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 non-zero; if overwrite is zero,
then the value of name is not changed.
The unsetenv() function deletes the variable name from the
The setenv() function returns zero on success, or -1 if there was
insufficient space in the environment. The unsetenv() function returns
zero on success, or -1 on error, with errno set to indicate the cause
of the error.
SUSv3 specifies that if name contains an '=' character, then
setenv() should fail with the error EINVAL; however, glibc
currently (v2.3.3) allows an '=' sign in name. Portable applications
should avoid this!