GETENV(3)

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

LIBRARY
libc

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

DESCRIPTION
These functions clear, set, unset and fetch environment variables from the host environment list.

The clearenv() function clears the environment of all name-value pairs.

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 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.

If corruption (e.g., a name without a value) is detected while making a copy of environ for internal usage, then setenv(), unsetenv() and putenv() will output a warning to stderr about the issue, drop the corrupt entry and complete the task without error.

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 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, 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 environment.

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

STANDARDS
The getenv() function conforms to isoC. The setenv(), putenv() and unsetenv() functions conforms to p1003.1-2001. The clearenv() function was to be included in p1003.1-1996 but was delayed and never included in future standards.

HISTORY
The functions setenv() and unsetenv() appeared in 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 specification. The clearenv() function first appeared in

BUGS
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. The clearenv() function marks items inactive in the list, but does not remove them from memory.