GETENV(3)

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

LIBRARY
libc

SYNOPSIS
#include <stdlib.h>
char *
getenv(const char *name);
int
getenv_r(const char *name, char *buf, size_t len);
int
setenv(const char *name, const char *value, int overwrite);
int
putenv(char *string);
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 getenv() or getenv_r() given argument name may be appended with an equal sign “=”.

The getenv() function obtains the current value of the environment variable name. If the variable name is not in the current environment, a NULL pointer is returned.

The getenv_r() function obtains the current value of the environment variable name and copies it to buf. If name is not in the current environment, or the string length of the value of name is longer than len characters, then -1 is returned and errno is set to indicate the error.

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 it will set the environment variable “name” equal to “value” by altering an existing entry, or creating a new one if an existing one does not exist. The actual string argument passed to putenv() will become part of the environment. If one changes the string, the environment will also change.

The unsetenv() function deletes all instances of the variable name pointed to by name from the list.

RETURN VALUES
The functions getenv_r(), setenv(), putenv(), and unsetenv() return zero if successful; otherwise the global variable errno is set to indicate the error and a -1 is returned.

If getenv() is successful, the string returned should be considered read-only.

ERRORS

[EINVAL]
    The name argument to setenv() or unsetenv() is a null pointer, points to an empty string, or points to a string containing an “=” character. The value argument to setenv() is a null pointer. The string argument to putenv() is a null pointer, or points to a string that either starts with a “=” character or does not contain one at all.

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

The function getenv_r() can return the following errors:

[ENOENT]
    The variable name was not found in the environment.

[ERANGE]
    The value of the named variable is too long to fit in the supplied buffer.

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

STANDARDS
The getenv() function conforms to ansiC. The putenv() function conforms to xpg4. The unsetenv() function conforms to p1003.1-2001.

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