man3/clearenv.3

   1 .\" Copyright 2001 John Levon <moz@compsoc.man.ac.uk>
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  25 .\" Additions, aeb, 2001-10-17.
  26 .TH CLEARENV 3  2016-03-15 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 clearenv \- clear the environment
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <stdlib.h>
  32 .sp
  33 .B "int clearenv(void);"
  34 .fi
  35 .sp
  36 .in -4n
  37 Feature Test Macro Requirements for glibc (see
  38 .BR feature_test_macros (7)):
  39 .in
  40 .sp
  41 .BR clearenv ():
  42     /* Glibc since 2.19: */ _DEFAULT_SOURCE
  43         || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
  44 .SH DESCRIPTION
  45 The
  46 .BR clearenv ()
  47 function clears the environment of all name-value
  48 pairs and sets the value of the external variable
  49 .I environ
  50 to NULL.
  51 After this call, new variables can be added to the environment using
  52 .BR putenv (3)
  53 and
  54 .BR setenv (3).
  55 .SH RETURN VALUE
  56 The
  57 .BR clearenv ()
  58 function returns zero on success, and a nonzero
  59 value on failure.
  60 .\" Most versions of UNIX return -1 on error, or do not even have errors.
  61 .\" Glibc info and the Watcom C library document "a nonzero value".
  62 .SH VERSIONS
  63 Available since glibc 2.0.
  64 .SH ATTRIBUTES
  65 For an explanation of the terms used in this section, see
  66 .BR attributes (7).
  67 .TS
  68 allbox;
  69 lb lb lb
  70 l l l.
  71 Interface       Attribute       Value
  72 T{
  73 .BR clearenv ()
  74 T}      Thread safety   MT-Unsafe const:env
  75 .TE
  76
  77 .SH CONFORMING TO
  78 Various UNIX variants (DG/UX, HP-UX, QNX, ...).
  79 POSIX.9 (bindings for FORTRAN77).
  80 POSIX.1-1996 did not accept
  81 .BR clearenv ()
  82 and
  83 .BR putenv (3),
  84 but changed its mind and scheduled these functions for some
  85 later issue of this standard (cf. B.4.6.1).
  86 However, POSIX.1-2001
  87 adds only
  88 .BR putenv (3),
  89 and rejected
  90 .BR clearenv ().
  91 .SH NOTES
  92 On systems where
  93 .BR clearenv ()
  94 is unavailable, the assignment
  95 .nf
  96
  97     environ = NULL;
  98
  99 .fi
 100 will probably do.
 101
 102 The
 103 .BR clearenv ()
 104 function may be useful in security-conscious applications that want to
 105 precisely control the environment that is passed to programs
 106 executed using
 107 .BR exec (3).
 108 The application would do this by first clearing the environment
 109 and then adding select environment variables.
 110
 111 Note that the main effect of
 112 .BR clearenv ()
 113 is to adjust the value of the pointer
 114 .BR environ (7);
 115 this function does not erase the contents of the buffers
 116 containing the environment definitions.
 117
 118 The DG/UX and Tru64 man pages write: If
 119 .I environ
 120 has been modified by anything other than the
 121 .BR putenv (3),
 122 .BR getenv (3),
 123 or
 124 .BR clearenv ()
 125 functions, then
 126 .BR clearenv ()
 127 will return an error and the process environment will remain unchanged.
 128 .\" .LP
 129 .\" HP-UX has a ENOMEM error return.
 130 .SH SEE ALSO
 131 .BR getenv (3),
 132 .BR putenv (3),
 133 .BR setenv (3),
 134 .BR unsetenv (3),
 135 .BR environ (7)