GETENV(3)

NAME
getenv, putenv, setenv, unsetenv - environment variable functions

LIBRARY
libc

SYNOPSIS
#include <stdlib.h>
char *
getenv(const char *name);
int
putenv(char *string);
int
setenv(const char *name, const char *value, int overwrite);
int
unsetenv(const char *name);

DESCRIPTION
These functions set, unset and fetch environment variables from the host environment list. For compatibility with differing environment conventions, the given arguments name and value may be appended and prepended, respectively, with an equal sign “=”. The behavior is undefined when an equal sign appears at any other location in name.

The getenv() function obtains the current value of the environment variable, name.

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.

RETURN VALUES
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.

ERRORS

[EINVAL]
    The function unsetenv() failed because name was not found in the environment list.

[ENOMEM]
    The function setenv() or putenv() failed because it was unable to allocate memory for the environment.

LEGACY SYNOPSIS
#include <stdlib.h>

void
unsetenv(const char *name);

unsetenv() doesn't return a value.

COMPATIBILITY
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.

SEE ALSO
csh(1), sh(1), execve(2), compat(5), environ(7)

STANDARDS
The getenv() function conforms to isoC.

BUGS
Successive calls to setenv() or putenv() assigning a differently sized value to the same name will result in a memory leak. The FreeBSD semantics for these functions (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.

HISTORY
The functions setenv() and unsetenv() appeared in AT&T UNIX. The putenv() function appeared in 4.3BSD Reno.