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       setenv - change or add an environment variable


       #include <stdlib.h>

       int setenv(const char *name, const char *value, int overwrite);

       int unsetenv(const char *name);

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

       setenv(), unsetenv():
           _BSD_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600


       The  setenv()  function adds the variable name to the environment with the value value, if
       name does not already exist.  If name does exist in the environment,  then  its  value  is
       changed  to value if overwrite is nonzero; if overwrite is zero, then the value of name is
       not changed.  This function makes copies of the strings pointed to by name and  value  (by
       contrast with putenv(3)).

       The  unsetenv() function deletes the variable name from the environment.  If name does not
       exist in the environment, then the function succeeds, and the environment is unchanged.


       The setenv() function returns zero on success, or -1 on error, with errno set to  indicate
       the cause of the error.

       The  unsetenv()  function  returns  zero  on  success,  or  -1 on error, with errno set to
       indicate the cause of the error.


       EINVAL name is NULL, points to a string of length 0, or contains an '=' character.

       ENOMEM Insufficient memory to add a new variable to the environment.


       4.3BSD, POSIX.1-2001.


       POSIX.1-2001 does not require setenv() or unsetenv() to be reentrant.

       Prior to glibc 2.2.2, unsetenv() was prototyped  as  returning  void;  more  recent  glibc
       versions follow the POSIX.1-2001-compliant prototype shown in the SYNOPSIS.


       POSIX.1-2001  specifies  that if name contains an '=' character, then setenv() should fail
       with the error EINVAL; however, versions of glibc before 2.3.4  allowed  an  '='  sign  in


       clearenv(3), getenv(3), putenv(3), environ(7)


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