28 "GNU" "Linux Programmer's Manual" |
| .SH NAME |
| getlogin, getlogin_r, cuserid \- get username |
| .SH SYNOPSIS |
| .B #include <unistd.h> |
| .sp |
| .B "char *getlogin(void);" |
| .br |
| .BI "int getlogin_r(char *" buf ", size_t " bufsize ); |
| .sp |
| .B #include <stdio.h> |
| .sp |
| .BI "char *cuserid(char *" string ); |
| .sp |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .sp |
| .BR getlogin_r (): |
| _REENTRANT || _POSIX_C_SOURCE\ >=\ 199506L |
| .br |
| .BR cuserid (): |
| _XOPEN_SOURCE |
| .SH DESCRIPTION |
| .BR getlogin () |
| returns a pointer to a string containing the name of |
| the user logged in on the controlling terminal of the process, or a |
| null pointer if this information cannot be determined. |
| The string is |
| statically allocated and might be overwritten on subsequent calls to |
| this function or to |
| .BR cuserid (). |
| .PP |
| .BR getlogin_r () |
| returns this same username in the array |
| .I buf |
| of size |
| .IR bufsize . |
| .PP |
| .BR cuserid () |
| returns a pointer to a string containing a username |
| associated with the effective user ID of the process. |
| If \fIstring\fP |
| is not a null pointer, it should be an array that can hold at least |
| \fBL_cuserid\fP characters; the string is returned in this array. |
| Otherwise, a pointer to a string in a static area is returned. |
| This |
| string is statically allocated and might be overwritten on subsequent |
| calls to this function or to |
| .BR getlogin (). |
| .PP |
| The macro \fBL_cuserid\fP is an integer constant that indicates how |
| long an array you might need to store a username. |
| \fBL_cuserid\fP is declared in \fI<stdio.h>\fP. |
| .PP |
| These functions let your program identify positively the user who is |
| running |
| .RB ( cuserid ()) |
| or the user who logged in this session |
| .RB ( getlogin ()). |
| (These can differ when set-user-ID programs are involved.) |
| .PP |
| For most purposes, it is more useful to use the environment variable |
| \fBLOGNAME\fP to find out who the user is. |
| This is more flexible |
| precisely because the user can set \fBLOGNAME\fP arbitrarily. |
| .SH RETURN VALUE |
| .BR getlogin () |
| returns a pointer to the username when successful, |
| and NULL on failure, with |
| .I errno |
| set to indicate the cause of the error. |
| .BR getlogin_r () |
| returns 0 when successful, and nonzero on failure. |
| .SH ERRORS |
| POSIX specifies |
| .TP |
| .B EMFILE |
| The per-process limit on the number of open file descriptors has been reached. |
| .TP |
| .B ENFILE |
| The system-wide limit on the total number of open files has been reached. |
| .TP |
| .B ENXIO |
| The calling process has no controlling terminal. |
| .TP |
| .B ERANGE |
| (getlogin_r) |
| The length of the username, including the terminating null byte (\(aq\\0\(aq), |
| is larger than |
| .IR bufsize . |
| .LP |
| Linux/glibc also has |
| .TP |
| .B ENOENT |
| There was no corresponding entry in the utmp-file. |
| .TP |
| .B ENOMEM |
| Insufficient memory to allocate passwd structure. |
| .TP |
| .B ENOTTY |
| Standard input didn't refer to a terminal. |
| (See BUGS.) |
| .SH FILES |
| .TP |
| \fI/etc/passwd\fP |
| password database file |
| .TP |
| \fI/var/run/utmp\fP |
| (traditionally \fI/etc/utmp\fP; |
| some libc versions used \fI/var/adm/utmp\fP) |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR getlogin () |
| T} Thread safety T{ |
| MT-Unsafe race:getlogin race:utent |
| .br |
| sig:ALRM timer locale |
| T} |
| T{ |
| .BR getlogin_r () |
| T} Thread safety T{ |
| MT-Unsafe race:utent sig:ALRM timer |
| .br |
| locale |
| T} |
| T{ |
| .BR cuserid () |
| T} Thread safety MT-Unsafe race:cuserid/!string locale |
| .TE |
| |
| In the above table, |
| .I utent |
| in |
| .I race:utent |
| signifies that if any of the functions |
| .BR setutent (3), |
| .BR getutent (3), |
| or |
| .BR endutent (3) |
| are used in parallel in different threads of a program, |
| then data races could occur. |
| .BR getlogin () |
| and |
| .BR getlogin_r () |
| call those functions, |
| so we use race:utent to remind users. |
| .SH CONFORMING TO |
| .BR getlogin () |
| and |
| .BR getlogin_r (): |
| POSIX.1-2001, POSIX.1-2008. |
| |
| System V has a |
| .BR cuserid () |
| function which uses the real |
| user ID rather than the effective user ID. |
| The |
| .BR cuserid () |
| function |
| was included in the 1988 version of POSIX, |
| but removed from the 1990 version. |
| It was present in SUSv2, but removed in POSIX.1-2001. |
| .LP |
| OpenBSD has |
| .BR getlogin () |
| and |
| .BR setlogin (), |
| and a username |
| associated with a session, even if it has no controlling terminal. |
| .SH BUGS |
| Unfortunately, it is often rather easy to fool |
| .BR getlogin (). |
| Sometimes it does not work at all, because some program messed up |
| the utmp file. |
| Often, it gives only the first 8 characters of |
| the login name. |
| The user currently logged in on the controlling terminal |
| of our program need not be the user who started it. |
| Avoid |
| .BR getlogin () |
| for security-related purposes. |
| .LP |
| Note that glibc does not follow the POSIX specification and uses |
| .I stdin |
| instead of |
| .IR /dev/tty . |
| A bug. |
| (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8 |
| all return the login name also when |
| .I stdin |
| is redirected.) |
| .LP |
| Nobody knows precisely what |
| .BR cuserid () |
| does; avoid it in portable programs. |
| Or avoid it altogether: use |
| .I getpwuid(geteuid()) |
| instead, if that is |
| what you meant. |
| .B Do not use |
| .BR cuserid (). |
| .SH SEE ALSO |
| .BR geteuid (2), |
| .BR getuid (2), |
| .BR utmp (5) |