GETENV(3) | Library Functions Manual | GETENV(3)

# NAME

`getenv`, `getenv_r`, `putenv`, `setenv`, `unsetenv` — environment variable functions

# LIBRARY

Standard C Library (libc, -lc)

# 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 ANSI X3.159-1989 (“ANSI C89”).

The `putenv`() function conforms to X/Open Portability Guide Issue 4 (“XPG4”).

The `setenv`() and `unsetenv`() functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).

# HISTORY

The functions `setenv`() and `unsetenv`() appeared in Version 7 AT&T UNIX. The `putenv`() function appeared in 4.3BSD-Reno.

May 25, 2025 | NetBSD 11.0