dlinfo.3: ATTRIBUTES: Note function that is thread-safe

[thirdparty/man-pages.git] / man3 / getenv.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (C) 2007, 2012 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's "POSIX Programmer's Guide" (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 19:30:29 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Fri Feb 14 21:47:50 1997 by Andries Brouwer (aeb@cwi.nl)
.\"
.TH GETENV 3  2015-03-02 "GNU" "Linux Programmer's Manual"
.SH NAME
getenv, secure_getenv \- get an environment variable
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "char *getenv(const char *" name );

.BI "char *secure_getenv(const char *" name );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR secure_getenv ():
_GNU_SOURCE
.SH DESCRIPTION
The
.BR getenv ()
function searches the environment list to find the
environment variable
.IR name ,
and returns a pointer to the corresponding
.I value
string.

The GNU-specific
.BR secure_getenv ()
function is just like
.BR getenv ()
except that it returns NULL in cases where "secure execution" is required.
Secure execution is required if one of the following conditions
was true when the program run by the calling process was loaded:
.IP * 3
the process's effective user ID did not match its real user ID or
the process's effective group ID did not match its real group ID
(typically this is the result of executing a set-user-ID or
set-group-ID program);
.IP *
the effective capability bit was set on the executable file; or
.IP *
the process has a nonempty permitted capability set.
.PP
Secure execution may also required if triggered
by some Linux security modules.

The
.BR secure_getenv ()
function is intended for use in general-purpose libraries
to avoid vulnerabilities that could occur if
set-user-ID or set-group-ID programs accidentally
trusted the environment.
.SH RETURN VALUE
The
.BR getenv ()
function returns a pointer to the value in the
environment, or NULL if there is no match.
.SH VERSIONS
.BR secure_getenv ()
first appeared in glibc 2.17.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw25 lb lb
l l l.
Interface	Attribute	Value
T{
.BR getenv (),
.BR secure_getenv ()
T}	Thread safety	MT-Safe env
.TE
.SH CONFORMING TO
.BR getenv ():
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

.BR secure_getenv ()
is a GNU extension.
.SH NOTES
The strings in the environment list are of the form \fIname=value\fP.

As typically implemented,
.BR getenv ()
returns a pointer to a string within the environment list.
The caller must take care not to modify this string,
since that would change the environment of the process.

The implementation of
.BR getenv ()
is not required to be reentrant.
The string pointed to by the return value of
.BR getenv ()
may be statically allocated,
and can be modified by a subsequent call to
.BR getenv (),
.BR putenv (3),
.BR setenv (3),
or
.BR unsetenv (3).

The "secure execution" mode of
.BR secure_getenv ()
is controlled by the
.B AT_SECURE
flag contained in the auxiliary vector passed from the kernel to user space.
.SH SEE ALSO
.BR clearenv (3),
.BR getauxval (3),
.BR putenv (3),
.BR setenv (3),
.BR unsetenv (3),
.BR capabilities (7),
.BR environ (7)