.\" Copyright 2001 John Levon <moz@compsoc.man.ac.uk>
.\"
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+.\"
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.\" responsibility for errors or omissions, or for damages resulting from
.\" have taken the same level of care in the production of this manual,
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.\" professionally.
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.\" Additions, aeb, 2001-10-17.
-.TH CLEARENV 3 2001-10-17 "Linux Programmer's Manual"
+.TH CLEARENV 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
clearenv \- clear the environment
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
-.sp
-.BI "int clearenv(void);"
+.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 \fBclearenv\fP() function clears the environment of all name-value
+The
+.BR clearenv ()
+function clears the environment of all name-value
pairs and sets the value of the external variable
.I environ
to NULL.
-.SH "RETURN VALUE"
-The \fBclearenv\fP() function returns zero on success, and a non-zero
+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 non-zero value".
-.SH AVAILABILITY
-Not in libc4, libc5. In glibc since glibc 2.0.
-.SH "CONFORMING TO"
-Various Unix variants (DG/UX, HP-UX, QNX, ...).
+.\" 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 \fBclearenv\fP() and \fIputenv\fP(),
+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 (cf. B.4.6.1).
+later issue of this standard (see \[sc]B.4.6.1).
However, POSIX.1-2001
-only adds \fIputenv\fP(), and rejected \fBclearenv\fP().
+adds only
+.BR putenv (3),
+and rejected
+.BR clearenv ().
.SH NOTES
-Used in security-conscious applications. If it is unavailable
-the assignment
-.RS
-.nf
- environ = NULL;
-.fi
-.RE
+On systems where
+.BR clearenv ()
+is unavailable, the assignment
+.PP
+.in +4n
+.EX
+environ = NULL;
+.EE
+.in
+.PP
will probably do.
-.LP
-The DG/UX and Tru64 manpages write: If
+.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 (),
-.BR getenv (),
+.BR putenv (3),
+.BR getenv (3),
or
.BR clearenv ()
functions, then
will return an error and the process environment will remain unchanged.
.\" .LP
.\" HP-UX has a ENOMEM error return.
-.SH "SEE ALSO"
+.SH SEE ALSO
.BR getenv (3),
.BR putenv (3),
.BR setenv (3),
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