GETENV(3) BSD Library Functions Manual GETENV(3)NAME
getenv, putenv, setenv, unsetenv — environment variable functions
Standard C Library (libc, -lc)
getenv(const char *name);
setenv(const char *name, const char *value, int overwrite);
unsetenv(const char *name);
These functions set, unset and fetch environment variables from the host
The getenv() function obtains the current value of the environment vari‐
able, name. The application should not modify the string pointed to by
the getenv() function.
The setenv() function inserts or resets the environment variable name in
the current environment list. If the variable name does not exist in the
list, it is inserted with the given value. If the variable does exist,
the argument overwrite is tested; if overwrite is zero, the variable is
not reset, otherwise it is reset to the given value.
The putenv() function takes an argument of the form ``name=value'' and
puts it directly into the current environment, so altering the argument
shall change the environment. If the variable name does not exist in the
list, it is inserted with the given value. If the variable name does
exist, it is reset to the given value.
The unsetenv() function deletes all instances of the variable name
pointed to by name from the list.
The getenv() function returns the value of the environment variable as a
NUL-terminated string. If the variable name is not in the current envi‐
ronment, NULL is returned.
The setenv(), putenv(), and unsetenv() functions return the value 0 if
successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
[EINVAL] The function getenv(), setenv() or unsetenv() failed
because the name is a NULL pointer, points to an empty
string, or points to a string containing an “=” char‐
The function putenv() failed because string is a NULL
pointer, string is without an “=” character or “=” is
the first character in string. This does not follow
the POSIX specification.
[ENOMEM] The function setenv(), unsetenv() or putenv() failed
because they were unable to allocate memory for the
[EFAULT] The functions setenv(), unsetenv() or putenv() failed
to make a valid copy of the environment due to the
environment being corrupt (i.e., a name without a
value). A warning will be output to stderr with
information about the issue.
SEE ALSOcsh(1), sh(1), execve(2), environ(7)STANDARDS
The getenv() function conforms to ISO/IEC 9899:1990 (“ISO C90”). The
setenv(), putenv() and unsetenv() functions conforms to IEEE Std
The functions setenv() and unsetenv() appeared in Version 7 AT&T UNIX.
The putenv() function appeared in 4.3BSD-Reno.
Until FreeBSD 7.0, putenv() would make a copy of string and insert it
into the environment using setenv(). This was changed to use string as
the memory location of the ``name=value'' pair to follow the POSIX speci‐
Successive calls to setenv() that assign a larger-sized value than any
previous value to the same name will result in a memory leak. The
FreeBSD semantics for this function (namely, that the contents of value
are copied and that old values remain accessible indefinitely) make this
bug unavoidable. Future versions may eliminate one or both of these
semantic guarantees in order to fix the bug.
BSD June 20, 2007 BSD