man3/getenv.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\" and Copyright (C) 2007, 2012 Michael Kerrisk <mtk.manpages@gmail.com>
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  24 .\" References consulted:
  25 .\"     Linux libc source code
  26 .\"     Lewine's "POSIX Programmer's Guide" (O'Reilly & Associates, 1991)
  27 .\"     386BSD man pages
  28 .\" Modified Sat Jul 24 19:30:29 1993 by Rik Faith (faith@cs.unc.edu)
  29 .\" Modified Fri Feb 14 21:47:50 1997 by Andries Brouwer (aeb@cwi.nl)
  30 .\"
  31 .TH GETENV 3  2012-08-14 "GNU" "Linux Programmer's Manual"
  32 .SH NAME
  33 getenv, secure_getenv \- get an environment variable
  34 .SH SYNOPSIS
  35 .nf
  36 .B #include <stdlib.h>
  37 .sp
  38 .BI "char *getenv(const char *" name );
  39
  40 .BI "char *secure_getenv(const char *" name );
  41 .fi
  42 .sp
  43 .in -4n
  44 Feature Test Macro Requirements for glibc (see
  45 .BR feature_test_macros (7)):
  46 .in
  47 .sp
  48 .BR secure_getenv ():
  49 _GNU_SOURCE
  50 .SH DESCRIPTION
  51 The
  52 .BR getenv ()
  53 function searches the environment list to find the
  54 environment variable
  55 .IR name ,
  56 and returns a pointer to the corresponding
  57 .I value
  58 string.
  59
  60 The GNU-specific
  61 .BR secure_getenv ()
  62 function is just like
  63 .BR getenv ()
  64 except that it returns NULL in cases where "secure execution" is required.
  65 Secure execution is required if one of the following conditions
  66 was true when the program run by the calling process was loaded:
  67 .IP * 3
  68 the process's effective user ID did not match its real user ID or
  69 the process's effective group ID did not match its real group ID
  70 (typically this is the result of executing a set-user-ID or
  71 set-group-ID program);
  72 .IP *
  73 the effective capability bit was set on the executable file; or
  74 .IP *
  75 the process has a nonempty permitted capability set.
  76 .PP
  77 Secure execution may also required if triggered
  78 by some Linux security modules.
  79
  80 The
  81 .BR secure_getenv ()
  82 function is intended for use in general-purpose libraries
  83 to avoid vulnerabilities that could occur if
  84 set-user-ID or set-group-ID programs accidentally
  85 trusted the environment.
  86 .SH "RETURN VALUE"
  87 The
  88 .BR getenv ()
  89 function returns a pointer to the value in the
  90 environment, or NULL if there is no match.
  91 .SH "CONFORMING TO"
  92 .BR getenv ():
  93 SVr4, POSIX.1-2001, 4.3BSD, C89, C99.
  94
  95 .BR secure_getenv ()
  96 is a GNU extension.
  97 .SH VERSIONS
  98 .BR secure_getenv ()
  99 first appeared in glibc 2.17.
 100 .SH NOTES
 101 The strings in the environment list are of the form \fIname=value\fP.
 102
 103 As typically implemented,
 104 .BR getenv ()
 105 returns a pointer to a string within the environment list.
 106 The caller must take care not to modify this string,
 107 since that would change the environment of the process.
 108
 109 The implementation of
 110 .BR getenv ()
 111 is not required to be reentrant.
 112 The string pointed to by the return value of
 113 .BR getenv ()
 114 may be statically allocated,
 115 and can be modified by a subsequent call to
 116 .BR getenv (),
 117 .BR putenv (3),
 118 .BR setenv (3),
 119 or
 120 .BR unsetenv (3).
 121
 122 The "secure execution" mode of
 123 .BR secure_getenv ()
 124 is controlled by the
 125 .B AT_SECURE
 126 flag contained in the auxiliary vector passed from the kernel to user space.
 127 .SH "SEE ALSO"
 128 .BR clearenv (3),
 129 .BR getauxval (3)
 130 .BR putenv (3),
 131 .BR setenv (3),
 132 .BR unsetenv (3),
 133 .BR capabilities (7),
 134 .BR environ (7)