man3/putenv.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  29 .\"     Single UNIX Specification, Version 2
  30 .\" Modified Thu Apr  8 15:00:12 1993, David Metcalfe
  31 .\" Modified Sat Jul 24 18:44:45 1993, Rik Faith (faith@cs.unc.edu)
  32 .\" Modified Fri Feb 14 21:47:50 1997 by Andries Brouwer (aeb@cwi.nl)
  33 .\" Modified Mon Oct 11 11:11:11 1999 by Andries Brouwer (aeb@cwi.nl)
  34 .\" Modified Wed Nov 10 00:02:26 1999 by Andries Brouwer (aeb@cwi.nl)
  35 .\" Modified Sun May 20 22:17:20 2001 by Andries Brouwer (aeb@cwi.nl)
  36 .TH PUTENV 3  2019-03-06 "GNU" "Linux Programmer's Manual"
  37 .SH NAME
  38 putenv \- change or add an environment variable
  39 .SH SYNOPSIS
  40 .nf
  41 .B #include <stdlib.h>
  42 .PP
  43 .BI "int putenv(char *" string );
  44 .\" Not: const char *
  45 .fi
  46 .PP
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .BR putenv ():
  53 _XOPEN_SOURCE
  54     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  55     || /* Glibc versions <= 2.19: */ _SVID_SOURCE
  56 .SH DESCRIPTION
  57 The
  58 .BR putenv ()
  59 function adds or changes the value of environment
  60 variables.
  61 The argument \fIstring\fP is of the form \fIname\fP=\fIvalue\fP.
  62 If \fIname\fP does not already exist in the environment, then
  63 \fIstring\fP is added to the environment.
  64 If \fIname\fP does exist,
  65 then the value of \fIname\fP in the environment is changed to
  66 \fIvalue\fP.
  67 The string pointed to by \fIstring\fP becomes part of the environment,
  68 so altering the string changes the environment.
  69 .SH RETURN VALUE
  70 The
  71 .BR putenv ()
  72 function returns zero on success,
  73 or nonzero if an error occurs.
  74 In the event of an error,
  75 .I errno
  76 is set to indicate the cause.
  77 .SH ERRORS
  78 .TP
  79 .B ENOMEM
  80 Insufficient space to allocate new environment.
  81 .SH ATTRIBUTES
  82 For an explanation of the terms used in this section, see
  83 .BR attributes (7).
  84 .TS
  85 allbox;
  86 lb lb lb
  87 l l l.
  88 Interface       Attribute       Value
  89 T{
  90 .BR putenv ()
  91 T}      Thread safety   MT-Unsafe const:env
  92 .TE
  93 .SH CONFORMING TO
  94 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
  95 .SH NOTES
  96 The
  97 .BR putenv ()
  98 function is not required to be reentrant, and the
  99 one in glibc 2.0 is not, but the glibc 2.1 version is.
 100 .\" .LP
 101 .\" Description for libc4, libc5, glibc:
 102 .\" If the argument \fIstring\fP is of the form \fIname\fP,
 103 .\" and does not contain an \(aq=\(aq character, then the variable \fIname\fP
 104 .\" is removed from the environment.
 105 .\" If
 106 .\" .BR putenv ()
 107 .\" has to allocate a new array \fIenviron\fP,
 108 .\" and the previous array was also allocated by
 109 .\" .BR putenv (),
 110 .\" then it will be freed.
 111 .\" In no case will the old storage associated
 112 .\" to the environment variable itself be freed.
 113 .PP
 114 Since version 2.1.2, the glibc implementation conforms to SUSv2:
 115 the pointer \fIstring\fP given to
 116 .BR putenv ()
 117 is used.
 118 In particular, this string becomes part of the environment;
 119 changing it later will change the environment.
 120 (Thus, it is an error to call
 121 .BR putenv ()
 122 with an automatic variable
 123 as the argument, then return from the calling function while \fIstring\fP
 124 is still part of the environment.)
 125 However, glibc versions 2.0 to 2.1.1 differ: a copy of the string is used.
 126 On the one hand this causes a memory leak, and on the other hand
 127 it violates SUSv2.
 128 .PP
 129 The 4.4BSD version, like glibc 2.0, uses a copy.
 130 .PP
 131 SUSv2 removes the \fIconst\fP from the prototype, and so does glibc 2.1.3.
 132 .PP
 133 The GNU C library implementation provides a nonstandard extension.
 134 If
 135 .I string
 136 does not include an equal sign:
 137 .PP
 138 .in +4n
 139 .EX
 140 putenv("NAME");
 141 .EE
 142 .in
 143 .PP
 144 then the named variable is removed from the caller's environment.
 145 .SH SEE ALSO
 146 .BR clearenv (3),
 147 .BR getenv (3),
 148 .BR setenv (3),
 149 .BR unsetenv (3),
 150 .BR environ (7)