man3/clearenv.3

.\" Copyright 2001 John Levon <moz@compsoc.man.ac.uk>
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.\" Additions, aeb, 2001-10-17.
.TH CLEARENV 3  2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
clearenv \- clear the environment
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.B "int clearenv(void);"
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR clearenv ():
    /* Glibc since 2.19: */ _DEFAULT_SOURCE
        || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
.SH DESCRIPTION
The
.BR clearenv ()
function clears the environment of all name-value
pairs and sets the value of the external variable
.I environ
to NULL.
After this call, new variables can be added to the environment using
.BR putenv (3)
and
.BR setenv (3).
.SH RETURN VALUE
The
.BR clearenv ()
function returns zero on success, and a nonzero
value on failure.
.\" Most versions of UNIX return -1 on error, or do not even have errors.
.\" Glibc info and the Watcom C library document "a nonzero value".
.SH VERSIONS
Available since glibc 2.0.
.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 clearenv ()
T}	Thread safety	MT-Unsafe const:env
.TE
.sp 1
.SH CONFORMING TO
Various UNIX variants (DG/UX, HP-UX, QNX, ...).
POSIX.9 (bindings for FORTRAN77).
POSIX.1-1996 did not accept
.BR clearenv ()
and
.BR putenv (3),
but changed its mind and scheduled these functions for some
later issue of this standard (see \[sc]B.4.6.1).
However, POSIX.1-2001
adds only
.BR putenv (3),
and rejected
.BR clearenv ().
.SH NOTES
On systems where
.BR clearenv ()
is unavailable, the assignment
.PP
.in +4n
.EX
environ = NULL;
.EE
.in
.PP
will probably do.
.PP
The
.BR clearenv ()
function may be useful in security-conscious applications that want to
precisely control the environment that is passed to programs
executed using
.BR exec (3).
The application would do this by first clearing the environment
and then adding select environment variables.
.PP
Note that the main effect of
.BR clearenv ()
is to adjust the value of the pointer
.BR environ (7);
this function does not erase the contents of the buffers
containing the environment definitions.
.PP
The DG/UX and Tru64 man pages write: If
.I environ
has been modified by anything other than the
.BR putenv (3),
.BR getenv (3),
or
.BR clearenv ()
functions, then
.BR clearenv ()
will return an error and the process environment will remain unchanged.
.\" .LP
.\" HP-UX has a ENOMEM error return.
.SH SEE ALSO
.BR getenv (3),
.BR putenv (3),
.BR setenv (3),
.BR unsetenv (3),
.BR environ (7)