putenv(3)

1putenv(3)                  Library Functions Manual                  putenv(3)
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NAME

6       putenv - change or add an environment variable
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <stdlib.h>
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14       int putenv(char *string);
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16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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18       putenv():
19           _XOPEN_SOURCE
20               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
21               || /* glibc <= 2.19: */ _SVID_SOURCE
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DESCRIPTION

24       The  putenv()  function  adds or changes the value of environment vari‐
25       ables.  The argument string is of the form name=value.   If  name  does
26       not already exist in the environment, then string is added to the envi‐
27       ronment.  If name does exist, then the value of name in the environment
28       is  changed  to value.  The string pointed to by string becomes part of
29       the environment, so altering the string changes the environment.
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RETURN VALUE

32       The putenv() function returns zero on success.  On failure, it  returns
33       a nonzero value, and errno is set to indicate the error.
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ERRORS

36       ENOMEM Insufficient space to allocate new environment.
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ATTRIBUTES

39       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
40       tributes(7).
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42       ┌────────────────────────────────┬───────────────┬─────────────────────┐
43       │Interface                       │ Attribute     │ Value               │
44       ├────────────────────────────────┼───────────────┼─────────────────────┤
45       │putenv()                        │ Thread safety │ MT-Unsafe const:env │
46       └────────────────────────────────┴───────────────┴─────────────────────┘
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STANDARDS

49       POSIX.1-2008.
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HISTORY

52       POSIX.1-2001, SVr2, 4.3BSD-Reno.
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54       The putenv() function is not required to be reentrant, and the  one  in
55       glibc 2.0 is not, but the glibc 2.1 version is.
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57       Since  glibc  2.1.2,  the  glibc  implementation conforms to SUSv2: the
58       pointer string given to putenv() is used.  In particular,  this  string
59       becomes  part  of  the  environment;  changing it later will change the
60       environment.  (Thus, it is an error to call putenv() with an  automatic
61       variable  as  the argument, then return from the calling function while
62       string is still part of the environment.)  However, from glibc  2.0  to
63       glibc 2.1.1, it differs: a copy of the string is used.  On the one hand
64       this causes a memory leak, and on the other hand it violates SUSv2.
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66       The 4.3BSD-Reno version, like glibc 2.0, uses a copy; this is fixed  in
67       all modern BSDs.
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69       SUSv2 removes the const from the prototype, and so does glibc 2.1.3.
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71       The  GNU C library implementation provides a nonstandard extension.  If
72       string does not include an equal sign:
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74           putenv("NAME");
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76       then the named variable is removed from the caller's environment.
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