GETENV(3) | Library Functions Manual | GETENV(3) |
getenv
, putenv
,
setenv
, unsetenv
—
environment variable functions
Standard C Library (libc, -lc)
#include
<stdlib.h>
char *
getenv
(const
char *name);
int
setenv
(const
char *name, const char
*value, int
overwrite);
int
putenv
(char
*string);
int
unsetenv
(const
char *name);
These functions set, unset and fetch environment variables from the host environment list.
The
getenv
()
function obtains the current value of the environment variable,
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 is equivalent
to:
setenv(name, value, 1);
The string pointed to by
string becomes part of the environment. A program
should not alter or free the string, and should not use stack or other
transient string variables as arguments to
putenv
().
The setenv
() function is strongly preferred to
putenv
().
The
unsetenv
()
function deletes all instances of the variable name pointed to by
name from the list. Note that only the variable name
(e.g., "NAME") should be given; "NAME=value" will not
work.
The getenv
() function returns the value of
the environment variable as a NUL
-terminated string.
If the variable name is not in the current
environment, 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
]getenv
(),
setenv
() or unsetenv
()
failed because the name is a
NULL
pointer, points to an empty string, or points
to a string containing an “=
”
character.
The function putenv
() failed because
string is a NULL
pointer
or string is without an
“=
” character.
ENOMEM
]setenv
(),
unsetenv
() or putenv
()
failed because it was unable to allocate memory for the environment.#include
<stdlib.h>
void
unsetenv
(const
char *name);;
unsetenv
()
doesn't return a value.
putenv
() no longer copies its input
buffer. This often appears in crash logs as a crash in
getenv
(). Avoid passing local buffers or freeing the
memory that is passed to putenv
(). Use
setenv
(), which still makes an internal copy of its
buffers.
unsetenv
() no longer parses the variable
name; e.g., unsetenv ("FOO=BAR") no longer works. Use
unsetenv("FOO"). unsetenv
() also now
returns a status value and will set errno to EINVAL if
name is not a defined environment variable.
The getenv
() function conforms to
ISO/IEC 9899:1990 (“ISO C90”).
The setenv
(), putenv
() and
unsetenv
() functions conforms to
IEEE Std 1003.1-2001 (“POSIX.1”).
The functions setenv
() and
unsetenv
() appeared in
Version 7 AT&T UNIX. The
putenv
() function appeared in
4.3BSD-Reno.
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.
June 20, 2007 | macOS 14.6 |