#include <stdlib.h>

       int putenv(char *string);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       putenv(): _SVID_SOURCE || _XOPEN_SOURCE

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

       The putenv() function returns zero on success, or nonzero if  an  error

       ENOMEM Insufficient space to allocate new environment.

       SVr4, POSIX.1-2001, 4.3BSD.

       The  putenv()  function is not required to be reentrant, and the one in
       libc4, libc5 and glibc 2.0 is not, but the glibc 2.1 version is.

       Description for libc4, libc5, glibc: If the argument string is  of  the
       form  name,  and  does  not contain an '=' character, then the variable
       name is removed from the environment.  If putenv() has  to  allocate  a
       new  array  environ,  and  the  previous  array  was  also allocated by
       putenv(), then it will be freed.  In no case will the old storage asso-
       ciated to the environment variable itself be freed.

       The  libc4  and  libc5  and  glibc 2.1.2 versions conform to SUSv2: the
       pointer string given to putenv() is used.  In particular,  this  string
       becomes  part  of  the  environment;  changing it later will change the
       environment.  (Thus, it is an error is to call putenv() with  an  auto-
       matic  variable  as the argument, then return from the calling function
       while string  is  still  part  of  the  environment.)   However,  glibc
       2.0-2.1.1  differs: a copy of the string is used.  On the one hand this
       causes a memory leak, and on the other hand it  violates  SUSv2.   This
       has been fixed in glibc 2.1.2.

       The 4.4BSD version, like glibc 2.0, uses a copy.

       SUSv2 removes the const from the prototype, and so does glibc 2.1.3.

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